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Old 02-12-2011
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After i lifted and regeared, i changed my tone ring to 198 and my tires to 635 and its dead on to my GPS
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Old 02-12-2011
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Originally Posted by woofer700 View Post
Ever put a GPS, in your truck? mine reads 2.5 to 3 mph fast, with stock size tires. after i put on the 33 inch tires for a 1 day fun trip, it read .5 to 1 mph slow, compared to the GPS. Our speedos read fast anyway, so you think your doing better than you are. not sure why manufactors do this, but its fairly common across the board.
Maybe so it looks like we put more miles on our trucks more quickly so the warranty goes away quicker? Probly good for the MPG numbers they like to lie about.
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Old 02-12-2011
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Originally Posted by Toreador4x4 View Post
ALSO if you went from a 7.5 to an 8.8, the 8.8 has 108 teeth on the tone ring and a 7.5 has 102 teeth. this will increase your speedometer by 5.8% . now you swapped from 28.5 to 31" tall tires, this makes your speedometer read 8.7% slower (given that your 31's are actually 31" in diameter which im sure they arent) so overall your speedometer reads 2.9% slower than it did before you did anything. at 60 MPH ground speed, thats only 1.74MPH difference
^^ Now that is interesting. Surprised Bob didn't mention it. That might explain it to. Since I went from a 7.5 to an 8.8 that is what messed with my speedo before I did tires. I thought it was the gears but the tone right was what made the difference. I do run a GPS on log trips but I haven't checked since I put on bigger tires. After the 8.8 swap when I was going 40 according to the speed I was actually going 35 as per GPS.
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Old 02-12-2011
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Originally Posted by Toreador4x4 View Post
ALSO if you went from a 7.5 to an 8.8, the 8.8 has 108 teeth on the tone ring and a 7.5 has 102 teeth. this will increase your speedometer by 5.8% .
Exactly right and thanks for reminding me, Justin. I remember seeing that choice option in the NGS setup. So, if a 98~00 8.8 axle gets regeared or a 98~00 7.5 axle gets regeared, the speedometer calibration would not affected. But if the entire axle assembly in a 98~00 is swapped from 7.5 to 8.8 or vice versa, the tone ring difference must be taken into account.
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Old 02-13-2011
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I went from 235/75R15's to 31s and before I went approx. 425KM before filling up, now I got roughly 350KM. I didn't change anything in the speed or odometer systems except move the needle on my speedo when I went to an Expo gauge cluster. Before I changed clusters at 100KM/H my speedo showed 105KM/H so that is a 5% increase.
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