Rear diff ratio

[chopthebass]

I was hoping to get a response to my question about the use of 3.55 diff ratio.
Please see thread below. The suggestion is that 'with that much engine I should go with 331/327 or even 308'
How will it perform with each ratio?


http://thefactoryfiveforum.com/showthread.php?16521-About-time-I-started-my-build!

[2FAST4U]

3.55 is fine. Quicker acceleration. Depends on what your 5th gear is. I'm running 4.10 in my car and my OD gear is .63

[chopthebass]

3.55 is fine. Quicker acceleration. Depends on what your 5th gear is. I'm running 4.10 in my car and my OD gear is .63

My 5th is also 0.63
Quicker acceleration is always good! Not interested in extending the top speed.

Thanks

[Avalanche325]

You only get quicker acceleration until your tires go up in smoke.

Lower gears are what you use with lower HP and torque. If you are going to have a 427, I don't think you are going to have a HP shortage.

[Jeff Kleiner]

You need to look at both ends of the equation. Here's an important and often ignored question; what ratio is your overall first gear. With your combination I'd prefer to see something under 10:1 overall. Anything over that is going to start getting pretty short (but will make lots of smoke and win burnout contests).

3.55 rear with 3.3 first gear=11.71:1 overall; 6000 RPM=~37 MPH...with .63 fifth gear 70 MPH=2100 RPM
3.55 rear with 3.0 first gear=10.65:1 overall; 6000 RPM=~42 MPH...with .63 fifth gear 70 MPH=2100 RPM
3.27 rear with 3.3 first gear=10.79:1 overall; 6000 RPM=~41 MPH...with .63 fifth gear 70 MPH=1940 RPM
3.27 rear with 3.0 first gear= 9.81:1 overall; 6000 RPM=~45 MPH...with .63 fifth gear 70 MPH=1940 RPM

Good luck,
Jeff

[smithbks]

This is always an interesting question. I have a 427 with 500+HP and I agonized over this. In the end I am resigned to the fact that there is no perfect choice. I chose a 3.27 rear with the .84 OD after long talks with Mike Forte about what I wanted and needed. I was setting my car for cruising comfort at 70 MPH because that is what I think I will do the most. I have a buddy that will go to the track with me and he is setting his car with a 4.10 or 3.73. If I get real serious about the track I will have to change my rear end out. But for the time being I want to enjoy driving it and getting used to it.

So my advice to you is to figure out what you most want to do with your car. It will always be a trade-off but lean towards your goal.

[chopthebass]

So my advice to you is to figure out what you most want to do with your car. It will always be a trade-off but lean towards your goal.

I won't be taking to the track and racing it. Street use only.

[chopthebass]

You need to look at both ends of the equation. Here's an important and often ignored question; what ratio is your overall first gear. With your combination I'd prefer to see something under 10:1 overall. Anything over that is going to start getting pretty short (but will make lots of smoke and win burnout contests).

3.55 rear with 3.3 first gear=11.71:1 overall; 6000 RPM=~37 MPH...with .63 fifth gear 70 MPH=2100 RPM
3.55 rear with 3.0 first gear=10.65:1 overall; 6000 RPM=~42 MPH...with .63 fifth gear 70 MPH=2100 RPM
3.27 rear with 3.3 first gear=10.79:1 overall; 6000 RPM=~41 MPH...with .63 fifth gear 70 MPH=1940 RPM
3.27 rear with 3.0 first gear= 9.81:1 overall; 6000 RPM=~45 MPH...with .63 fifth gear 70 MPH=1940 RPM

Good luck,
Jeff

Thanks Jeff,
I want to get this right at the build stage, and I spoke to my engine builder, who assembled my rear end, and he commented that the TKO has a first gear ratio of 2.87, and with a rear tire of 26.5" he calculated 1st gear @ 6000 = 46.6mph, and 5th (0.64) 70mph = 2000RPM. Sounds good to me!

[Jeff Kleiner]

Thanks Jeff,
I want to get this right at the build stage, and I spoke to my engine builder, who assembled my rear end, and he commented that the TKO has a first gear ratio of 2.87, and with a rear tire of 26.5" he calculated 1st gear @ 6000 = 46.6mph, and 5th (0.64) 70mph = 2000RPM. Sounds good to me!

That does compute. What tire do you plan to run @ 26.5 diameter?

Jeff

[skullandbones]

This is always an interesting question. I have a 427 with 500+HP and I agonized over this. In the end I am resigned to the fact that there is no perfect choice. I chose a 3.27 rear with the .84 OD after long talks with Mike Forte about what I wanted and needed. I was setting my car for cruising comfort at 70 MPH because that is what I think I will do the most. I have a buddy that will go to the track with me and he is setting his car with a 4.10 or 3.73. If I get real serious about the track I will have to change my rear end out. But for the time being I want t enjoy driving it and getting used to it.

So my advice to you is to figure out what you most want to do with your car. It will always be a trade-off but lean towards your goal.

I can see why you would want to get it right the first time. I planned on going to a higher rear gear but now that I have put about 3K on the roadster, I have found that I sort of like the 3.27 even with my .84 overdrive. It is a trade off but since I want to do some longer road trips, I just can't talk myself into going with a gear like 3.70 just to pep up my 350 hp 302. It's nice having the comfort of the lower gear ratio on trips where the car acts more like a normal ride. So I plan on increasing the HP a bit to make the lower gears more fun when the time comes. With these light cars, it doesn't take much to break the traction even with less than 400 HP. Good luck,

WEK.:cool:

[Bob Cowan]

There is no perfect gear. Everything you choose will be some kind of compromise. Just like everything else on the car.

Take a look at the top gear. Will that put you in the power band where you'll be driving most? For example, if most of your cruising will be at 55mph and less (for example), the a 3.08 gear will be almost useless to you.

I'v tried 4.10, 3.55, and 3.27. Surprisingly, there's a big difference between 3.27 and 3.55. After driving it for a while, I think I'll go back to 3.55. I liked that better for nearly every situation.

[CHOTIS BILL]

I went about it a little differently. I always wanted a car with a speedometer that went to 200 MPH but putting one in a Bugeye Sprite seemed a little farfetched. But putting a 200 MPH speedo in a Type 65 seemed appropriate so I got a GPS 200 MPH speedometer from Speedhut and checked that one off my Bucket List. When it came time to choosing a rear end gear I thought it only right to match the potential top speed of the car to the speedo so with 25.7” tires, a red line on the engine of 6500, and a 5th gear ratio of .68 the logical choice was 3.55.

HTH :)

Bill Lomenick

[chopthebass]

That does compute. What tire do you plan to run @ 26.5 diameter?

Jeff

295/50-15 total diameter = 26.6inches
Billboards

[CHOTIS BILL]

If anyone is interested I have a spreadsheet that you can change all the drivetrain info and it shows all the info to compare different setups. Just e-mail me at blomenick at deltatau dot com.

Bill Lomenick

[CraigS]

My original point was this; there isn't a lot of point in having power multiplied by gear ratio that grossly exceeds the traction you have available. So a 355 w/ (I am guessing) 500 hp is going to light up the tires real easily in the lower 2-3 gears. Of course this depends on the tires. Drag radials work better but usually aren't a good combination because you will have sticky rears combined w/ standard traction fronts. Not good for braking or for cornering. So that's why I suggested at least looking at taller rear gears. Now that I see you have the .63 5th I would temper my thoughts a little. Here is why. The .63 gives an almost un-natural gap from 4th to 5th. So you are driving at maybe 50-55 and think about a shift into 5th. Well the engine rpms drop a lot. Will your engine be happy running at a steady super low load speed at maybe 1600 rpm? So this is a tradeoff and it's not always easy to make the decision w/ gear/speed calculators. Also, now that I see your tall 1st gear (T-5s and TKO500s have something in the 3.30 to 1 range in 1st) that 355 is looking a lot better.

[edwardb]

I find this calculator to be very useful. Handy also because it already has the values for the various Tremec models. http://www.tremec.com/calculadora.php

I agree with Jeff's point, and echoed by others. Look at both ends of the spectrum. 1st as well as 5th. I had 3.55 gears on my 306 powered Mk3 with a T-5. Was probably the right combination, but still ran out of first really fast. 5th gear was good, although a big drop from 4th, so only a highway gear. For my Mk4 with a much stronger engine, after a lot of research went with the 3.27 diff and TKO600 with .82 5th. Street driving only. I'm hoping to get the best of both worlds. A "reasonable" 1st gear and 5th that I can use for more than just highway. We'll see when it's on the road in the Spring.

[myjones]

I went about it a little differently. I always wanted a car with a speedometer that went to 200 MPH but putting one in a Bugeye Sprite seemed a little farfetched. But putting a 200 MPH speedo in a Type 65 seemed appropriate so I got a GPS 200 MPH speedometer from Speedhut and checked that one off my Bucket List. When it came time to choosing a rear end gear I thought it only right to match the potential top speed of the car to the speedo so with 25.7” tires, a red line on the engine of 6500, and a 5th gear ratio of .68 the logical choice was 3.55.HTH :)
Bill Lomenick

I have a similar bucket list and agree with the earlier comment that it's all a compromise.
I already have a car with a 200 speedo but I hope to get one for the 33 as well. Aero/safety is now the 33's limiting factor.
I did the speedo calculator for a target top end at redline, I did the cruise RPM's at 70 in my .69 OD mode
BUT I also did the calcs for 55 in Drive at 1<>1 and looked at those ratios as well. You don't always get to cruise at 70
and on trips you will spend some time doing 55. Plus with an auto I also get to pick the stall speed and lock or unlock the TC
With 27" tires the 327 ratio hit my best sweet spot and the stroker 426 Hemi will pull just fine in D at 55 or OD at 70 without
limiting the top end as much the 355's would. I do plan to dial in the custom 4 link to set dynamic rise as close as possible to
minimize that blowing away the tires in the first few gears. < that just seems like guys trying to get everyone else to put in a smaller motor :rolleyes:
DB
Hemi33

[32J]

I see that it's been nearly 6 years since this post. Were you happy with your choice of the 3.55 rear and the 0.82 5th? I'm debating between a 0.64 5th with a 3.27 rear, and 0.82 5th with a 3.55 rear. Coyote Gen III, street only. No autocross or drag. Some highway. Thanks!

32J

[CobraboyDR]

3.73 with a .63 OD is the best of all worlds, quick on the street and long legs on the slab.

[GoDadGo]

3.73 with a .63 OD is the best of all worlds, quick on the street and long legs on the slab.

3.73 Gears with a 25.7" tire and .63 O.D. equates to 2,150 RPM at 69.95 MPH

3.55 Gears with the same combination equates to 2,050 RPM at 70.08 MPH

[CobraboyDR]

3.73 Gears with a 25.7" tire and .63 O.D. equates to 2,150 RPM at 69.95 MPH

3.55 Gears with the same combination equates to 2,050 RPM at 70.08 MPHNothing wrong with 2150 at 70mph. Heck, I'm not even sure a speedo and tach are that accurate to tell the difference in action.

3.73 gets the sled into the meat of the torque curve faster, especially in a smaller block. Heck, my old C2 'Vette had a 4.11 rear end...and no OD. It could scoot between stoplights, for sure, but you really didn't want to take it onto the slab for long.

No problem with 3.55, though. Different strokes.

A 295/50R15 is 26.6 in diameter. That makes a difference, too, tire size.

[GoDadGo]

Nothing wrong with 2150 at 70mph. Heck, I'm not even sure a speedo and tach are that accurate to tell the difference in action.

3.73 gets the sled into the meat of the torque curve faster, especially in a smaller block. Heck, my old C2 'Vette had a 4.11 rear end...and no OD. It could scoot between stoplights, for sure, but you really didn't want to take it onto the slab for long.

No problem with 3.55, though. Different strokes.

A 295/50R15 is 26.6 in diameter. That makes a difference, too, tire size.

4th will yield 125 MPH @ 6,000 RPM with 3.73's so let the drag racing begin.

Try 285/40-17 (26.1" diameter) with 3.73's in front of a .75 (5th) and .50 (6th) Overdrives.

1,700 @ 70 MPH or 3,000 @ 125 MPH

https://youtu.be/9WEe6-wdNtA