is my mpg really this bad (Warning involves math)

[thedave360]

so i finally got the speedometer to work.

 

I filled up the tank, and noted the mileage. After I made a few 20 mile trips over a few days, and a bunch of slow off road driving, I filled up again and figured out the mileage. Now, the difference in mileage was 133, and I filled up with 12 gallons...this comes out to 11 mpg. BUT, I have 31" tires and my speedo hasn't been adjusted. So I made a trip that I know is 2 miles, and the odo only measured 1.8 miles. Doing some math, I adjusted my mileage to 147 miles, this comes out to 12ish mpg.

 

The truck seems to run fine other than the crappy mileage. Any other ideas, am I missing something??

 

 

92 'manche 4.0 auto 4" lift 31" tires

[HOrnbrod]

Have you regeared or is your rear diff ratio still the normal 3.55 for the AW4? That's pretty tall gearing for 31" tires, and will hurt your mileage as the engine will be lugging in a less economical power band. I did the same (31" tires) and mileage sucked, especially for slow local driving. I regeared to a 4.10 ratio that is much closer to the stock stock final drive ratio. Mileage is now 15-17 in town, and 19-21 highway. And that's w. the stroker. :D

 

What was your mileage before the 31" tire change?

[Dave Stark]

I was only getting 10hwy & 8 city in my XJ with 31's on it, so don't feel lonely. :brows:

[mvusse]

20 mile trips are not long enough to get optimal mileage, and you probably burned a few gallons off road while only covering 5 miles or so. Drive 200 miles or so straight on the highway ~60mph and I bet you'll see at least 20.

 

Assuming you have a 4.0, try a set of new Ford injectors from precision. Much better spray pattern, resulting in better mileage and smoother idle for most people. Some have seen upwards of 15% improvement in mileage.

[Eagle]

You did the math right. 31" tires are about 10% larger (in terms of distance per revolution) than stock tires (assuming that stock was 215s or 225s), so 133 + 10% would be around 147.

[thedave360]

Thanks for the input guys. If it gets 11-12 mpg, I can live with that because its not my DD...I just wanted to make sure I didn't have something wrong or was messing up the math for the tires.

 

....Assuming you have a 4.0, try a set of new Ford injectors from precision. Much better spray pattern, resulting in better mileage and smoother idle for most people. Some have seen upwards of 15% improvement in mileage.

 

How do I know which injectors from ford will fit my 4.0? I assume this would also come with a nice increase in power.

[mvusse]

http://www.precisionautoinjectors.com/p ... roductId=7

[dfreeman616]

i put those injectors in my truck, and i'd recommend it to anyone. i wasn't too particular about my MPG calculations, but it's somewhere around a 10% increase for me, aside from the better performance.