Hi all,

In terms of changing gear ratio and how it affects the RPMs, can anyone check these figures as I've been getting various numbers depending on the calculator I've been using.

I don't have a tach but looking at these numbers would this make sense.

So here's the info
3.55 rear end ratio
46RE auto trans
29 inch tires
70mph
= 2000rpm?

Seems to low

On the calculator, if I changed to 2.76 rear end ratio, (which I'm thinking of doing for higher MPG) then it says approx. 1,550rpm.

Is this right?

Does anyone have a better calculator online they know is correct? This was app I found that takes into account the 46RE ratios etc

Thanks all

 

What I am seeing, looking at a chart, (not a calculator.....) 65mph with 29" tires would yield 2675 RPM. (so, multiply by .69 to account for the O/D, and you end up with 1846 RPM.) Dropping to 3.21 gear ratio, yields 2418 RPM, (or around 1670 RPM) So, a drop of around 176RPM..... You really don't want to go any lower than the 3.21, as the engine will be turning to slow to develop any power to push the truck along at freeway speeds. It's entirely possible that even going to 3.21 will put the engine far enough out of its comfort zone, that it will constantly be gear hunting... in which case, your gas mileage may actually be WORSE.

Then we come to the cost/benefit thing. You are going to spend 6 to 8 hundred dollars (assuming 2 wheel drive.) to change gears, and get the speedometer accurate again...... That would pay for a LOT of gas..... It would take an awfully long time for the .5 to 1 MPG increase to pay for itself.......

 

Thanks for your reply, thats really interesting.

Can i view this chart please? Is there a link I can follow to find it?

Good to know the revs are lower than I thought, it feels higher than that but it if the numbers check out.

I understand what your saying about the mpg and the hunting etc. It has plenty of power and its very flat where I drive so wondered about the mpg benefits. I suppose a drop of a few hundred rpm would be small, I was thinking the rpm drop would be much more significant.

Thanks

 

This one. I think.... There are tons of them out there on the web.

If it's pretty flat, you might be able to get away with going to even lower gears. Might try just bumping tire size a bit first though.....

 

Formula,
RPM = MPH x axle gear ratio x transmission gear ratio x 336 divided by tire diameter.
you can also use this
http://www.grimmjeeper.com/gears.html

 

Great information!