What gear ratio to run with a T56?

[Dino]

I picked up a T56 6 speed with 0.74 5th and 0.50 6th gear. The rear tire is just under 28" tall. Mild 440 with stock 727 and 3.23 gears is the current setup. The T56 will allow me to run a much shorter gear so I've been playing around with a gear ratio calculator. Much of my driving will be in town and rural roads, so anywhere from 35 to 55 mph. I do take the highway at times but I try to keep it under an hour. When keeping up with traffic I'm in the 3300 rpm range. Not too bad really but I'd like it a little lower. I do plan on doing way way more highway in the future. If I get a 4.10 I'd be turning 2100 rpm doing 85 in 6th, and about 57 in 5th. That seems perfect, or is 2100 too low?
What can I expect the car to act like in town going from 3.23 to 4.10? Any downsides?

I added shift speeds at 2500 RPM for 3.23, 3.55, 3.73, 3.91, and 4.10 gears.

[70 sublime]

The first 4 gears of the 6 speed will be close to a reagular 4 speed ratio

You might be doing a lot more gear shifting to get up to the 35 mph range or skip a couple gears along the way if you have the 410 vs 323 rear

 

[Dino]

The first 4 gears of the 6 speed will be close to a reagular 4 speed ratio

You might be doing a lot more gear shifting to get up to the 35 mph range or skip a couple gears along the way if you have the 410 vs 323 rear

 

Good points!

I added a few screenshots to see the difference in shift speeds. I picked 2500 RPM for consistency.

[Kern Dog]

I could have swapped in a T 56 if I wanted but I saw no need for a .50 top gear. I'm not looking to go 180 mph nor am I interested in 1200 rpm engine speeds at 70 mph. Any old school engine usually has a wilder than stock cam that needs to rev a bit to make decent power.
I thought about axle gearing before I bought my Tremec 5 speed. I thought of how the late model Challengers had a 3.92 while their transmissions had similar gearing 1st through 4th. Then I thought of how they have the modern engines with astounding cylinder head flow, variable valve timing, active exhaust and other things that broaden their torque curve.
The 440/495 big block is a different animal than a 3rd generation Hemi. I stuck with the 3.55s I had. The car would be a bit more snappy with a 3.91 but I don't mind how it runs now.

[Dino]

I consider the T56 to be a 5 speed for most of the time, with the exception being long highway drives. I'm in south Michigan and like to drive up north, or to Chicago or maybe Indianapolis. Those are 4 hour drives, and some are much longer like when I want to go to the upper peninsula. Around town there's no reason to go beyond 5th gear because it already has a .74 overdrive. I took this screenshot off my EFI app last night. This is with stock 727 and 3.23 gears. I usually drive 80-90 on the highway so you can see that rpm would get old on long drives. Same speed with a 4.10 or 3.91 and the .50 OD gives me very nice cruising rpm.

[cdr]

I would go 4.10, also need a very good drive shaft because of critical speed of the shaft, Mark Williams makes the best they can spin the shaft for balancing at the proper RPM that your Drive shaft spins , other shops can only spin them 500 rpm, trust me on this, I wasted lots of money on a proper drive shaft, one local built & one from Strange Engineering , NEVER again  

[Mike DC]

Google "Blocklayer rpm gear".  It has some glitches but it's my favorite one of these calculators.  It gives a good visual graph of the spreads between gears.  When you open the page, you have to click to see a pop-up mini ad for a few seconds.  Then the page is free for the rest of the day.  No signing in or any other BS. 


Axle gearing:
(I assume you have the "close ratio" in the lower gears, with the 2.66 1st gear.)  I vote 3.91 or 4.10. 

If you run it too tall (like a 3.55) then the 6th gear gets impractical.  The engine would be near stalling every time highway traffic slows down to 50-60 in a crowded stretch.

With 4.56 gears the 1st gear is getting annoyingly low.  The engine would hit 4000 rpm at like 20-something miles an hour.


Other thoughts:

- Like CDR said, the driveshaft is gonna be spinning pretty fast when you combine tall overdrives + short axle gears.  Don't underestimate this issue.  IIRC Ford used to put alloy driveshafts in the police versions of Crown Vics because the stock steel CV driveshafts weren't happy at 120+.  That's with 3.27 axle gears and 26-27" tires.     
 
- Think hard before cutting the car up for a 6spd.  The new TKX version of the 5spd addressed some complaints with the TKOs, and both the Tremec 5spds require way less cutting up the car. I also like the "wide ratio" gear options better on the 5spds.  I'm only convinced that 6 gears is better on a racetrack.  I'm not convinced it's better than 5 (well-spaced) gears for street cruising.

[John_Kunkel]

Think about it, the .050 6th gear with a 4.10 axle gives you a final ratio of 2.05...kinda useless below 80 mph. With that trans, I'd go with a 4.56 or deeper rear.

[Mike DC]

Think about it, the .050 6th gear with a 4.10 axle gives you a final ratio of 2.05...kinda useless below 80 mph. With that trans, I'd go with a 4.56 or deeper rear.

Depends on how comfortable the motor is at low revs. 

Dodge Vipers were the original purpose for Tremec's 6spd tranny.  Those cars came with 335/35r17 rear tires (26.23") and 3.07 axle gears. 

Do the math, and that's like picking a 3.23 axle for Dino's combo. 


Granted, modern computerized engines are more tolerant of a wide range of revs (better "dry" intake designs, etc).  And the Viper's top overdrive gear was really high to improve the car's gas mileage.   But still, it was an OEM setup for a street car.  They sold it to rich guys who didn't own hand tools.   
 

[John_Kunkel]

quote author=Mike DC link=msg=1764262 date=1752177985]

Think about it, the .050 6th gear with a 4.10 axle gives you a final ratio of 2.05...kinda useless below 80 mph. With that trans, I'd go with a 4.56 or deeper rear.

Depends on how comfortable the motor is at low revs. 

Dodge Vipers were the original purpose for Tremec's 6spd tranny.  Those cars came with 335/35r17 rear tires (26.23") and 3.07 axle gears. 

Ever drive a Viper? I had one and, even though the motor is comfortable at low revs, the throttle response in top gear is mush...even at 80 mph. Lots of Viper owners who weren't racing them changed to 3.55 or 3.73 gears so they could comfortably use 6th gear.

The Viper Truck got it right, it had 4.56 gears.

[Mike DC]

Ever drive a Viper? I had one and, even though the motor is comfortable at low revs, the throttle response in top gear is mush...even at 80 mph. Lots of Viper owners who weren't racing them changed to 3.55 or 3.73 gears so they could comfortably use 6th gear.

The Viper Truck got it right, it had 4.56 gears.

Yeah I know the stock Viper gearing was too tall.    I'm just pointing out that it was still driveable enough for dealerships even with a 3.07 rear. 

Chrysler probably built the Vipers with tall axle gears to please the EPA while knowing that gearheads would lower it later.  With the stock 3.07 axle, and in 6th gear, the engine would be almost idling at 60 mph.  That was the speed of the EPA 'highway' mileage test.  In real life a driver would still have the car in 5th gear at that speed.  It also bears noting that the Viper's stock 1st gear wasn't very low overall.  So it would still be in a usable range even after the owner lowered the axle gear.         


BTW, Dino's combo with a 3.91 = a Viper with a 3.73.  The speed & rev numbers are a close match.     

[Dino]

Thanks for the advice fellas, I really appreciate it. I can actually pick up a center section with sure grip 4.56 locally for $850. It's a 742 case but I'm not sure which sure grip. Power lock probably. I do have my eye on the Dr. Diff Sure-Trac II.

That said I do like the idea of cruising in the 1800-2200 range on the freeway. That's 73-89 mph. I'm rarely below 75 here and usually mid 80s.

3.91, 4.10, or even 4.56 would probably fit the bill. Leaning towards the 4.10 right now.

Thanks for the heads up on the driveshaft. I never thought of it beyond getting a new one. 

[Mike DC]

   

3.91, 4.10, or even 4.56 would probably fit the bill. Leaning towards the 4.10 right now.

There is also a 4.30 ratio being made for the Mopar 8.75" rearend. 

In the big picture any of these choices you're batting around is pretty good at the top end. 

3.91 gear:  1900 rpm @ 80 mph.
4.56 gear:   2200 rpm @ 80 mph.