MPG calculation - Gramps - Help me out on this one :) -
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Unread 04-04-2008, 11:35 PM   #1
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2007 MK Compass 
Join Date: Feb 2008
Location: Antelope, CA
Posts: 135
MPG calculation - Gramps - Help me out on this one :)

My math was all wrong .... so I was getting 274 miles on a tank of gas.

Well, if my speedometer was not accurate with my tire size, then I was using bad data.

Stock: 215/55/18
speedometer @ 60mph (if calibrated)
revs per mile - 761.6

Patriot Wheel: 215/65/17
speedometer @ 58.5 - 2.53% slower
revs per mile - 742.8

New tires: 245/65/17
speedometer @ 55.4 - 8.135% slower
revs per mile - 704.3

So using my GPS which should be the same accuracy irregardless of the tires/wheel size. When it shows me doing 60 and my speedometer shows me only doing 55 mph, I figure that I have driven about 5 miles per hour further than the odometer is showing. So at the end of a tank, lets say 5 hours driving @ 60 mph average, my odometer would be 25 miles less than I actually traveled.

This might be hard to follow, I am not good at explaining things

So let's say I did 275 miles on that fill up and it was 12 gallons. That is 22.9 mpg ... but wait, it should really be 300 miles and 25 mpg .... right??

Why does this even matter? I have no idea ... it is just one of those things that I have been thinking about all night for some stupid reason.

'07 Compass Limited 4X4: K&N Air charger, Borla cat back exhaust, Rocky Road Lift kit, Firestone Destination A/T 245/65/17's, Red painted brake calipers, Pioneer AVIC Z2, Sirius, iPod interface, Polk Audio 6X9s (all 4 doors), Polk Audio Tweeters (dash), Polk Audio 6.5s (rear hatch), 2 Kicker CVR 10" Sub woofers in custom enclosure (hidden), Eclipse XA4000 4 channel & XA1000 1 channel amplifiers.

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Unread 04-05-2008, 03:49 AM   #2
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2008 MK Compass 
Join Date: Feb 2008
Location: NH
Posts: 68
I hate when that happens....

Yes your fuel economy is better than you thought. The specifics of calculating the error would depend on whether the error is consistent or not. I have seen cars that at 30 are going 31 and at 60 are going 58. You might want to take it in and see if they can re-program the speedometer to match the tires....
2008 Jeep Compass Limited 4x4 CVT
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Unread 04-05-2008, 05:33 AM   #3
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2007 MK Compass 
Join Date: Aug 2006
Location: Madison/Janesville, Wisconsin
Posts: 5,624
LOL! This is right up there with the Chewbacca defense and Cliff's Buffalo Theory.

Revs per mile is one way to do the calcs but I prefer working with tire height.

Originally calibrated to a 27.3" tire (confirmed by your nav unit); current tire 29.5".

Speedo correction: 27.3 divided by 29.5 = 92.5% or at 60mph X .925 the speedo reads 55.5.

Distance traveled is just the opposite, 29.5 divided by 27.3 = 1.08% or 274 x 1.08 = 295.2mi.

MPG: 295.2 (actual miles) divided by 12gal = 24.66 mpg

Doin' good!!

Side note: My wife is really liking those tires on her Grand, A LOT!!! and I just bought some Patriot 16" steels, now I'VE got more options to play with!

Artificial intelligence is no substitute for natural stupidity

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