Swapping Rear End Ratios

[NC Cobra]

I would welcome some input on how difficult it is to swap out rear end ratios in our cars. Do you need to drop the pumpkin to get proper access? I have the 2015 Ford Super 8.8 in a IRS configuration.

[CraigS]

I sure would drop it. As w/ the solid axle I suppose it would be possible to leave it in the car but, since you need to remove both axles and the drive shaft anyway, I'd drop it. A friend did it to his on the bench. It is mostly like doing a solid axle but, IIRC, he needed to make himself a case spreader tool. I'd look for instructions on the Ford website and pay attention to look for differences between al and iron cases. Also, I don't remember the details, but not all ratios can be swapped for another ratio.

[NC Cobra]

Thanks Craig. Think Im ok on the ratios. Going from 3.31 to 3.55. For what its worth, here is a photo. 142037

[rich grsc]

Yes it'll have to come out. No way to spread the case bolted into the frame.

[NAZ]

I suggest you get a service manual before deciding that you want to tackle this as a DIY project as there are some tools you will need that most folks don't have.

It's not complicated but does require special tools and knowledge to set them up correctly. I was originally trained by Ford at their Engineering Center to overhaul Ford rear ends but have set-up hypoid angle drives from single digit HP ratings to huge drives you could literally crawl into to work on -- all have commonalities to their basic set-up. The work is precise and folks that work to a standard of "that looks close enough" should not attempt this.

If you want to learn a new skill and can work with close tolerances and have the proper tools, buy a manual or search the internet for a Ford Performance set-up procedure on your axle -- read and understand it, then decide if this is a job you want to tackle. You could take the center section to a Ford Dealer, a race shop, or even off-road shop that has experience doing gear changes. The ratio switch you are looking at will really wake up your car.

[Railroad]

Another option would be to buy the complete rear in the ratio you desire. You can probably find a buyer for the 3.31.
It would be tuff to pull, change gears, install and find out the gear install was not right.

[BradCraig]

Another option would be to buy the complete rear in the ratio you desire. You can probably find a buyer for the 3.31.
It would be tuff to pull, change gears, install and find out the gear install was not right.

Correct...for the price of a salvage center it makes sense.

[Hoooper]

The rear cover of the diff is bolted to the car. Without dropping the whole thing you would be supporting the entire weight plus all the prying you have to do to loosen bolts and get the carrier out and that would all be on the two front cast iron wings. Removing and installing the diff isn't that hard, i suspect even if you could do the work work it still in it would be more of a pain than just pulling it

[NC Cobra]

Thanks very much guys. I already have a new ring and pinion gear from Mike Forte and am in line at the local speed shop to have it installed. This was more of a “how hard could it be” type of question that comes up when you search on line and have too much time on your hands. Sounds like this is one of those 400 level classes in the school of hard knocks waiting to happen. Think I’m going to let the pros do the job right the first time. I may grab a 6 pack and head over to watch him pull it however. Sounds like an interesting opportunity to learn a few things and I have found liquid appreciation to be a good form of tuition.

[NC Cobra]

Naz, thanks for the advice. Over the years I have learned to listen when people with experience offer caution it pays to follow their advice. As a client of mine from West Texas once told me “God gave you two ears and one mouth. Use them proportionately.”

Interesting to hear that you spent some time in Dearborn as well. I spent 1.5 years over in the Blue Lagoon with Ford heavy Truck before the division was sold. Curious as to what type of drives were you working on that was so large. Guessing it had to be mining, rail or ship orientated. My Dad used to tell stories about the old lake freighters that had piston cylinders so large that they had doors through the engine block. Fascinating to watch machinery that large in operation. The power just boggles the mind - and deafens your ears too!

[NAZ]

NC, I used to rebuild very large drives on industrial machinery in a previous life. And as a millwright, I used to remove & install them. What's a millwright? https://en.wikipedia.org/wiki/Millwright

[CraigS]

NC, I used to rebuild very large drives on industrial machinery in a previous life. And as a millwright, I used to remove & install them. What's a millwright? https://en.wikipedia.org/wiki/Millwright

Thanks for posting this NAZ. I have heard the term many times but never bothered to check the actual meaning. So now a burning question. What is NAZ? I thought maybe northern AZ but Flagstaff is only a little above center of the state.

[NAZ]

Craig, you thought correct -- NAZ is for Northern AZ. When I was a Physical Plant Director at ADOT I was responsible for facilities in the northern half of the State, or NAZ (Northern Arizona). Once you get north of Flagstaff things get fairly sparse and by the time you reach the Arizona Strip, you'd think you were in Baja Utah. And the northeast corner of the state is mostly Rez. I live in the second largest county in the country, in the forest about 10-miles north of Flag and there is lots of open country up here. I can swing my back gate open and ride my horses for hundreds of miles, but at my age I'm happy just to ride a few miles looking for sheds and then back to the barn.

[GoDadGo]

Suggestion:..Please run your numbers, or I can do it for you, to figure out if the altered ratio puts your engine in the Sweetest RPM Possible for highway cruising.
................I went on the conservative side when I picked 3.73's and now wish I would have gone with 3.90's.
................It would have made a nice enough difference to get me to a sweeter cruising RPM.

[NC Cobra]

Steve,
Thank you for your offer. I’m running a Forte SBF 306 with 311 hp & 322 torque per the dyno. Car has a T5 and 3.11 ratio with 375 Street Comps @ 25.6” dia. I’ve run the #’s on this calculator https://www.blocklayer.com/rpm-gear.aspx and 3.55 it seems to be the right answer to my concerns with the car lugging when I hit 5th gear on the highway. At 70 mph I’m turning about 1’700 to 1,800 rpm and it doesn’t feel much better till I’m in the upper 80 mph range. With a 3.55 gear set that rpm comes up to around 2,200 @ 70 and the numbers don’t seem to dramatically change in the lower 4 gears which I’m generally happy with. I’ll also be changing up my tires as the 245/275” street comps are 4.5 years old and the compound is hard enough that I can spin them at 1/4 throttle easily in the first two gears. I tend toward the road racer in my driving style - North Carolina mountain roads will do that to you- so hole shots are not my focus with the car. (But they are fun) Having done some research on the site, it seems that several respected builders have been happy with 3.55 with cars similar to mine. What I don’t have is seat of the pants insight as to if the 3.55 or 3.73 ratio would meet my objectives for a street car that does some HPDE events, the occasional AX event for skills, and eventually a few hill climbs like the tail of the dragon event. Any input prior to wrenches being turned would be appreciated.

[GoDadGo]

NC,

I ran the numbers using the gear ratios for the T-5 that are currently displayed on the Tremec Website and here is what I came up with.

3.31 RING.............2.95 / 1st...............1.94 / 2nd...............1.34 / 3rd...............1.00 / 4th................63 / 5th
-------------------------------------------------------------------------------------------------------------------------------
1,200 RPM..............9.38 MPH...............14.27 MPH..............20.66 MPH..............27.68 MPH.............43.94 MPH
2,200 RPM.............17.20 MPH..............26.16 MPH..............37.98 MPH..............50.75 MPH.............80.56 MPH
-------------------------------------------------------------------------------------------------------------------------------


3.55 RING.............2.95 / 1st...............1.94 / 2nd...............1.34 / 3rd...............1.00 / 4th................63 / 5th
-------------------------------------------------------------------------------------------------------------------------------
1,200 RPM..............8.73 MPH...............13.27 MPH..............19.21 MPH..............25.74 MPH.............40.86 MPH
2,200 RPM.............16.00 MPH..............24.33 MPH..............35.22 MPH..............47.20 MPH.............74.92 MPH
-------------------------------------------------------------------------------------------------------------------------------


3.73 RING.............2.95 / 1st...............1.94 / 2nd...............1.34 / 3rd...............1.00 / 4th................63 / 5th
-------------------------------------------------------------------------------------------------------------------------------
1,200 RPM..............8.31 MPH...............12.63 MPH..............18.28 MPH..............24.50 MPH.............38.89 MPH
2,200 RPM.............15.23 MPH..............23.15 MPH..............33.52 MPH..............44.92 MPH.............71.30 MPH
-------------------------------------------------------------------------------------------------------------------------------

My big takeaway regarding this subject is hitting a Sweet Spot at a Typical Interstate Speed Limit of 70 MPH, assuming this is the case.

Steve

[NC Cobra]

Steve,
I see you are an early riser as well. Thanks for taking the time to run those values, very much appreciated. Your math agrees with the calculator I used. What Im struggling with is how quickly I’m going to run up to that 15 or 16 mph in first gear and how much of that expensive new rear tire I’m going to leave behind on the pavement. ��. Would you mind sharing your thoughts on why you wanted a deeper gear? NAZ made the comment that the swap to 3.55 would really wake the car up and Im concerned that the #’s aren’t telling the whole story.
Thanks
Jeff

[GoDadGo]

Jeff,

The use of the car is everything, but we all know the deeper the gear the easier it will be to spin the tires which is a problem we all have with these little cars.

When you are changing the rear end ratio you are changing the torque multiplier to the tires so the percentage change in the ratio equates to real-world power at the pavement.
I'm running 3.73's because my 5th gear is a .75 with 6th being a .50 overdrive which created a 1st gear versus 6th gear dilemma...I chose the 3.73 gear assuming I'd hit around 124 MPH at the top of 4th gear at 6000 RPM...This ratio gave me a little wiggle room for the top of the 1/4 Mile since my redline is 6,500.00 RPM...6th gear also gets me to 1,700 RPM at 70 MPH or 1,800 RPM at 80 MPH.

Torque Is What Moves Cars so when you think about it that way, the Horse Power Numbers are just a reference point.

Steve

[BradCraig]

Where in NC are you? I’m right outside Charlotte.

[NAZ]

You built a “performance” vehicle and if performance is important you should choose your gearing wisely. Look at your engines torque curve and think about the following.

Brake specific fuel consumption (BSFC) is a measure of how efficient the engine is combusting fuel at a given RPM. The formula for calculating BSFC = fuel pounds per hour brake horsepower and many dyno programs will report this important variable. The lower the BSFC number, the less fuel the engine is using to develop HP. Therefore, the lower the BSFC number, the more efficient the engine is. Typically, the lowest BSFC will appear at or near peak torque as that is where the best volumetric efficiency is.


For a high performance street car or for my tow rigs I like my highway cruise (75 MPH here in AZ) RPM to be just below my peak torque. If I have beaucoup torque and not a need for that much I may adjust the cruise RPM down a bit.

[CraigS]

One thing you have going for you regardless of all the math is the 355 has been by far the most popular diff ratio in FFRs for ages. Particularly w/ a 302 like yours. The problem going in either direction from can be two fold. Assuming reasonably good tires, go to 373 and first becomes shorter, and has more torque multiplication, so it becomes almost a throw away gear. Just spins up the tires. Even in normal around town driving it feels like you need to shift half way across the street from the stop sign. OTOH, the other gears do feel better, but now 5th may spin the engine more than you really want. You already know what your 331 feels like. That 1800 on the highway sucks. I think you will be very happy w/ the 355.

[rich grsc]

So why does 1800 suck? I can drive all day long at that rpm

[NC Cobra]

Brad,
I’m in Ballentyne, on the South end of town. How bout you?
Cheers
Jeff

[NC Cobra]

So why does 1800 suck? I can drive all day long at that rpm

Thinking the difference in our experience is that your engine is putting out more torque than mine. In my car it feels gear bound and outside of the sweet spot in the torque curve.

[NC Cobra]

You built a “performance” vehicle and if performance is important you should choose your gearing wisely. Look at your engines torque curve and think about the following.

Brake specific fuel consumption (BSFC) is a measure of how efficient the engine is combusting fuel at a given RPM. The formula for calculating BSFC = fuel pounds per hour brake horsepower and many dyno programs will report this important variable. The lower the BSFC number, the less fuel the engine is using to develop HP. Therefore, the lower the BSFC number, the more efficient the engine is. Typically, the lowest BSFC will appear at or near peak torque as that is where the best volumetric efficiency is.


For a high performance street car or for my tow rigs I like my highway cruise (75 MPH here in AZ) RPM to be just below my peak torque. If I have beaucoup torque and not a need for that much I may adjust the cruise RPM down a bit.

Naz
Thanks for the info. I know I have those data points in my dyno results and will have to review that when I get back home from this business trip. Appreciate the opportunity to learn more about BSFC, first time I’ve heard the term and am very interested to test my assumptions against the numbers.
Cheers,
Jeff

[NAZ]

1800 RPM cruise? That's diesel truck cruise :)

[NC Cobra]

One thing you have going for you regardless of all the math is the 355 has been by far the most popular diff ratio in FFRs for ages. Particularly w/ a 302 like yours. The problem going in either direction from can be two fold. Assuming reasonably good tires, go to 373 and first becomes shorter, and has more torque multiplication, so it becomes almost a throw away gear. Just spins up the tires. Even in normal around town driving it feels like you need to shift half way across the street from the stop sign. OTOH, the other gears do feel better, but now 5th may spin the engine more than you really want. You already know what your 331 feels like. That 1800 on the highway sucks. I think you will be very happy w/ the 355.

Craig,
Thanks very much. The scenario you describe regarding first gear is exactly my concern. I know I’m moving the torque curve in the right direction; but with two variables (100 tw tires & ratio change) I find myself second guessing myself as to how far I need to go. Think I will validate my assumptions with the method proposed by Naz and most likely stick with the conventional wisdom of previous 302 owners at 3.55.
Cheers,
Jeff

[NC Cobra]

1800 RPM cruise? That's diesel truck cruise :)

Hey, down-speeding diesel engines is all the rage these days. Drivers hate it but the accounts love the fuel bills.

[Big Blocker]

NAZ,

I knew there was something I liked about you . . . Quote: "I was originally trained by Ford at their Engineering Center".

So was I, albeit way back in the late 60's. I officially went to the FORD-SHELBY Technical Center in Santa Fe Springs, Ca.

Guess we're partners in crime, huh?

Doc

[NAZ]

NAZ,

I knew there was something I liked about you . . . Quote: "I was originally trained by Ford at their Engineering Center".

So was I, albeit way back in the late 60's. I officially went to the FORD-SHELBY Technical Center in Santa Fe Springs, Ca.

Guess we're partners in crime, huh?

Doc


Yes we are Doc. It was 1969 or 1970 and I was working at Keystone Ford in Norwalk, CA when I got picked to go through the training. Ford had an Engineering Center on the campus of the Pico Rivera assembly plant and we trained there for a week then went back to our dealerships for three-weeks to work in whatever section that we had been studying, i.e., A/C, transmission, alignment, etc. That training cycle continued for maybe a year or more. We normally worked off commission only but for the week I was in training I got a meager salary. Great training by the engineering staff.

[GoDadGo]

1800 RPM cruise? That's diesel truck cruise :)

A .50 6th gear in Redbone's ZF Gear Box gets me to 1,800-RPM @ 80-MPH or 1,700-RPM @ 70-MPH.

[NAZ]

1700 @ 70 MPH? That's just above idle speed on my car. You guys must be building those hyper-mileage vehicles, not performance cars. Next you'll be telling us your exhaust only registers 60dB.

[GoDadGo]

1700 @ 70 MPH? That's just above idle speed on my car. You guys must be building those hyper-mileage vehicles, not performance cars. Next you'll be telling us your exhaust only registers 60dB.

Naz,

It's more of a Hyper-Cruising Machine even with the 3.73 rear end.

ZF S6-40 General Specifications:
Gear ratios - 1st-2.68, 2nd-1.80, 3rd-1.29, 4th-1.0, 5th-0.75, 6th-0.50, Reverse-2.50
Description - 6 Speed 95 mm Fully Synchronized
Weight (w/shifter) 145 lbs Plus Flywheel & Clutch

3,000 RPM Equates to 125-MPH / 6th Gear Super Cruise

It actually feels a lot faster than my old C-4 Corvette at the same speed and no I didn't stay there for very long.

The good news is the motor was really, really, really happy at 3,000 RPM since the operation range is 2,300-6,500 RPM.

Steve

[NC Cobra]

Naz,

It's more of a Hyper-Cruising Machine even with the 3.73 rear end.

ZF S6-40 General Specifications:
Gear ratios - 1st-2.68, 2nd-1.80, 3rd-1.29, 4th-1.0, 5th-0.75, 6th-0.50, Reverse-2.50
Description - 6 Speed 95 mm Fully Synchronized
Weight (w/shifter) 145 lbs Plus Flywheel & Clutch

3,000 RPM Equates to 125-MPH / 6th Gear Super Cruise

It actually feels a lot faster than my old C-4 Corvette at the same speed and no I didn't stay there for very long.

The good news is the motor was really, really, really happy at 3,000 RPM since the operation range is 2,300-6,500 RPM.

Steve


Steve,
That must be what happens when you add that cheater 6th gear. You need to label it hyperdrive and watch the world blur as you get the rpms up!
Cheers,
Jeff

[CraigS]

So why does 1800 suck? I can drive all day long at that rpm

It depends on your driving style and location. My torquey 408 was just fine at that speed. A mild 302 will feel a little weak when you come to a hill. And w/ a stock T-5 w/ the huge gap between 4th and 5th, shifting down to climb a grade isn't much fun.

[rich grsc]

Maybe true, but I don't have a mild 302. I don't need to down shift if I'm at or above 1800.

[Paul2STL]

I would welcome some input on how difficult it is to swap out rear end ratios in our cars. Do you need to drop the pumpkin to get proper access? I have the 2015 Ford Super 8.8 in a IRS configuration.
Its not that difficult. I swapped out my 3.31 pumpkin out for a 3.73 one I got with low miles on Ebay. I know this doesn't help you since you have already bought the gear set, but I found it cheaper to buy the whole differential then buying the gears and having some one install them. The only tuff part is getting it out of the mounts from under the car. You have to do a little finagualing to get it out and back in. In all it took me about 3 hours to include jacking up the car and disconnecting the upper spindles. The cost of someone having to do it for you will be spendie in my opinion.

[NC Cobra]

Brake specific fuel consumption (BSFC) is a measure of how efficient the engine is combusting fuel at a given RPM. The formula for calculating BSFC = fuel pounds per hour brake horsepower and many dyno programs will report this important variable. The lower the BSFC number, the less fuel the engine is using to develop HP. Therefore, the lower the BSFC number, the more efficient the engine is. Typically, the lowest BSFC will appear at or near peak torque as that is where the best volumetric efficiency is.


For a high performance street car or for my tow rigs I like my highway cruise (75 MPH here in AZ) RPM to be just below my peak torque. If I have beaucoup torque and not a need for that much I may adjust the cruise RPM down a bit.

Naz,
Sorry it took so long to look at this; but life does get in the way sometimes. Checked my dyno results and min BSFC occurs at 4,900 rpm. For reference my max torque occurs at 4,500 rpm and max hp at 5,600 rpm. Kind of happy with that as it seems my seat of the pants shift point of 4,500 rpm has some validity and the min BSFC is not too far off.

I did run the calcs for speed at peak torque as well and had to chuckle a bit. With the 3.31 gears I would theoretically be going 165 mph at 4,900 rpm. Drops to a more modest 153 mph with the 3.55 gear set. :p. Obviously the calculator doesn’t factor in the wind resistance for our flying brick; but my seat of the pants ASSessment is that I would be in triple digit territory for sure. Sounds like I’d better load up some Sammy Hagar (cant drive 55) and start contributing to my local Sherrifs Association if I’m cruising at max turque! :cool: FYI at a typical cruising speed of 75 mph I’ll theoretically be spinning 2,300 rpm which I’m fairly comfortable with from a real life perspective.

Final decision was to stick with the 3.55 gears and have my local speed shop do the install as I don't have the tools and experience is expensive when you are learning on your own. Thanks to everyone for their input and I’ll let you know how it feels when the rubber gets back on the road.
Cheers,
Jeff

[NAZ]

Naz,
Sorry it took so long to look at this; but life does get in the way sometimes. Checked my dyno results and min BSFC occurs at 4,900 rpm. For reference my max torque occurs at 4,500 rpm and max hp at 5,600 rpm. Kind of happy with that as it seems my seat of the pants shift point of 4,500 rpm has some validity and the min BSFC is not too far off.

I did run the calcs for speed at peak torque as well and had to chuckle a bit. With the 3.31 gears I would theoretically be going 165 mph at 4,900 rpm. Drops to a more modest 153 mph with the 3.55 gear set. :p. Obviously the calculator doesn’t factor in the wind resistance for our flying brick; but my seat of the pants ASSessment is that I would be in triple digit territory for sure. Sounds like I’d better load up some Sammy Hagar (cant drive 55) and start contributing to my local Sherrifs Association if I’m cruising at max turque! :cool: FYI at a typical cruising speed of 75 mph I’ll theoretically be spinning 2,300 rpm which I’m fairly comfortable with from a real life perspective.

Final decision was to stick with the 3.55 gears and have my local speed shop do the install as I don't have the tools and experience is expensive when you are learning on your own. Thanks to everyone for their input and I’ll let you know how it feels when the rubber gets back on the road.
Cheers,
Jeff


Good choice. Very practical.

The only practical cars I own are the OEM built ones I haven't modified -- yet. Did I mention that my Hot Rod is running 3600 RPM at 75 MPH?

[Avalanche325]

Checked my dyno results and min BSFC occurs at 4,900 rpm. For reference my max torque occurs at 4,500 rpm and max hp at 5,600 rpm. Kind of happy with that as it seems my seat of the pants shift point of 4,500 rpm has some validity and the min BSFC is not too far off.

You are short shifting and leaving some acceleration on the table. You want to shift when the NEXT gear is at or coming into maximum torque, not the gear that you are in. Another way of saying it is that the next gears torque should be higher than the gear you are currently in. That will give you higher overall average torque delivery. That is usually at or near redline for performance cars.

[GoDadGo]

25.59" Tire (315/35-17 or 275/40-17)

3.55 RING.............2.95 / 1st..............1.94 / 2nd..............1.34 / 3rd..............1.00 / 4th...............63 / 5th
----------------------------------------------------------------------------------------------------------------------------
6,000 RPM............48.01 MPH.............71.48 MPH............99.74 MPH............128.67 MPH...........171.56 MPH

I'm assuming your REDLINE is 6,000.

[NAZ]

Here's the science behind shift points for max acceleration. https://www.youtube.com/watch?v=zZBqb0ZJSwU If you get a couple minutes into this video and your head is spinning, just shift every gear at red line and you'll be close.

[NC Cobra]

Thanks, I learned something today.

[CraigS]

Maybe true, but I don't have a mild 302. I don't need to down shift if I'm at or above 1800.

Jeez rich, this thread is about the OP's gear choice, for his engine. It's not about your engine.

[GoDadGo]

Thanks, I learned something today.

I Did Too So I Overlayed My Torque Curve With My Shift Points.
In my case, the torque curve is Crazy Flat once the engine is running between 2,250 to 5,750 R.P.M.'s
I Never Truly Understood the relationship between the transmission gear ratios and the torque curve of the engine.
Even though the Sweet Spot of my engine's torque curve is rather large, the Close-Ratio ZF S6-40 transmission can keep the engine where it is happiest.
Thanks For Starting This Thread & It Is Great That "The NAZ" Always Finds Ways To Give Us Teachable Mechanical Moments!

[rich grsc]

Jeez rich, this thread is about the OP's gear choice, for his engine. It's not about your engine.
What your issue? I'm not the only person talking about how their car drives with a specific gear set, and it was a response to another post.

[Hoooper]

For a high performance street car or for my tow rigs I like my highway cruise (75 MPH here in AZ) RPM to be just below my peak torque. If I have beaucoup torque and not a need for that much I may adjust the cruise RPM down a bit.

That makes sense for diesels since most make peak torque well under 2000 RPM, or maybe you with a 2 speed trans which obviously severely limits your options for gearing, but for most of us around here with 5 or 6 gears it would be pretty irritating and extremely pointless to be cruising on the freeway at 4000-5000 RPM.