Re: 304/290/200R4 - now V8 Fuel Mileage

[farna@xxxxxxx]

A good site for gear/tire calculations: http://www.4lo.com/
That's why I don't have 60 or 65 series tires on my Classic. I think they would look better, but would knock me out of the cruising rpm range I want to be in. I could drop to 3.31 gears (instead of 3.55) and stay in the same range with a 60 series tire instead of a 70 series. That's one reason I wanted 3.31 gears -- could use the 65 seriess and gain a few mph at my target cruising rpm and look a bit better! 

My tire size: 205/70, 15". Diameter (avg.) is 26.3"
              205/65 15 is 25.5" (avg.)

w/3.55 gears and 205/70 tires I'm running ~63 mph @ 2000 rpm.
Drop to 205/65 tires and speed drops to 61.1 mph @ 2000 rpm. 
I'd need a 3.44 axle to maintain the same speed, or a 24.5" diameter tire w/3.31 gears (205/60-15 is ~24.7", and would be 63.5 mph @ 2000 rpm).

On the 4Lo site don't forget to enter 1 for the transfer case ratio (1:1) and enter your high gear ratio. I used 0.70 for the OD gear in the AW4. High gear in all three speed autos and most three/four speed manuals is of course 1:1. You can figure speed in any gear if you know the ratio. I used "average" (avg.) and "approximate" (tilde [~] means approximately) for tire sizes as there is a little variation among manufacturers. Should be +/- about 0.125" though. 


On May 12, 2005 Jim Boone wrote:

> On May 12, 2005 Jim B wrote:
> 
> > A: You also have to take into account for the size of tires on your vehicle
> > when calculating gear ratios. Too many offroaders wonder where their power
> > went when they slap oversize tires on. I know my own power dropped a lot
> > when I put 33" on (from 26") but going to 3.73 helped that. When I switch to
> > AW4, I'll probably have to drop down to 4.10 to keep adequate power.
> 
> Great point;  I did figure in for 25" tires when I posted a day or two ago.  I think that the Jeepers have more to gain/lose than we do as going from 26" to 33" or more is quite a difference.  With a passenger car, and esp an older AMC that you're going to use 14" or 15" wheels, it's more of a fine tuning tool.
> 
> I think the rear end we're talking about in this instance (with an OD trans aka 200R4) is right at 3.54; however, if you wanted/needed to go a little off of that, you could change the size of the tires.
> 
> Jim Boone
> Mims, FL
> 
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