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I am not good at math at all but I think I have this right. Please let me know where (not if) I made a error. I entered my first fill up into fuelly.com and the calculation is the same as my computer in my Jeep even though I have not adjusted the tire size or anything like that. The CPU and fuelly were at 14.3 How does that work? My mileage is still off but the MPG average is right on. Here is how I came up with the correct number to enter into fuelly so let me know if I am messed up.

Stock tire = 29.5" - 92.63" circumference
New tire = 31.5" - 98.91" circumference

The CPU says I drove 226 miles since last fill up so here is how I came up with the number of revolutions the tire made during that time.

226miles
x5280feet/mile
1,193,280feet

1,193,280feet
x 12inches/foot
14,319,360inches

14,319,360inches
92.63inches = 154,586.6revs

That means that the computer thinks the tire made 154,586.6 revolutions since the last fill up. From there I think I can calculate the actual miles like this:

154,586.6revs
x 98.91inches<--new tire circumference
152,901,164.1inches

152,901,164.1inches
12inches/foot = 1,274,180.3feet

1,274,180feet
5280feet/mile = 241.3miles

This means that I actually went 241.3 miles on my last fill up even though the computer in the jeep says I went 226 miles. The computer also calculated the average MPG to 14.3. so.....

241.3miles
16.892gallons = 14.3MPG

If all of that is correct then how is it that the XK knows the correct MPG but not the correct mileage? or maybe it is just a strange coincidence.
 

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Beats the heck out of me? Maybe the Nav unit told your computer how far you went?

Seriously, though, it looks like your calculations are right but I don't know how the on board computer knows.

To simplify your calcs in the future, just multiple your miles by 1.07 That's 98.91/92.63

For every mile your odometer reads, you are going 1.07 miles.
 

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Discussion Starter #4
Beats the heck out of me? Maybe the Nav unit told your computer how far you went?

Seriously, though, it looks like your calculations are right but I don't know how the on board computer knows.

To simplify your calcs in the future, just multiple your miles by 1.07 That's 98.91/92.63

For every mile your odometer reads, you are going 1.07 miles.
Thanks GP, I will use that for future entries into fuelly but I just wanted to be as sure as possible that things were right. I don't know if the nav units can communicate with with the computer in the XK but I don't have a nav unit so I cannot say for sure.
 

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I doubt the NAV and the trip computer are integrated on those that have them. I was kinda kidding. But it is mysterious that your trip computer got 14.3. Maybe one of the Mopar experts out there can explain it.
 

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Some math for you

Circumference of a circle is found by:



so a 32" tire is

3.14*32.05=100.688"

5280'*12"=63360 " in one mile

63360/100.688=629.27 revolutions per mile.

for some reason, BFG shows 649 rev/mile at 45 mp/h
 

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683.665/629.27= 1.086

that is the difference of the tires.

so now all you ever need to do is multiply your mileage shown by that number.

255*1.086=276.93

this of course does not take into account for tire wear and tire pressures which change the circumference of the tire.
 

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Discussion Starter #12
From what you have said I am confident in my calculations. I know my tire is actually 31.6" new and there is some wear. so to get from the mileage that my computer says to the actual mileage I multiply by 1.0677 or there about. My question is why is my computer's estimated MPG not 6% (about) off of actual? I would think that based on the 6% difference in mileage calculation the MPG would reflect that also.
 

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Some math for you

Circumference of a circle is found by:



so a 32" tire is

3.14*32.05=100.688"

5280'*12"=63360 " in one mile

63360/100.688=629.27 revolutions per mile.

for some reason, BFG shows 649 rev/mile at 45 mp/h
Calcs of this kind can have compounded rounding errors but I think the 629 revs/mile is accurate for 32".

255/70/17 which is 31.06" is 649 revs/mile. Maybe BFG used that instead of 255/75/17 (32").
 

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I don't know how they calculate the mileage. It could be calculated on factors like pedal position, g-force, load etc. or it could be off the mileage alone.

i found my mileage is usually not correct.

From what you have said I am confident in my calculations. I know my tire is actually 31.6" new and there is some wear. so to get from the mileage that my computer says to the actual mileage I multiply by 1.0677 or there about. My question is why is my computer's estimated MPG not 6% (about) off of actual? I would think that based on the 6% difference in mileage calculation the MPG would reflect that also.
 

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i usaually just round 3 decimal places out. there is not much more need then that in calcs of this type.

Calcs of this kind can have compounded rounding errors but I think the 629 revs/mile is accurate for 32".

255/70/17 which is 31.06" is 649 revs/mile. Maybe BFG used that instead of 255/75/17 (32").
 

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i usaually just round 3 decimal places out. there is not much more need then that in calcs of this type.
Yep, I agree. Especially on trips of 1 or 2 tankfulls. I just wanted to point it out. When you multiply a rounded number by a rounded number it compounds it.
 

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I got a head ache just trying to keep up with you guys :)
 

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Discussion Starter #18
Yep, I agree. Especially on trips of 1 or 2 tankfulls. I just wanted to point it out. When you multiply a rounded number by a rounded number it compounds it.
that is correct I used all the numbers to the last decimal place to do the actual calculations but I rounded the numbers when I typed it out to save time.
 

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Why not drop by your dealer and get the computer reset to you new tire size?
 
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