[Swagger_Wagon]

So today before I left for home from college I filled up my gas tank. I was just shy of the empty line. The pump stopped at exactly 15 gallons. So I figured I have a 16 gallon tank. The ride to home from school is 238 miles and I used ALMOST a whole tank. Get to about the last dash before the empty line. So I do the math and figure I'm cranking out about 15 miles per gallon, which is pretty ****ty considering it's a straight drive on the highway doing about 75 the entire way until I hit 75 and 85 (if you've ever been unfortunate enough to drive through/around ATL, you know what kind of traffic is present on those highways ). But just a second ago I checked my email and saw that Chrysler returned my request for the original build sheet of my XJ and see that I have a 20 gallon tank... the math with that size tank factored in comes out to just under 12 miles per gallon... What's goin' on here? I plan on running a can of sea foam through the TB, etc. Hopefully that'll help. Any ideas on what's going on here. And FWIW when I set my cruise control to 75 my rpm's stick at ~2,200. Is that a little high? I know I have bigger tires on my 3.55 gears as well as a roofrack with a fullsize spare and I expected that to kick the mileage down a little bit but not this much Any ideas? And by no means am I expecting something crazy like 20 mpg but somewhere around 16-17 would be amazing. Although I think those #s may be wishful thinking.

[CJ7-Tim]

Sorry to say, you flunked this quiz. Miles per Gallon is figured by filling the tank (use medium not full blast) and noting the odometer miles. Drive until you need to re-fill. Fill the tank, note the odometer miles again and do the math. Ideally, you will do this for several fill-ups to average out the effects of wind as you drive.

In your case, you will have to determine a correction factor for your oversize tires. This will apply to miles traveled between fill-ups and will increase your MPG calculation.

Lift, oversize tires, stuff of the roof, all effect your MPG's due to added wind resistance and extra weight. With incorrect gearing, you are not using the most efficient portion of the engines power band. There are plenty of mpg threads here on the Forum, but like they say in the adverts, your actual mileage may vary, batteries not included.

[drvn93]

one way to calculate the odometer difference is to drive to a nearby city in a reg car and gauge the miles from 2 points. next time you go, fill up your jeep and take the same drive gaugeing the miles between the same 2 points. then divide the 2 for the percentage difference. in my case with 31" tires on 3.55 it was 1.12168%.
if i want to figure out the miles I just mutiply by that percentage.

its kindof a rough estimate but it helps

[Paradise XJ]

...and then there are mileage markers on most highways. A simple way to measure (or figger) difference in what your odometwer says and what you're actually driving.

My odometer reads 9/10th's for each mile driven, consistently, so 10% off. My stock tires were 215/75/R15 with 3.55 gearing. I now have 31/10.5/15's. If I've driven 275 miles by the odometer reading, I've actually driven 27.5 miles more....so 302.5 miles on 15.8 gallons gives me roughly 19.1 mpg (I'm pretty happy with that number)

[CJ7-Tim]

With 31's my speedometer says 60mph and my GPS says 64 mph. My odoemter is off by the same amount, about 7%.

[Swagger_Wagon]

Ah alright thanks for the help guys. But I'm not going off the mileage from the ODO. I got the 238 mile number from MapQuest..

[yellowford]

You can't go off of the gas gauge it is not an accurate measurment of how much gas you actually have, the only way to actually figure it is to go and fill up again then see how much gas you used.

[_litz]

And ... don't forget -- XJ gas gauges are notoriously inaccurate (some were even recalled) ...

On my '97, I find I can do about 200 miles on the first 1/2 a tank (by the gauge) ... and it drops like the proverbial rock for the next 100 miles, since I usually get 300ish miles per tank.

My experience (and I've put all 180k of the miles on my Jeep, so I have quite a bit of experience at this) is that 1/4 tank = find a gas station. 1/8 tank + warning light = find a gas station NOW.

I've put over 18 gallons into the thing w/the needle still above E.

- _litz

[Swagger_Wagon]

alright thanks for all the advice!

[Kettles]

As everyone else said, dont use the gas guage for math. Use numbers for math. The gas pump gives you numbers so use those.
Miles per tank is a terrible number to reference, it is essentially meaningless. Miles per gallon is the way to go. Make sure to take into account your tire size when doing calculations if you are using the ODO and not a gps unless you have corrected gearing or the speedometer gear.

FWIW one line above E (where the light comes on) for me is about 4 gallons left (16 used), E is closer to 2 (18 used). But I have had a few times where its been below E and still only took like 16 gallons. Point is the gauge is not accurate.

[Paulio91184]

i know what u mean guys, my tank was on E and only took 15 gallons, it is consistently 1/4 tank off...at full it reads 3/4..3/4 reads 1/2 and so on

[Swagger_Wagon]

So the consensus seems to be, I really shouldn't trust that damn gas gauge lol. I was at the last dash, the one that's right before when the empty light comes on, and I just put in 15.5 gallons... So just doing a quick little rough estimate, not factoring in tire size, wind, taking 2 trips, etc. I'm getting ~17 mpg which is VERY reasonable because the first 100 miles of the trip home from school get's pretty windy and there's quite a few little long rolling hills (I hate those, I have my pedal to the floor 80% of the time on those hills and I'm winding it past ~3,300 rpm, everything sounds like it's gonna fall apart )

Edit: and btw, when I was looking at my build sheet I saw I have 3.55 gears. Does that number apply to all XJs with the 8.25? I know 3.07 gearing is common on TJs with the turdy five.

[Kettles]

AFAIK all the autos had 3.55 (axle independant), the 3.07 would be for a manual. Tow package might have had 3.73 but I dont know.

[Swagger_Wagon]

Originally Posted by Kettles

AFAIK all the autos had 3.55 (axle independant), the 3.07 would be for a manual. Tow package might have had 3.73 but I dont know.

Oh ok, I don't have a hitch and it wasn't listed on the build sheet so I don't have the tow package. Is there a reason why the sticks have lower (numerically) gears?

[Kettles]

The ratios on the tranny are different.