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My tires and speed...


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Okay, need a little clarification...OEM tire size lists at:245/75/R16 on my b pillar stickerI have 285/75/R16's on the truck and have had them since I got the truck.I went and did it on a calcu-mutilator and it says I go 55.6 mph with my current size compared to going 60 with OEM size.I cannot remember how this correlates to my actual speed and what the speedo says. And believe it or not, 4 areas I go through list all my speeds differently when I pass their speed sign/radars.I get them either saying I'm going 7 over or some say I'am going actual speed or going 3-5 over.Its confusing!IIRC, this should mean I'm going 60mph when my speedo says I'm going 55-56mph, right?

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You have to go by RPM's to really check, when the TC is locked up. So in overdrive if you have 3.55 gears at 1600RPM, the stock tires will be at 59.2mph, but you will really be going 63.8 because the nonstock 285's are bigger.In other words, if the speedo says 59 at 1600RPM, then you know its set up for those other tires. If it says 63.8 at 1600, its calibrated for the bigger tires.

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Most of the side-of-the-road "Your Speed is.." signs are not correctly calibrated, but give a good generalization..

Like ISX said, a lockup rpm with final drive and tire diameter calculation is best, but generally, a good GPS will also show you a variance. Again, it depends on the quality/accuracy of the GPS device.

Tire Size Comparison

[B][U]Specification[/U]  [U]Sidewall[/U]     [U]Radius[/U]    [U]Dia.[/U]     [U]Circ.[/U]      [U]Revs/Mile[/U]  [U]Difference[/U] [/B]245/75-16      7.2in        15.2in    30.5in    95.7in    662        0.0% 285/75-16      8.4in        16.4in    32.8in    103.1in   614        7.8%
7.8% @ 60mph (via speedo) is 64.7mph (actual)

However, remember that this is not linear on the speedometer (i.e. @ 70mph speedo, you're doing 75.5mph actual. 35pmh = 37.8, etc.. the 7.8% is an approximation of tire size. actual is determined by going YOUR percentages..

Say, a new 245/75-16 has a circum. of 95.7". And say your 285/75s have a circ. of 103.1..

big # minus small # = difference

difference divided by small # equals percentage in decimal. Move dec. point two places to the right = percentage

103.1 - 95.7 = 7.4/95.7 = .077, or 7.8%

60mph + 7.8% = 4.68 (or 4.7) totals 64.7

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Not sure if my '97 is at all unique in this regard, but my odometer is 1% off by GPS over 100 miles, but the speedometer is off by 5-6% (both read high, so the speed is about 5% "faster" than the odo)Something to consider when judging accuracy. not sure which is aimed to be more accurate with factory calibration.

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my odometer differs from my speedo as well. even so, my OCH mileage differs from my odometer, as well, by roughly .5mi over 250mi on the odometer. (odo this weekend read 250mi, OCH showed 250.5mi.)

Same here the ScanGauge II differs from my cluster speedometer and odometer...
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