My 98 XJ, 4.0 I6, 4x4 automatic, 4.88 gears, 33" tires, 5.5 inch lift...gets insanely GOOD fuel economy...WAY more than I expected!

I filled it to the top when I first got it last week. And then I was down to just over a quarter of a tank so I filled it up again. I had driven 303.1 miles, and it took 13.37 gallons to fill it back up to the top. This was all back road miles, with no speeds over 55mph.

That comes out to 22.67 MPG!!! That is WAY better than I was expecting to get, especially with the big tires!

Is that about average? I don't recall my stock '99 getting much better than 18, but that was a long time ago so I don't remember for sure.

Kev

 

Check your speedo with your phones GPS.....did you correct it for tires/gears ??

I was getting numbers like that with my 96, till I checked the speedo against the GPS....and the factory speedo was off by a lot. With 31" tires, I should have been reading 8% low, but @ 60 mph on the speedo, I am doing 61.8 mph by gps......with no corrections to the speedo. So the factory speedo was off by ~11%-12% high.

My mpg also went down with the lift due to air resistance...same tire size & gears


If you went from 3.55 to 4.88 gears, you have a major change due to that (37% increase in engine rpm for the same speed = speedo reads high), and the tire size also makes a change come into play (14% increase in distance traveled for the same indicated speed = speedo reads low). You need to correct for both of those, but your speedo thinks you are going a lot farther than you really are due to the changes, and the best way to check is using gps vs speedo speed to see how far off you are, then correct from there.

I use an app that gives you a HUD display to compare speeds (DigiHUD Speedometer). My guess is you are off by about ~30% on MPG between the 2 changes (tires & gears).



.

 

So how would I correct? 33" MT tires, and 4.88 gears. Shouldn't those just about cancel each other out? Are there different senders to adjust for these differences? And is there a calculator to determine what sender one would need to make the odometer/speedometer more accurate? At 55 mph, it feels like I'm going right around that speed. What I do know for sure is that I only had to gas the Jeep up once this week...where with the old K5 Blazer, it seemed like I was dropping money in the tank every time I took it out...lol!

Kev

 

The speedo gear in the trans should be changed out is what they are hinting at.

 

I figured as much. But how do I determine which one I would need? Is there a formula or calculator to use? Thanks!

Kev

 

https://www.quadratec.com/c/referenc...ar-tooth-chart

 

Perfect! With 4:88 gearing and 33s, it says I need a 42 tooth gear. Now...should I go with the height marked on the tire (33"), or should I measure the height with a tape measure since they are brand new MT type tires with big *****?

Kev

 

measure always......and measure from the center of the hub to the ground...

and from what I see, you need a 41 tooth gear, not 42.

 

41 is what I saw as well.

Use the chart, it is ballpark, but different tire manufacturers have different sizes, meaning they list it as a 33" tire, but it might actually be closer to 32 or 34, depending. Take into account tire wear as well...the chart will get you as close as possible, without doing trig to figure out your gear tooth requirements, haha

 

I have a 1998 auto on 33's with stock gearing (3.55), after market bumpers, winch, full size spare, and a roof rack and I'm getting between 10-15MPG.
I'm going to say your 22mpg calculation is high. That would be more than the stock vehicle would have gotten when it was brand new. I believe 20mpg combined was advertised.

 

Thanks again! Typo. I meant 41.

Kev

 

My XJ with a 4 1/2" lift, roof rack, winch bumper plus added weight of extra gear it gets 15-17 mpg. I have corrected the speedometer gear and use a GPS also for more data both are within + or - 1 for speed checked against each other.

This with a 33x12:50" tire locked and 4.56 gears in my opinion this mpg is maxed out for me and most other XJ's like it. The way you drive has a lot to do with your mpg as much as anything else.

 

Okay. I'm an idiot. My gearing is 4.56, NOT 4.88 as my faulty memory told me. I wrote it down on a piece of scrap paper when I traded my K5 for this XJ, and found the piece of paper. So yeah. 4.56 gearing. And while my Mastercraft Courser MXT tires say 33x12.5x15r, I measure them at just around 32.25 inches from side to side, and exactly 32 inches from ground to top. So...should I go with a 39 tooth gear for 33", or a 40 tooth gear for 32"? I'm guessing I should go with the 40 since it measures at 32"?

 

measure from the center of the hub to the ground with the vehicle weight on it......that is the rolling radius.

Multiply that by 2 and that is the tire size the vehicle sees and you should use.

The weight of the tire flexes it, and while it may measure 33" top to bottom, if the hub to ground is 15.5", it will only travel the distance of a 31" tire per revolution.


What's this side to side stuff ?? I can make a 30" tire measure 34" if I do it side to side....all I have to do is squeeze it top to bottom more. The correct way to measure a tire is hub to ground.....not side to side. The change in tire height from the vehicle weight that makes the sidewall shorter on the bottom will make it longer on the sides & top to maintain even air pressure inside it.

On the truck in my avatar, I ran 40" swampers, and the tire itself read 40" tall, but the center of the hub to the ground was 19".....and when you calculated out the distance per rev, it was closer to a 38" tire than it was a 40".




.

 

Thanks man! I measured it just the way you said, and from center of hub to ground it is just a hair over 16 inches...so 32 total. Looks like I should go with the 40 tooth speedo gear. Thanks again man!

Kev