What effect do you guys think the axel ratio has on gas mileage?

The lower the number gears should yield the most MPG, however, one must consider the RPM that the motor is turning at say 70 MPH with the various gears 0 it must be in the most efficient operating range.

Small changes, the mileage differences are not noticeable. Huge changes they have a pronounced affect on mileage. The engine has a peak efficiency RPM. If you get too far below it then your burning extra gas to keep moving. Same thing happens if you go to far over the peak efficiency RPM. Your engine is turning easily, however the higher RPMs burn fuel quicker.

I have two different experiences that I can relate. My first was that I had an '86 2.3L Ranger with a 3.08 axle. The thing was pathetically slow off the line (actaully at all speeds). Keeping the tires their stock height, I swapped the rear end for a 3.73. It gave it a bit better acceleration off the line however my highway mileage dropped from 26 MPG to 22 MPG as I was now turning over 3500 RPM at 70 MPH. My second experience was when I went to larger tires on my Explorer. It had the same affect as going to a lower (numerically) gear ratio. It had the same affect as keeping stock sized tires and changing my rear end to a 3.89. I can't say that I noticed any change in my mileage either way. Probably the more effort required to get moving is offset by the lower RPMs on the highway.

That being said, if you went with the lowest (numerically) ratio that Ford offered with your size tires (or maybe even a bit lower) you should pick up a little bit of highway mileage. If you go way too low then the effort required to get up to speed, go over overpasses or pass other vehicles will offset any mileage gained.

I have a 4.0L SOHC with a 410 LIMITED SLIP DIFF. This engine really cranks off the line. Ain't to many cars who can beat it off the line, but you pay in gas mileage at highway speeds. So if you want to play you got to pay! Kapisch!

My 33"s with 3.73 gears, 15-16 mpg on the highway.

Same tires with new 4.30s gears, 18-21 mpg on the highway.

What RPM with the various gears?

At 70 mph, about 2100-2200 before, about 2500 now.

If I go faster than about 76 my mileage does start to go down, but below speed I'm very pleased, especially with my 33s and rack.

I get 18 mpg on highway, I have (as you can tell from sig) 30 x9.50's and 3.27 gear ratio on a 4x4 1st gen. When I change the fpr and fuel filter I may be able to get 20 mpg, which would be nice!

In my experience, there is not much difference in mileage between P235s and 30 x9.50's with 3.27 gear ratio. (there's only 3/8" diameter difference).