Speedometer and Gas Mileage Questions

I just received my rims and tires in the mail (31x10.5s on 15" cragar 8's) and I'm getting ready to install my lift once i get home from vacation.

How far off will my speedometer be with 31s and how much will my gas mileage decrease? (ballpark estimates, personal experiences) I have the d35 in the rear with the aw4. Will my gas mileage decrease more during city or highway driving. Will installing a roof rack and putting my spare up there decrease it a lot more?

It will be off by a percentage, so you can't say that it will read 5mph low or something like that.
I know that once I was traveling at what it said was about 72-73, and the cop's radar said I was going 78, I have a yellow paper that proves this speed
Here's several calculators, including one for this. http://www.rocky-road.com/calculator.html

So if they make 3.93 gears, that would make it correct. I got a 32 tooth speedo gear for 80 bucks. http://www.go.jeep-xj.info/HowtoSpeedoGears.htm Bring the part number with you to the dealer, as when I went to get it, his computer only showed several sizes, and of course 32 was not one of them. If your jeep pre 1990, use the P/N for 91-93 long shaft, as they claim to not make the old ones anymore.

I would think your gas milage drop would be proportional to the speedometer change. You are going further each tire revolution, but the odometer doesn't know that. Also the engine has to work harder in stop-and-go driving so that would cause it to drop more than highway miles. Maybe?

You're going to lose about 10% across the board in mileage. Ironically your speedo will be off by about the same. Hmm... I wonder if they're related?

Skibum12189 said:

Will installing a roof rack and putting my spare up there decrease it a lot more?

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My buddy lost 2MPG with a tire and roof rack.

i didn't see much loss when i had a 33 on my roof. maybe because my mpg sucks as it is but i'm leaving it up there. better than nothing.

im getting about 10mpg these days (about 3/4 of it highway) with a bunch of things affecting it: roof rack with 30" spare, 32s with stock gearing, stock speedo, 96' 4.0 w/ AW4 @ 160K. i need a new DD :eyes:

cphillips said:

im getting about 10mpg these days (about 3/4 of it highway) with a bunch of things affecting it: roof rack with 30" spare, 32s with stock gearing, stock speedo, 96' 4.0 w/ AW4 @ 160K. i need a new DD :eyes:

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Are you adjusting your odo reading for the bigger tire size when you calculate your mpg?