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Unread 01-09-2014, 09:53 AM   #1
Bockwinkle
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2012 JK Mileage Calculation

I purchased a 2012 JK with 743 miles on it with the following options:
3.73 gears
6spd manual
connectivity package
AC & soft top

It came with a set of 17" sport wheels with 255/75/r17 tires off of a 2012 Sport S JK.

I only have 1200 miles now and have synthetic oil in it.....the trip computer says I am averaging 17.6mpg since my oil change.

The ECU still thinks I have 225/75/r16s on it.....doing a little conversion math I am only turning 629 revolutions per mile instead of 689 revs per mile...or at an indicated 60mph I am actually travelling 65.7mph.

Gas consupmtion is gas consumption....but driving at a steady 60mph for an hour I would actually travel an extra 5.7 miles. Am I correct in assuming the displayed 17.6 mpg would actually be 17.6*1.09 = 19.3mpg? That would be assuming the figure was correct in the first place even though several people have indicated it reads low. So I might even be pushing 20mpg.

Sound right?

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Unread 01-09-2014, 10:10 AM   #2
TheBoss
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Sure and you could hit 26mpg with 315 70 17's using that logic. You need to get a recalibration unit like a ProCal to re set the tire size for the ECU .Your still in stock tire size range so it won't be off by much ,but it is a worthy investment. You could always hand calc the mpg when filling the tank vs miles traveled as well. Your dealership may even do this for you as well but there are other functions with the ProCal that you may find useful.
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Unread 01-09-2014, 10:42 AM   #3
Jeepforthewin
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I would opt and get the ProCal. The dealership will typically charge to recalibrate for larger tires. As for hand calculating, You will need to find the percentage difference between the stock tire size and current size and remember to add that to the miles traveled before you calculate. I usually have to add about 17-20 extra miles when I hand calculate since I havn't corrected for tire size yet.
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Unread 01-09-2014, 02:26 PM   #4
Rammer500
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Yes, That sounds right as you are traveling further than the Odometer is indicating.
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Unread 01-10-2014, 06:05 AM   #5
Michel347
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That sounds right, and around what I get with mine.
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Unread 01-10-2014, 10:51 AM   #6
Andyjr
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Another vote for the ProCal. I just put 35's on my JKU and I love how easy the ProCal is to adjust the tire size. If you have a smartphone there are many apps out there that have a GPS on them and it will tell your speed while driving.
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Unread 01-19-2014, 01:12 PM   #7
Bockwinkle
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Quote:
Originally Posted by TheBoss View Post
Sure and you could hit 26mpg with 315 70 17's using that logic. You need to get a recalibration unit like a ProCal to re set the tire size for the ECU .Your still in stock tire size range so it won't be off by much ,but it is a worthy investment. You could always hand calc the mpg when filling the tank vs miles traveled as well. Your dealership may even do this for you as well but there are other functions with the ProCal that you may find useful.
and why wouldn't it be true for a 5' diameter tire? the jeep ECU knows how much fuel it consumed per unit of operational time.....and you know how much further you actually travelled based on the increased rolling tire circumference......aren't those the ONLY two factors to consider?

assuming the Jeep ECU is actually capable of calculating how much gas the jeep is consuming
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Unread 01-19-2014, 02:20 PM   #8
73azbronco
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that's about right for these things, but I admit it sounds like your trying to cut the problem with a scalpel when the closest result you can get is cutting it with an axe.
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Unread 01-19-2014, 03:43 PM   #9
TheBoss
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Quote:
Originally Posted by Bockwinkle View Post

and why wouldn't it be true for a 5' diameter tire? the jeep ECU knows how much fuel it consumed per unit of operational time.....and you know how much further you actually travelled based on the increased rolling tire circumference......aren't those the ONLY two factors to consider?

assuming the Jeep ECU is actually capable of calculating how much gas the jeep is consuming
Your assuming that the ECU could accurately compute the fuel mileage enough for you to recalculate it accordingly. This has already been debated to no end on this and every other JK forum. You NEED to get a recalabration unit so that at a minimum your odometer is correct as I do believe there are legal implications involved in tampering with it. If you ever decide to sell it.
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