[Formulajones]

Originally Posted by HWYSTR455

Another reason I went with the road race gear set of 2.87, 1.89, 1.28, 1.00, .82 . As long as you don't stay in 1st much, it's fine, and linear.

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Those work fine if you're actually road racing and run some rear gear where the 1:1 isn't enough for the straights. That's really what the .82 overdrive is meant for. It wasn't really intended as a street cruising overdrive gear.

[ID67goat]

Originally Posted by Formulajones

The difference between a .64 and a .82 is HUGE!! You are talking 15 mph between the two. Instead of cruising at 70 mph at 2000 rpm, you are now pushing the engine to 25-2600 to go 70 mph. That's a 30% increase.

That's not much of an overdrive for the money being spent.

For his setup with the now offered .68 overdrive

1700 rpm will be 56 mph. Pontiac should pull that easily without a need to downshift, but the option to move the lever is always there.

2000 rpm will be 66 mph.

2200 is a nice spot to be on the highway that returns good mpg and less noise.
That'll be 73 mph.


That's about as perfect as it gets.

Agree with this. I think it just depends on how fast you want to go...

I have 461, 256@0.050 cam, 26" tire, 3.73 gears with T56 (0.74 and 0.50 OD gears).

For speed limits where I live, 5th gear is good for the 65mph zones (2300RPM) and 6th is good for the 80mph zones(2000RPM). While 6th might be a little too tall, I still have no problem with drivability at this RPM and it makes for a nice quiet ride that can keep up with traffic.

With the 4 speed it was getting a little embarrassing on the interstate getting passed by minivans and Prius' while I was buzzing around 3500-4000RPM.

[Formulajones]

We had a couple of cars with that same T56. My wife drove one of them. I really liked that .50 overdrive 6th gear. I ran 3.73's in one and 4.10's in the other.

Even with the 4.10's it would loaf along at 80 mph. It was perfect for the 75 mph limits we have here and the long distances between towns, plus it worked really nice with the 2.66 first gear too. I always found myself just skipping 5th gear all together, rarely ever used it at all.

[Cliff R]

Here is an easy formula to determine engine RPM's at any MPH.

MPH x GEAR RATIO x 336 divided by TIRE DIAMETER

This provides the RPM's in high gear or 1 to 1. Take the result and multiply by the OD ratio to get the RPM's in OD at the same MPH.

For example:

70mph x 3.42 x 336 divided by 26" equals 3093 RPM's. So I know that in high gear I'm running 3093 @ 70mph. So if I'm installing a 700-R4 with a .70 OD ratio I simply plug in the following numbers.

3093 x .70 (OD RATIO) equals 2165 RPM's. Pretty simple stuff.

Put you numbers in and you can pick the OD ratio accordingly. Keep in mind that this applies directly to manual transmissions or OD's with the torque converter clutch applied. If you are using an OD automatic transmission w/o lock-up you may see about 50-100rpm's higher in OD as there is a small amount of converter slip depending on the efficiency of the converter you are using. If you see much more than that the converter is too "loose" or poorly coupled for what you are doing...........Cliff