accual mph
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From: New London, Mo
accual mph
i have an 87 f250 with a c6 and 3.73s it has the white speedometer gear. it origonally had 265s on it and now it has 36s on it. how do i figure out how far the speedometer is off do to the indrease in tire size.
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Joined: Dec 2007
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From: Colorado Springs, CO
tire size x .03937 = inches wide. (example 265 x .03937 = 10.433")
inches wide x aspect ratio (ex 10.433 x .65 = 6.78")
product x 2 = tire from rim to ground (ex 6.78 x 2 = 13.56")
product + rim size = diameter (ex 13.56" + 16" = 29.56")
so, your 36" tires are 6.44" greater diameter than stock (in example)
29.56" x 3.1415 = 92.86" circumference
36" x 3.1415 = 113.1" circ.
21.8% off of stock tires, speedo will be 21.8% off (too slow), so at an indicated 60mph you are actually going 73mph.
If my math is wrong, somebody will be along shortly to correct it!
inches wide x aspect ratio (ex 10.433 x .65 = 6.78")
product x 2 = tire from rim to ground (ex 6.78 x 2 = 13.56")
product + rim size = diameter (ex 13.56" + 16" = 29.56")
so, your 36" tires are 6.44" greater diameter than stock (in example)
29.56" x 3.1415 = 92.86" circumference
36" x 3.1415 = 113.1" circ.
21.8% off of stock tires, speedo will be 21.8% off (too slow), so at an indicated 60mph you are actually going 73mph.
If my math is wrong, somebody will be along shortly to correct it!
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From: dryden,michigan
i dont know why but mine opposite then yours i gota do 65 to do 60 i dont know why this is stupid but i have 285 75 16 is there a way to get mine to be right?? seems like somone maybe had big tires on my truck got it adjusted and now i have smaller then what they had???
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From: Nutter Fort, WV
http://www.transmissioncenter.net/sp...n_______va.htm
They have a lot of speedo gears here and a few calculators to help make the correct selection.
They have a lot of speedo gears here and a few calculators to help make the correct selection.
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what i did was look at mile markers... they arent always accurate BUT if you notice that you get say 9/10's of a mile on your odometer per mile marker you know your off by 1/10th roughly so on a 200 mile trip you actually driven 220 miles...i did it becuz ive always been a MPG freak so i know if something is wrong if im losing MPG's
Gps Solution
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From: Nutter Fort, WV
Vicpo,
Welcome to FTE and the IDI diesel forum.
In the above example where you read 9/10 of a mile per mile, the speedometer was off by 10%.
So when the speedometer was showing 50 MPH, you were actually going 55 MPH.
When the odometer showed 100 miles, multiply by 1.1 for an actual 110 mile trip.
Before you can use a formula you have to know how much it is off.
If you have GPS, the speedo is showing 60 MPH and the GPS is showing 66 MPH, divide actual GPS by indicated speed.
66/60=1.1
So the speedometer is off 10%.
Welcome to FTE and the IDI diesel forum.
In the above example where you read 9/10 of a mile per mile, the speedometer was off by 10%.
So when the speedometer was showing 50 MPH, you were actually going 55 MPH.
When the odometer showed 100 miles, multiply by 1.1 for an actual 110 mile trip.
Before you can use a formula you have to know how much it is off.
If you have GPS, the speedo is showing 60 MPH and the GPS is showing 66 MPH, divide actual GPS by indicated speed.
66/60=1.1
So the speedometer is off 10%.
Last edited by Dave Sponaugle; Dec 24, 2007 at 03:12 PM.
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