From a thread post from RetiredWarHorses.........maybe this is the answer.
Easier is too tell what model year the truck is...
85-94 A0-A1 have 2:56
all A2’s have 2:73
ECV on up, 3:08
If RetiredWarHorses posted it, you can pretty much take it as gospel for these trucks.
Another question..........if the truck isn't used for hauling or heavy loads and I want better highway cruising.........the 2.56 is best gear ratio??
I think it will affect the 'take off' by being slower (especially if heavy).......but better top end. Correct?
Short answer is: yes, it will cruise at highway speed more efficiently all else being the same.
Slightly longer answer: depending on what gearing you are starting with, while it will be more efficient, it won't be massively so. Any truck that didn't come with 2.56 gearing already came with a 4 speed transmission rather than a 3 speed, so changing the gear ratio won't do a whole lot.
Long answer is this (use the math for basic reference, it might be off but it should be close enough)
As long as it is within the power band of the engine. If the engine RPMs are too high or too low it will burn more fuel at the same speed than if it is within that band. Also what you consider "highway cruising".
With the ECV and newer they are turbo motors so they might have a little different sweet spot.
It's not a huge difference though.
2.56 final gear ratio is 4.86. at 2500 RPMs in 3rd gear (direct drive) that would be 56.6 mph.
2.73 final gear ratio is 5.19. at 2500 RPMS in 3rd gear (direct drive) that would be 53.0 mph
3.08 final gear ratio is 5.85. at 2500 RPMs in 3rd gear (direct drive) that would be 47.0 mph.
peak power is something like 2k rpms, best efficiency between 1800-2100 or so I've read.
So it would make a noticable difference putting the 2.56 gears into a 3.08 geared truck for top speed, but not as much for a 2.73.
Remember though, the trucks with the 2.73 and 3.08 gearing came with a 4 speed transmission. Those trucks are much closer to the peak power output of the motor running 60ish to begin with. Changing out the diff gearing wouldn't that much difference unless your plan is to cruise at 75+ MPH.
A2 trucks (2.73 gears) have a listed top speed of 78mph. I don't know what it is for the 3.08 trucks, but if it's the same RPM limit on the ECV/REV trucks as the A2, then it should be around 70 mph I would think.
Changing either the 3.08 or 2.73 to 2.56 would give a top speed of 83mph, so not a lot of gain on the A2, but noticeable on the ECV/REV.
As you said though, it would be slower to accelerate with those gears, and would also create more heat in the engine and transmission (not as big of a deal in a civilian rig weighing 7k lbs, but a big deal when the truck weighs 12-14k lbs or more and is operating in 130 degree deserts)
Note: so it's getting late and I'm past my bedtime. I didn't necessarily research everything posted above on everything, but I tried.
The information I found shows a 37 inch tire has 545 rotations per mile. Ours are closer to 36.5" diameter, and wear affect that, but it's close enough.
I recall the geared hubs being 1.9 ratio, so that is what I used. I might be wrong on this, which if I am let me know and I'll try to fix my math.
I also don't pay a huge amount of attention to the transmission gear ratios, which is why I said "3rd gear/direct drive". if it isn't direct drive at third let me know.
Not sure what the overdrive ratio is in overdrive on the 4 speed transmissions, so just used the 78 mph as a baseline and did the math from there, using the same percentage difference between speeds in 4th as in 3rd gear.
Most people say 55mph top sustained speed for the 3 speed transmission, and 2500 RPMs for my math seemed close enough for this purpose if all the other math is close enough.
brain is all fuzzy, I'm going to bed.
