Rear End Gear Ratio Dilemma

[lastdime]

My roadster has a 460 SCJ engine with 550 hp @6250 RPMs and 545 ft./lbs. of torque @ 4800 RPMs. My cam is a Ford Racing M-8501-C460 with significant horsepower above 3000 RPMs and great low-end torque. I have no problem accelerating in 5th gear from 1800 RPMs. The transmission is a TKO 600 with .82 5th gear overdrive and my rear end gears are 3:55. Tire height is 25.13 inches.

So what's the dilemma? I love the performance and it is super fast (0-60 MPH in 3.6 seconds). Don't know what it does in the quarter mile but it's a rocket. And.....I'm tired of 3300 RPMs at 80 MPH in 5th gear. I've used the TREMEC Gear Ratio Calculator but it is way off the actual speed/RPM test drives (as in 4-6 MPH higher from 3000 - 5000 RPMs).

If the rear end is dropped to 3:27 gears it would still be doing 2870 RPMs at 80 MPH which is better, but not good enough. With 3:08 gears it would do 80 MPH at 2700 RPMs and 70 MPH at 2370 RPMs. This is all figured with the TREMEC calculator, which because of actual driving experience, I don't trust. I can currently drive on posted 50 MPH roads in 5th gear with no problem. If it were a problem I could just shift down to 4th.

So all you knowledgeable gear heads out there.....give me some feedback to mull over.

Thanks.

Mark

[BradCraig]

My roadster has a 460 SCJ engine with 550 hp @6250 RPMs and 545 ft./lbs. of torque @ 4800 RPMs. My cam is a Ford Racing M-8501-C460 with significant horsepower above 3000 RPMs and great low-end torque. I have no problem accelerating in 5th gear from 1800 RPMs. The transmission is a TKO 600 with .82 5th gear overdrive and my rear end gears are 3:55. Tire height is 25.13 inches.

So what's the dilemma? I love the performance and it is super fast (0-60 MPH in 3.6 seconds). Don't know what it does in the quarter mile but it's a rocket. And.....I'm tired of 3300 RPMs at 80 MPH in 5th gear. I've used the TREMEC Gear Ratio Calculator but it is way off the actual speed/RPM test drives (as in 4-6 MPH higher from 3000 - 5000 RPMs).

If the rear end is dropped to 3:27 gears it would still be doing 2870 RPMs at 80 MPH which is better, but not good enough. With 3:08 gears it would do 80 MPH at 2700 RPMs and 70 MPH at 2370 RPMs. This is all figured with the TREMEC calculator, which because of actual driving experience, I don't trust. I can currently drive on posted 50 MPH roads in 5th gear with no problem. If it were a problem I could just shift down to 4th.

So all you knowledgeable gear heads out there.....give me some feedback to mull over.

Thanks.

Mark

You sure you don't have a 3.73 rear?

[NAZ]

What's the objection to 3,300 RPM? What are you trying to achieve? Sounds like you know how to calculate RPM / speed / ratios so what input are you looking for?

There are quite a few ratios available for the 8.8 so have many options but I suggest you start with a problem statement and segue to possible solutions (not limited to rear end ratios). Don't forget to look at your torque curve for all the possible cruise RPMs you may select and keep in mind two very important things: torque & HP ratings are always at WOT and will not represent output at cruise RPM throttle opening and at at your density altitude actual WOT numbers will be noticeably less than theoretical.

[Jim1855]

Lastdime,

I'm kinda partial to 3.27s, TKO-600 w/ .64 OD, 315 or 335/35-17. 80mph in 5th at 2,200, 157 in 4th at something close to 7,000. Use 4th running on the track or otherwise to go fast. Lots of miles (65k) with this combo around the neighborhood, road tracks or cross country. Same pieces except 3.31 going into the new build.

427w 500+ HP & TQE at flywheel. 440 something at the wheels. Not to far off from your numbers, plenty of torque at the bottom and power at the top.

Worked for me, YMMV.

Jim

PS Naz, I sure wouldn't want to listen to 3,300rpm for ten hours on a road trip. 2,200 and ear plugs was enough, felt like my head was in a vice at the end of the day.

[Itchief]

Mark

You can calculate the rpm at 80 mph with the new 3.27 gear using the following where x = new rpm

3300/3.55 = x/3.27

930 = x/3.27

930 x 3.27 = x

3041 = x So at 80 mph with the 3.27 gear the rpm should be 3041

A 3.08 gear would put you at about 2860 rpm

Rick

[NAZ]

Lastdime,

I'm kinda partial to 3.27s, TKO-600 w/ .64 OD, 315 or 335/35-17. 80mph in 5th at 2,200, 157 in 4th at something close to 7,000. Use 4th running on the track or otherwise to go fast. Lots of miles (65k) with this combo around the neighborhood, road tracks or cross country. Same pieces except 3.31 going into the new build.

427w 500+ HP & TQE at flywheel. 440 something at the wheels. Not to far off from your numbers, plenty of torque at the bottom and power at the top.

Worked for me, YMMV.

Jim

PS Naz, I sure wouldn't want to listen to 3,300rpm for ten hours on a road trip. 2,200 and ear plugs was enough, felt like my head was in a vice at the end of the day.

That made me laugh. Yes Jim, I can really understand that -- I run 3.90:1 rear ratio and a 6000 RPM stall converter (and wear ear plugs). But I've learned not to assume too much. I put up with the high revs as performance trumps comfort in my case.

You're combo is a better choice for a street car as the .64:1 ratio OD is way better at reducing the drone of loud exhaust. The TKO-600 part number TCET5008 would be my choice for Mark's ride and I'd stay with the 3.55:1 as that would maintain the acceleration performance while keeping the cruise RPMs a more reasonable 2450 @ 80 MPH. But for me, if I want creature comfort I'll drive the Mercedes. The hot rod is for fun not comfort and the fun factor is linked to the raw power and loud exhaust. But I never drive more than 10-miles into town for cruise night. I keep getting older but I refuse to grow up.

[SJDave]

I just sold a Mark 3.1 that had a Smeding Performance 427, 620 HP ...650 ft lbs of torque. Same tranny, but had 3.73 gears in it...couldn't stand to drive it on the freeways...over 3000 rpm. I was tempted to sell the .82 OD TKO 600 and get a .64 5th gear, that is really the best solution. But time and money. Had a local rear end shop swap to 2.73 rear gears, made the highway cruising so nice at roughly 2100 rpm at 75 mph. Engine had FAST EFI fuel injection with 8 Stack intake, took very little tuning effort to get it to run great with this combo. Yeah it lost come torque, but it had way too much anyway for a street car even with Toyo 888s on it. First gear was unusable before, I always started in second. First gear at 6500 rpm now is just a hair over 60 mph, and if you carefully apply the throttle you have some wheelspin but not a cloud of smoke. Full throttle, cloud of smoke still. Maybe go to the 3:08 as a compromise for now.

[mike223]

MK 4 Roadster #8195, 460 Super Cobra Jet Engine, Wilwood Brakes, IRS, TKO 600, 18" Halibrand Wheels

Mark

T-56

All problems solved and it shifts better too.

I think it also uses the same bellhousing.

[NAZ]

T-56

All problems solved and it shifts better too.

I think it also uses the same bellhousing.

Good choice! The TUET11011 would lower the cruise RPM significantly.

https://www.tremec.com/menu/magnum/

[Jim1855]

Sorry Mike, the bell housing changes for the T56. Will probably need a new throw-out bearing and certainly a shorter driveshaft.

I have the TKO-600 and am looking at switching to the T56 Magnum. I have heard many times that the shifting is significantly better and the combination of the two overdrives has real appeal. But, there're always "buts", the T56 is wider, longer and heavier. I think the basic cost of T56 and a QuickTime bellhousing is about $3,900.

The OD gears can be changed in the TKO-600, this might help the most. Mike Forte can direct the process.

Jim

[mike223]

I have the TKO-600 and am looking at switching to the T56 Magnum. I have heard many times that the shifting is significantly better...



For anyone who is interested - this video does a good job of explaining the difference in synchro rings (why T5 + T56 shift better).

The first part is conventional operation, he gets into the newer rings about 10 minutes in.

https://www.youtube.com/watch?v=MXsRfbOiBhE

[j.miller]

I just sold a Mark 3.1 that had a Smeding Performance 427, 620 HP ...650 ft lbs of torque. Same tranny, but had 3.73 gears in it...couldn't stand to drive it on the freeways...over 3000 rpm. I was tempted to sell the .82 OD TKO 600 and get a .64 5th gear, that is really the best solution. But time and money. Had a local rear end shop swap to 2.73 rear gears, made the highway cruising so nice at roughly 2100 rpm at 75 mph. Engine had FAST EFI fuel injection with 8 Stack intake, took very little tuning effort to get it to run great with this combo. Yeah it lost come torque, but it had way too much anyway for a street car even with Toyo 888s on it. First gear was unusable before, I always started in second. First gear at 6500 rpm now is just a hair over 60 mph, and if you carefully apply the throttle you have some wheelspin but not a cloud of smoke. Full throttle, cloud of smoke still. Maybe go to the 3:08 as a compromise for now.



I'm with SJDave. If you're going to knock it down then knock it down 273-308 (I lean toward the 273). There was a time when the speed limit was 65 and everybody (mostly) abided by it. Now days runnin 75 will get your arse run over and 85 is the norm so to get the R's down at that speed ya gotsta make some big cuts. The least expensive would be a gear change......or write your congressman and try to bring back 55max speed (you would not be popular with the gang for that move) Sooooo Gear change or be the second most hated man in America...da Bat (jus tryin to make the choice easy for ya)

[Erik W. Treves]

With that much torque and that engine which is probably making 400+ down low - I wouldn't be afraid for a second to throw in a set of 2.73's - you should get a lot more use out of 1st gear and your 5th gear will be much better.

[GTBradley]

I'm with SJDave. If you're going to knock it down then knock it down 273-308 (I lean toward the 273). There was a time when the speed limit was 65 and everybody (mostly) abided by it. Now days runnin 75 will get your arse run over and 85 is the norm so to get the R's down at that speed ya gotsta make some big cuts. The least expensive would be a gear change......or write your congressman and try to bring back 55max speed (you would not be popular with the gang for that move) Sooooo Gear change or be the second most hated man in America...da Bat (jus tryin to make the choice easy for ya)

https://youtu.be/RvV3nn_de2k

[rich grsc]

With that much torque and that engine which is probably making 400+ down low - I wouldn't be afraid for a second to throw in a set of 2.73's - you should get a lot more use out of 1st gear and your 5th gear will be much better.
So true. Just don't understand using anything over 3:27 with a big block, unless it's primarily a track car.

[Railroad]

A lot of the 80's and 90's 302 Mustangs used to be running around with the 2.73 gears. They had enough tq, 300 ft lbs to make them feel peppy. Your should have no issues with over 500 ft lbs and and 1000 lbs lighter.

[CraigS]

Which IRS? If it's the 2015 based version 315 diffs are readily available and cheap since it seems no one wants them. If it's the T-bird version 308 gears are readily available too. But I'd seriously look at changing the 5th gear. Get the 80 mph rpm you want and everything else stays the same.

[mcwho]

My roadster has a 460 SCJ engine with 550 hp @6250 RPMs and 545 ft./lbs. of torque @ 4800 RPMs. My cam is a Ford Racing M-8501-C460 with significant horsepower above 3000 RPMs and great low-end torque. I have no problem accelerating in 5th gear from 1800 RPMs. The transmission is a TKO 600 with .82 5th gear overdrive and my rear end gears are 3:55. Tire height is 25.13 inches.

So what's the dilemma? I love the performance and it is super fast (0-60 MPH in 3.6 seconds). Don't know what it does in the quarter mile but it's a rocket. And.....I'm tired of 3300 RPMs at 80 MPH in 5th gear. I've used the TREMEC Gear Ratio Calculator but it is way off the actual speed/RPM test drives (as in 4-6 MPH higher from 3000 - 5000 RPMs).

If the rear end is dropped to 3:27 gears it would still be doing 2870 RPMs at 80 MPH which is better, but not good enough. With 3:08 gears it would do 80 MPH at 2700 RPMs and 70 MPH at 2370 RPMs. This is all figured with the TREMEC calculator, which because of actual driving experience, I don't trust. I can currently drive on posted 50 MPH roads in 5th gear with no problem. If it were a problem I could just shift down to 4th.

So all you knowledgeable gear heads out there.....give me some feedback to mull over.

Thanks.

Mark

My first thought is the rear ratio is higher than you think. I have a tko600 w .64 5th gear. My tire height is 26.5 and the trees calc is pretty close w my 3.27 rear ratio. At 60 mph a have to shift into 4th cause in 5th it's close to or below 2000. I have a 302 stroker (331) . I have the pre 2015 irs and ffr said they thought it was 3.27 and I verified that by counting revs of drive shFt vs axles.

[Jim1855]

There's a 3.31 for the 2015 IRS, I have this one. 3.27 was available for the old 8.8 T-Bird unit, I had that one. The difference between the two ratios hardly matters.

TKO-600 first to fourth were fine with 17's and the 3.27. The .64 OD was great on the highway. Well, at least I liked it.

Jim

[lastdime]

Great Song!!! I had forgotten all about it. It brings back a very sad time in our country's history. Here in Utah, 70 mph freeway driving and 80 mph once you get out of very populated areas. It's fantastic!

[lastdime]

Very sure. I had it changed out at a shop and saw that it is a 3:55 gear

[lastdime]

Love your perspective NAZ. Our Cobra group goes on several road trips each year (one being 6 days, 1,000+ miles) and it is too hard on the car and me. It's time to give another option a try.

[lastdime]

If I had the .64 OD instead of the .82, 3.27 would work well. With a 3:08 ratio torque shouldn't be a problem in the lower gears.

[lastdime]

One more reason for the 3:08.

[lastdime]

Sorry Mike, the bell housing changes for the T56. Will probably need a new throw-out bearing and certainly a shorter driveshaft.

I have the TKO-600 and am looking at switching to the T56 Magnum. I have heard many times that the shifting is significantly better and the combination of the two overdrives has real appeal. But, there're always "buts", the T56 is wider, longer and heavier. I think the basic cost of T56 and a QuickTime bellhousing is about $3,900.

The OD gears can be changed in the TKO-600, this might help the most. Mike Forte can direct the process.

Jim

You guys are really trying to complicate things for me.......and cost me a lot of money. :D

[lastdime]

Which IRS? If it's the 2015 based version 315 diffs are readily available and cheap since it seems no one wants them. If it's the T-bird version 308 gears are readily available too. But I'd seriously look at changing the 5th gear. Get the 80 mph rpm you want and everything else stays the same.

I have the 89 T-bird. If I can get the 3:08 I'll go with it. Any ideas where I can get it? It's been hard to find.

[GoDadGo]

With that much torque and that engine which is probably making 400+ down low - I wouldn't be afraid for a second to throw in a set of 2.73's - you should get a lot more use out of 1st gear and your 5th gear will be much better.

2.73 Gears with a .82 O.D. running a 25.6" tall tire will yield 2,058 RPM @ 70 MPH.

With those gears, you'd likely only use the first (1st) three (3) gears to traverse the quarter-mile.

In addition, having all that torque I don't think you'd have any issues with the car.

[Erik W. Treves]

exactly :)

https://thefactoryfiveforum.com/attachment.php?attachmentid=130097&d=1592407842

[SJDave]

I have the 89 T-bird. If I can get the 3:08 I'll go with it. Any ideas where I can get it? It's been hard to find.

This shop has been a round forever and does great work...you could pull the diff and send to them to do the swap and adjustment. In California unfortunately.

rearendspecialties.com

(408) 988-3619

[CraigS]

Summit has a 308 gear
https://www.summitracing.com/parts/fms-m-4209-88308/applications
but their application doesn't have enough detail to absolutely confirm it is for your diff. The 2015 IRS is also an 8.8 but is usually referred to as a Super 8.8. So be careful any gear you buy is for the older 8.8. I'd call Summit to confirm. Ah, I found it.
https://www.cjponyparts.com/ford-racing-ring-and-pinion-gear-set-8-8-rear-axle-1986-2014/p/M420988-V/
These guys do specifically say it's for your diff.

[rich grsc]

The older IRS rear ends (T-birds and Lincolns) used the exact same gear sets as the solid axles. I done know about the new IRS setups.

[GoDadGo]

3.08 Gears with a .82 O.D. running a 25.6" tall tire will yield 2,322 RPM @ 70 MPH.

[lastdime]

My roadster has a 460 SCJ engine with 550 hp @6250 RPMs and 545 ft./lbs. of torque @ 4800 RPMs. My cam is a Ford Racing M-8501-C460 with significant horsepower above 3000 RPMs and great low-end torque. I have no problem accelerating in 5th gear from 1800 RPMs. The transmission is a TKO 600 with .82 5th gear overdrive and my rear end gears are 3:55. Tire height is 25.13 inches.

So what's the dilemma? I love the performance and it is super fast (0-60 MPH in 3.6 seconds). Don't know what it does in the quarter mile but it's a rocket. And.....I'm tired of 3300 RPMs at 80 MPH in 5th gear. I've used the TREMEC Gear Ratio Calculator but it is way off the actual speed/RPM test drives (as in 4-6 MPH higher from 3000 - 5000 RPMs).

If the rear end is dropped to 3:27 gears it would still be doing 2870 RPMs at 80 MPH which is better, but not good enough. With 3:08 gears it would do 80 MPH at 2700 RPMs and 70 MPH at 2370 RPMs. This is all figured with the TREMEC calculator, which because of actual driving experience, I don't trust. I can currently drive on posted 50 MPH roads in 5th gear with no problem. If it were a problem I could just shift down to 4th.

So all you knowledgeable gear heads out there.....give me some feedback to mull over.

Thanks.

Mark

I can now report back on the outcome of my differential gear change. I decided to go with a 3:08 ratio and am glad I did. Still plenty of torque in first gear (tires can light up), but not as crazy as with the 3:55. At 2900 RPM it runs at 80 mph. That's 400 down from before. Much easier cruising drive but not a low as I had expected. The calculators I have use have been worthless. However, it looks like it was the right choice.

[lastdime]

I can now report back on the outcome of my differential gear change. I decided to go with a 3:08 ratio and am glad I did. Still plenty of torque in first gear (tires can light up), but not as crazy as with the 3:55. At 2900 RPM it runs at 80 mph. That's 400 down from before. Much easier cruising drive but not a low as I had expected. The calculators I have use have been worthless. However, it looks like it was the right choice.

I need to make a correction to my comment about the RPM calculators. As it turned out, my tach was off. I found that it was reading 200 rpm higher than the actual rpms. Speedhut is the manufacturer and they said they would send instructions on how to re-calibrate the tach, but, since I live near their location, I took it to them. They had it fixed within 15 minutes and even gave me a new glass cover that had a small crack in it at no charge. Now everything is right on what the calculators indicate it should be. My apologies to all those rpm calculators out there.

[BEAR-AvHistory]

Was one thing I did not think about so am running a 3:55. Would like to have the lower revs of a better gear set but drove a 67 StingRay for over 20 years with a 3:70 gear-set & a 4 speed manual. IIRC 60MPH was 3,000RPM, The Jersey Shore to NYC commute on the GSP & NJTP forced higher RPMs to keep up with traffic. Glad it was a coupe & I could roll up the windows.:D

Have found the COBRA wind noise & buffeting limited to 75/80MPH more than the engines revs. The COYOTE is pretty quiet when its not under load. That said will drop the rear ratio at some point in the future to increase the cars range.

Seems like a new project comes up every year.