I recently changed tire size on my truck, 95 302 Auto 2wd and it dropped my mileage from 15.5 to 13. I have 3.31 gears and do almost all highway driving 75 - 80. I had to run in to work today so I did a mpg test. Ran my normal route with OD cancelled. Put me at 3000 rpm at 75 mph on a 60 mile trip. Got a bit over 16 mpg. Will drive this coming week the same way to see if that holds.

Using this logic I'd get better mpg with 4.10 or 4.56 gears. Anyone have some practical experience running higher rpm and getting better mileage?

By the way tires are 30" diameter 255/65/17 so nothing huge by any measure.

 

What are you using to get your mpg?

 

I would calibrate your PSOM before you try calculating you MPG: http://www.supermotors.net/registry/media/76022

 

Thanks for the replies. Spedo is calibrated for tires. I usually run the week and fill up to check mileage, after 400 miles or so. I'll do that to see if the first small test was a fluke.

Edit. The trans seems to lock better in D vs OD, that affects mpg just not sure how.