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Old 11-12-2011, 10:45 AM   #11 (permalink)
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My car reads 4 -5 mph faster than it is, according to the radar speed alert signs and my Garmin GPS although my odometer reads true. It is my understanding that all cars from the factory are preset to read faster than actual to insure any speeding tickets are not caused by speedo.
Any non stock tire size will effect the speedometer and odometer.

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Old 11-12-2011, 04:36 PM   #12 (permalink)
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...for the numerically-inclined:

MPH = [ 60 / (GA) ][ RPM/rpm ]

...where:

MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile <--get this from your tire dealership
60 = conversion constant, minutes-per-hour
G = transmission GEAR ratio, ie: 1.00:1 <--usually highest gear
A = transmission AXLE ratio, ie: 3.91:1
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Old 11-18-2011, 02:42 AM   #13 (permalink)
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I made my weekly long trip again. Nor. Cal. does have mile markers.For every 10 miles, i gain 4/10ths.
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Old 11-18-2011, 02:20 PM   #14 (permalink)
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At the rate of gaining 4/10ths of a mile for every ten miles your odometer will be off 4,000 miles per 100,000 miles. Your car has less mileage on it then you thought and your mpg is not quite as good as you thought, you should be subtracting 00.4 off of your trip meter before calculating its fuel economy.Assuming the mile marker signs are correct..
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Old 11-18-2011, 03:09 PM   #15 (permalink)
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same as minusing 4 miles per hundred?
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Old 11-18-2011, 03:44 PM   #16 (permalink)
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My speedo is only off by a small amount (Ive calculated a .054 difference between stock tire size and the taller tires I have now) but my odometer seems to be nearly an entire 1/10th off when I use mile markers on the hwy. I use my odometer to roughly figure out my MPG over an entire tank but will prolly look into a more accurate measure soon.
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Old 12-21-2011, 08:22 AM   #17 (permalink)
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I use a GPS over long (50-100 mile) distances.
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Old 12-21-2011, 11:52 AM   #18 (permalink)
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...a tires' revolutions-per-mile (rpm) number are measured at 45 mph test speed and its rated load capacity, but that number varies with changes in speed and/or load.

...for example, below are the speedometer-vs.-actual results for my '70 AAR 'Cuda (S=speedometer; A = actual):

S = 100.0 mph, A = 102.0 mph, or 2.00% high.
S = _90.0 mph, A = _91.5 mph, or 1.67% high.
S = _80.0 mph, A = _81.2 mph, or 1.50% high.
S = _70.0 mph, A = _71.0 mph, or 1.43% high.
S = _60.0 mph, A = _60.5 mph, or 0.83% high.
S = _50.0 mph, A = _50.0 mph, or 0.00% no error.
S = _40.0 mph, A = _40.0 mph, or 0.00% no error.
S = _30.0 mph, A = _29.0 mph, or -3.33% low.

...thus, tire "error" is not static, it gets bigger above & below 45 mph!

Last edited by gone-ot; 12-21-2011 at 12:02 PM..
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Old 01-01-2012, 07:43 AM   #19 (permalink)
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to answer the question....
the gps is accurate.
Everything else is just for looks.
So what if the speedo is off....or the odometer is off.....
If you have a gps you have an accurate measure of distance and speed.
Think about it.
Is there an adjustment option on a gps?
no.
Is there an adjustment option on scangauge (which reads obdII ie: the enternal car numbers) yes.
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Old 01-01-2012, 10:46 AM   #20 (permalink)
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I don't think the speedometers are purposely set a fixed mph high. I think it has more to do with there being a manufacturing tolerance on the accuracy of the speedometers. Instead of being specified accurate +/- 5 %, I think they are accurate with a tolerance of + 10 % and - 0 %, which could also be interpreted as being 5 % high with a tolerance of +/- 5 %. The tolerance keeps costs down by allowing more speedometers to be accepted without costly individual tweaks. So everyone's speedometer will read a little different, though all will probably purposely read high to avoid speeding tickets and lawsuits, as mentioned earlier. Speedometers that read low are rejected and (hopefully) never shipped.

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