Hope my math is right! - Jeep Commander Forums: Jeep Commander Forum
Wheels / Tires This section contains discussion about wheels and tires for your Jeep Commander.

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post #1 of 37 (permalink) Old 03-13-2009, 09:12 AM Thread Starter
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Hope my math is right!

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.

Bright Silver 4.7L 2.25 RR/OME lift, Kumho KL71 265/70/17, Flashpaq, Lights, rack, intake, Super44, mopar skids, 4x belly guard, rear fog lights, MM9500 winch, GetLost4x4 winch mount

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post #2 of 37 (permalink) Old 03-13-2009, 09:21 AM
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Whoa. Way to much math, way to early in the morning for me.

06 Sport-4.7L with QT2

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post #3 of 37 (permalink) Old 03-13-2009, 09:26 AM
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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.

2007 XK Limited, 5.7L Hemi, QDII, Rusty's 2" lift, Spidertrax 1.5 inch wheel spacers, GetLost4x4's front hitch mount for portable winch.GY Duratracs 265/70R17s , Rola Roof Rack, diy built spare tire mount for the roof rack.
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post #4 of 37 (permalink) Old 03-13-2009, 09:30 AM Thread Starter
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Quote:
Originally Posted by GPintheMitten View Post
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.

Bright Silver 4.7L 2.25 RR/OME lift, Kumho KL71 265/70/17, Flashpaq, Lights, rack, intake, Super44, mopar skids, 4x belly guard, rear fog lights, MM9500 winch, GetLost4x4 winch mount

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post #5 of 37 (permalink) Old 03-13-2009, 09:52 AM
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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.

2007 XK Limited, 5.7L Hemi, QDII, Rusty's 2" lift, Spidertrax 1.5 inch wheel spacers, GetLost4x4's front hitch mount for portable winch.GY Duratracs 265/70R17s , Rola Roof Rack, diy built spare tire mount for the roof rack.
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post #6 of 37 (permalink) Old 03-13-2009, 09:54 AM
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you can go to the tire manufactures site to see all this info. you just hurt your brain for no reason.

Sizes & Specs - Mud-Terrain T/AŽ KM
http://www.bfgoodrichtires.com/specs...a-km/1152.html
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post #7 of 37 (permalink) Old 03-13-2009, 10:04 AM
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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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post #8 of 37 (permalink) Old 03-13-2009, 10:13 AM
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stock sized tire according to Goodyear

29.5

so.......


3.14*29.5= 92.677

63360/92.677=683.665 revs / mile
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post #9 of 37 (permalink) Old 03-13-2009, 10:17 AM
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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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post #10 of 37 (permalink) Old 03-13-2009, 10:19 AM
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if you want an even simpler way.

GPS!!!
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