90 Xj 2.5 liter 5 speed, on the stock tires it was getting 28 highway. Stock were 205/70

Now its less on 30x9.5's

 

1996 5 speed 4.0 all stock for now. I get 12.8 MPG driving with my foot on the floor all day. I drive 50/50 with city and highway. I could probably push 14 MPG if I drove it easy, but that just wouldn't be any fun

 

average about 12.5mpg. I use my jeep for work and do a ton of slow driving *10mph* which really kills the milage calculations. The surco rack doesn't help any either.

 

1990 4.0 with 30" tires, getting around 18 in town, 20 on the HW.

 

i stopped calculating my MPG. every time i calculated how much i got from a full tank of gas every fill up or every week, the more i got depressed! haha therefor i just fill her up. problem solved.

 

If larger tires than stock are used can the mileage be trusted? The speedometer is off so unless the corrected speedometer gear was installed how are people getting accurate readings?

 

you do the math and calculate to get an accurate figure

 

But if the speedometer is off that means the odometer is off also. Right? So if it says 280 miles on a tank of gas, it's really not 280. Right?

 

yes and you calculate the difference in tire size and figure out the actual miles traveled.

 

Yes, that's the only way to do it... How many people with bigger tires do you suppose did that though...

 

Most probably don't.

 

Now I'm going to have to go out and measure my tires to figure out how far it's off.

 

Ahaha, niiice!

 

Re: Calculating gas mileage with larger tires! HELP
Assuming you mean LT325/65R18, BFGoodrich says their AllTerrain has 599 tire revs/mile. Stock LT275/70R18 tires had 629 tire revs/mile.

629 minus 599 = 30

30 divided by 629 = 4.77 percent.

So your speedo, odo, and tripmeter are all slow by 4.77 percent. If you don't want to have the speedo calibrated to 601 tire revs/mile (the smallest number the dealer can do), then to compute accurate fuel mileage, you need to add 4.77 percent to the miles indicated by your tripmeter.

Example:

24.8 gallons of fuel

400.7 miles per tripmeter + 4.77% = 419.8 miles

419.8 miles divided by 24.8 gallons = 16.93 MPG

 

So using that calculator (with the trip computer mpg's) it turns my 15.3 on the trip computer into 16.09mpg accounting for the larger tires.

I still need to do a tune-up so hopefully I'll get mid 16's all around. Around 1mpg better than my last 3 Trucks... Chevy Suburbans! Driven under same conditions on the same routes...