Mathematicians, Help me Formulate my tire PSI! - Fuelly Forums

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Old 12-24-2009, 09:13 AM   #1
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Mathematicians, Help me Formulate my tire PSI!

Ok, here's some ideas ive been teetering with for the last few days,
considering the snow me having to let air out of my tires for saferty.. lol

Does anyone know the formula that can calculate tire height vs trip odometer mileage..

because

one thing that can cause your odometer to really rack up the miles is smaller tires(more spins per mile)

so having bigger/taller tires, ect can cause your mpg numbers to go down due to the less spins needed per mile..

yet adding psi will increase your mpg by a certain said amount( this varies by car)
so by adding psi are you not really getting exact mileage unless you've thrown in a math formula to account for higher tire height?

meaning that you might be getting even better mileage(providing you have a control mpg using stock psi/height that matches your car's computer's recognition as what a mile is) than you're odometer says you are..

wouldnt you think on some cars that this unaccounted for error could actually be a 1-2mpg difference, well I would need the formula to even hypothesize this, someone here has to know it...
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Old 12-24-2009, 10:36 AM   #2
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Good question. I've never considered the inflation-induced diameter change to be significant, but I haven't measured it.
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Old 12-24-2009, 11:00 AM   #3
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Originally Posted by spotaneagle View Post
Does anyone know the formula that can calculate tire height vs trip odometer mileage..
i think ive actually seen a formula for this before, but it wouldnt surprise me if you needed to make one for every different car
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Old 12-24-2009, 11:03 AM   #4
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Well, that issue can be done by many gear calculators. Height is only useful for determining circumference. Get the circumference and you have the distance the car will go with one tire revolution. You can feed size, diameter, or revolutions per mile into this gear calculator:
http://f-body.org/gears

Also feed in your diferential and at least one transmission gear and you'll get your answer.
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Old 12-24-2009, 12:05 PM   #5
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Trie diameter doesn't hange much with air pressure. Unlike a ballon, it has steel cords in the tread that don't stertch much.
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Old 12-24-2009, 02:05 PM   #6
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As above and remember too IF you are going to this length you might want to find / build an equation to take the effect of centrifugal forces into account as well.

Personally I think the differences would be so slight they would be lost in background "noise" and effects like road materials and surface finishes would also be an influence.

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Old 12-24-2009, 03:12 PM   #7
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Originally Posted by Sludgy View Post
Trie diameter doesn't hange much with air pressure. Unlike a ballon, it has steel cords in the tread that don't stertch much.
yea.. well an inch on my tires on my car is the difference from 29psi to38psi, I mean how tall the tire is while on the car.. and could potentially mean alot in terms of long term results measuring mpg, to the point where this has its own calculator

all you can be doing is giving your self a higher mpg readout and more accurate.. whats wrong with that?
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Old 12-24-2009, 07:21 PM   #8
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The easiest way to measure this is to use a ruler or tape measure and measure the height of the tire from the ground to the bottom of the rim at it's lowest point. While you're doing that, let out ten psi and compare the change. Use that measurement to calculate effective diameter/circumference. It'll be different for front and rear tires since they carry different ammounts of weight.
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Old 12-26-2009, 09:14 AM   #9
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That doesn't work because the tire CIRCUMFERENCE is NOT changing with different air pressure only the about of SAG in the sidewall on the bottom of the tire. You still travel the exact same distance per revolution no matter what the tire pressure.

Now for some bad news . . . apparently the tire rolling resistance increases with taller tires so putting taller tires may lower the engine RPM and effectively change the final gear ratio and Odometer reading but it also apparently increases rolling resistance too.
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Old 12-26-2009, 05:15 PM   #10
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Where did you get the idea that taller tires increase RR? As far as I can tell, shorter tires increase RR...but if you have some more concrete theory or some actual data, I'd be happy to learn.
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