This can't be right? - JeepForum.com

 
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post #1 of 14 Old 07-27-2011, 01:35 PM Thread Starter
freestylekid33
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This can't be right?

Yesterday i checked my fuel mileage and burnt 1.9 gallons in 53.4 miles. I know my speedo being off messes it up but it cant be that much? When my speedo says im doing 65 im doing 70 on my gps. Can anyone help me figure out what i am actually getting for MPG because there's no way i'm getting around 27 MPG in a xj on 32's with stock gears lol.

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post #2 of 14 Old 07-27-2011, 01:45 PM
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So you went to the gas station after a 53.4 mile trip and topped your tank back off with only 1.9 gallons?

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post #3 of 14 Old 07-27-2011, 01:56 PM Thread Starter
freestylekid33
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Quote:
Originally Posted by FleshEater View Post
So you went to the gas station after a 53.4 mile trip and topped your tank back off with only 1.9 gallons?
Yes sir that is correct, it doesnt sound right does it? I thought my jeep was gonna shrink into a prius on the ride home from the gas station
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post #4 of 14 Old 07-27-2011, 02:24 PM
Kevin108
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That's not how you check mileage.

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post #5 of 14 Old 07-27-2011, 03:24 PM
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Thats way too short of a trip to even get an idea. Every pump is going to cut off at a slightly different time, and therefor fill to a slightly different level. The best way is atleast 2 tanks full, averaged together.

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post #6 of 14 Old 07-27-2011, 08:12 PM Thread Starter
freestylekid33
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I figured it would be the same? I used the same pump at the same station... I guess ill try again and wait till i go further to fill it, is there any way to figure out how many miles i actually go with my odometer being off because of my tires?
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post #7 of 14 Old 07-27-2011, 08:31 PM
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With your actual speed being 1.077 times your speedo reading, your odo readings are likely off by a similar ratio. Thus 100 miles on the odo may equate to about 107.7 actually traveled.
I'd guess the gas pump hiccupped and shut off before your tank was completely full. My 96 will do the same if I run the pump at full blast.

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post #8 of 14 Old 07-28-2011, 06:53 AM Thread Starter
freestylekid33
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Ok cool thanks for the info, i guess ill fill it a couple times then and check it after that so if something like that happens it wont make near as big of difference lol...

Quote:
Originally Posted by chris87xj View Post
With your actual speed being 1.077 times your speedo reading, your odo readings are likely off by a similar ratio. Thus 100 miles on the odo may equate to about 107.7 actually traveled.
I'd guess the gas pump hiccupped and shut off before your tank was completely full. My 96 will do the same if I run the pump at full blast.
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post #9 of 14 Old 07-28-2011, 12:01 PM
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If your running larger tires on the highway with stock gears for 57 miles, I could see 27 mpg out of an XJ. As long as there was no hills and it was flat or down hill. Meh.

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post #10 of 14 Old 07-28-2011, 05:00 PM
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Quote:
Originally Posted by jeepdavetj View Post
If your running larger tires on the highway with stock gears for 57 miles, I could see 27 mpg out of an XJ. As long as there was no hills and it was flat or down hill. Meh.
Doesn't it work the other way around when you have bigger tires? Because your tires are going to make fewer rotations in a mile. making your speedo show that you have traveled less miles than you actually have and trowin off your calculation.

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post #11 of 14 Old 07-28-2011, 05:58 PM
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Take the circumference of a 28 inch tire then take it of a 31. The 31 is much longer. 1 rotation of the 31 means more distance traveled than the 28, but the speedo will still only count one roatation. My jeep with 31's goes 1.12 REAL miles for every 1 JEEP mile.

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post #12 of 14 Old 07-28-2011, 06:27 PM
cdawall
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i answered this on another forum

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Originally Posted by cdawall View Post
so quick break down to work it out since people are asking

200 miles on your stock setting odo for a jeep that came stock with 225/75R15 (28.29") after installing 285/75R16 (32.83") and used 12 gallons

32.83*pi=103" for a full revolution
28.29*pi=88.83" for a full revolution
63360" in a mile
63360*200=12672000 inches in 200miles
12672000/88.83=142654 revolutions of the tire in 200 miles
142654*103=14693414" actual inches traveled by a 285/75
4693414/63360=232 miles traveled
232/12=19MPG

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post #13 of 14 Old 07-28-2011, 08:35 PM
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That is the long way to do the math, the short way is to do it like this:

32.8" new tire / 28.3" old tire = 1.16 ratio between new and old
200 miles indicated * 1.16 = 232 miles acutally travelled
232/12 = 19MPG
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post #14 of 14 Old 07-28-2011, 08:38 PM
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Quote:
Originally Posted by the_weirdo View Post
That is the long way to do the math, the short way is to do it like this:

32.8" new tire / 28.3" old tire = 1.16 ratio between new and old
200 miles indicated * 1.16 = 232 miles acutally travelled
232/12 = 19MPG
yup but it is very very exact and someone asked i was specific.

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