U thank there is a problem?

[Eagle]

ok not a vacuum leak, what else could the bad mpg be from?

Oxygen sensor.

 

And I seriously doubt that anyone gets close to 30 MPG out of a 4.0L MJ. The very best my '88 XJ ever did was one tank on one trip when it was almost new, when I got 28 MPG. More typical is 18 around town to 21 or maybe 22 highway -- and I drive like an old lady. The '88 MJ typically runs a couple of MPG less than the XJ, probably because the aerodynamics are worse; the MJ runs around 17 to 18 in local driving, and 18 to 19 MPG highway.

[TrailReadyMJ]

Still running stock 3.55.

 

Plan is to upgrade to FS Ford axle (high pinion 44 front, matching 9" rear), so I'm not going to regear what I have. these 3.55s are going to go into Booger (4.0HO/AX-15, currently has stock 3.07 gears), and I'm building a 4.10 ratio front axle to go under Sparkles (4.0HO/AW4). Will need to get a 4.10 rear axle before I make it into a 4wd, as it is currently still stock at 3.55.

 

Ah, that makes sense then.

 

I hope the 4.10s are low enough for you, I wouldn't mind going 4.88s myself, because there are a lot of hills here. The 4.56s aren't bad at all, close to stock, but you also have to factor in the extra rolling resistance of the larger tires. A little lower would be nice at times. Anything will be better than stock though, I had to drive mine for about a week on 35s and 3.55s before my gears came in and it was very sluggish. After the regear it was night and day!

 

Oxygen sensor.

Very good point! That's the sensor that will have the most effect on millage :doh: .

[mvusse]

Don't know what I'll be running yet, but the 35s are to be just play tires, using the 33s on the road. Will probably go with 4.56.

 

Sparkles is getting 4.10s, minimal lift, possibly 3.5", but more likely 2 budget boost and 31" tires. It is to be my daughter's car when she turns 16, and even though she wants to be able to wheel it, it will also be her DD. An inexperienced driver (at least on the road, she has lots of experience getting the Pig muddy) in a tall rig with big tires is asking for trouble.

[TrailReadyMJ]

Oh, ok. It was early and I didn't put the names of your Jeeps together :doh: . I was thinking the 4.10s were going in your MJ.

4.10s will be perfect with the 31s :thumbsup: Should be a great combo for the inexperienced (street) driver!

 

A friend of mine never wanted to lift his XJ because he didn't want to mess up the ride and handling (he had a rural mail route). I finally talked him into a 3" lift and 31s, and he came back a week later commenting on how it was even better than stock! Of course his suspension was badly worn to begin with.

[Eagle]

If you can find the original tire size, get its circumference, and measure the circumference of your existing tires, then divide the two together, this will give you a percentage that your speed and odo is off and then you can add that percentage to your per tank milage for a more accurate reading.

You have the right idea, but using tire circumference isn't very accurate. Most of the tire manufacturers include in their specs a figure for revolutions per mile, which takes into account the amount of sidewall flex under a normal load as well as tread squirm. In making up my spreadsheet for comparing gear ratios and tire sizes, I used manufacturers' revolutions per mile for the calculations.

 

Even within one (nominal) tire size there's variation between/among brands. In fact, at one Time Cooper had a P235/75R15 and the same tire as a 235/75R15 LT. The LT had stiffer sidewalls and, even though ALL dimensions of the two tires were identical, the revs per mile were different.

 

Back to the bottom line. In round numbers, going from a stock 215/75R15 to a 30x9.50 is a difference of approximately 6%. Going from the same 215 to a 31x10.50R15 the difference is 11%. If you start with a 225/75, the difference will be slightly less.

 

In VERY round numbers, and for most practical purposes, I think it's valid to generalize and say that going from "stock" to 30s is about a 5% change, and going from "stock" to 31s is about a 10% change.

 

If you're running 31s and the gas mileage calculated from the odometer is only 12 MPG, then correcting by adding 10% only brings it up to 13.2 MPG. That's not very good for a 4.0L MJ.