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Jetsfan8099
Dodge Dakota
JOIN HERE
4/26/2007
07:59:08
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 Subject: Speedometer mathematics
IP: Logged
Message:
I see so many posts about wanting to change the speedometer when you increase your tire size. Here's an estimate...about 10%. Now here's a reason to not modify your speedometer, thus modifying your odometer. (By the way you are legally required to modify your odometer and speedometer when changing to a non-stock tire)
Some basic math on my 2004 Dakota 4X4:
stock tire = 30.6 inches in diameter
new tires = 33.0 inches in diameter
30.6" x 3.14 = 96.084" in circumference = 8 ft
33" x 3.14 - 103.62" in circumference = 8.63 ft
1 mile = 5280ft / 8ft = 660 rotations to go 1 mile.
660 rotations x 8.63ft = 5695.8ft
5695.8 - 5280 = 415.8 ft per mile you will travel farther with 33" tires.
I usually go 240 miles before I fill up my gas tank....thats an additional 99,792 ft = 18.9 miles.
18.9 / 240 = 7.8% or 5695.8 / 5280 = 107.8%
7.8% is the approximate amount of miles you are saving on your odometer by not adjusting it (thus the reason for it being illegal).
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Marv
Dodge Dakota
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5/02/2007
21:04:01
| RE: Speedometer mathematics
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Message:
Go to http://www.dakota-truck.net/TIRECALC/tirecalc.html
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Kowalski
GenIII
5/03/2007
06:19:34
|  RE: Speedometer mathematics
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Message:
"about 10 %"
There is not one answer that will work for everyone - this will vary depending on what your stock size was, and what size you changed to. Each person needs to do calculations for what they did.
Lead, follow, or get out of the way
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Jetsfan8099
Dodge Dakota
JOIN HERE
5/03/2007
20:46:41
| RE: Speedometer mathematics
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Message:
Kowalski - Obviously you are correct, but us also depends on what speed you are doing. 10% seems to be an easy number for calculating while your driving. My guess is that not to many folks can figure out 7.5% of 60mph while driving...10% would quickly tell you to add 6mph to your speed and it'll be a safe estimate.
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