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spl_kota
Dodge Dakota
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7/29/2007
12:11:17
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 Subject: Speedometer difference w/ new wheels?
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Message:
I put 255/50/17 inch RT wheels on my truck which came with 215/75/15 inch tires so I was just wondering how much difference there is between the two and approximately how much that difference would throw off my speedometer? Thanks for any info.
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daddio
Dodge Dakota
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7/29/2007
12:23:42
|  RE: Speedometer difference w/ new wheels?
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Message:
2.4% too fast. if your speedo reads 60 you're acutally going 58.6.
http://www.miata.net/garage/tirecalcold.html
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dakota49
Dodge Dakota
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7/29/2007
18:39:26
| RE: Speedometer difference w/ new wheels?
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Message:
spl_kota you didn't say what year it is, but if its newer than 91 you may be able to have the speedo recal. by the dealer.
Good Luck,
Dakota49
88 3.8 5 speed 4X4 16000 miles on reman engine.
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dakota49
Dodge Dakota
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8/01/2007
23:19:32
| RE: Speedometer difference w/ new wheels?
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Message:
spl_kota, here is a formula to figure out just where you are for tires and gear ratio's.
First measure the diamater, then times that by 3.1417 that will give you the circumfrence in inches. Then devide 63360 by the cir. and that will give you the rev's per mile. (of the tire) then, you can use any speed, just I will use 65 as an example.
say you have a 28" tire, so 28X3.1417=95.676
63360 devided by 95.676=662.2 (rev's per mile)
then 65X trans high gear ratio or skip it if you have a direct drive trans. Anyway
65X1Xrear gear ratio3:55X 662.2=152802.65
deived that by 60=2546.7 (engine RPM) then measure your other tire to find out how much difference you have between the two. You can use any speed, and if you don't know what your over drive ratio is, just use 1 (for 3rd gear in an auto, or 4th with a 5 speed.)
I know it sounds hard, but try it a few times and write it out and it gets really easy.
Good Luck,I used 3:55 because that's what my Dak has, just use what ever ratio your truck has, or you can figure out what a gear change can do as well as tire changes.
Dakota49
88 3.9 4X4 5 speed
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