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Old 12-14-2010, 11:38 AM   #1
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Default Speedometer and Odometer relationship/accuracy

Hey all,

Since I've been using a Garmin GPS while driving, I've noticed that my Bonne'* speedometer is faster than my GPS. I've got cruise control set at 70, but the GPS says I'm going 68.

I'm wondering if this affects my odometer reading as well. If the car thinks I'm putting on 70 miles an hour, but I'm really only putting on 68, will the odometer reflect this, or is it a separate computer?

My car has over 120000 miles now, and I'm starting to wonder if this is really accurate.

Thanks for the replies.
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Old 12-14-2010, 11:44 AM   #2
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The odometer will read the same as the speedo. It counts signals from a sensor in the differential. If your tire size is different from stock that will effect the readings. what size tire you have on the car now? A smaller then stock tire will have the speedo fast then your going.
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Old 12-14-2010, 11:49 AM   #3
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Lets assume that it is off, and has been off the entire life of the car. It would be a difference of 3500 miles, and that was rounding a lot of numbers a lot. That will not affect the value of the car enough to bother with. The speedometer'* on these cars are also not renowned for their accuracy. You may wish to take a trip, and if your gps has the ability, set a trip meter on both it and your car, and that will tell you how far off everything is.
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Old 12-14-2010, 11:55 AM   #4
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Another option is one I've used many times, well, before GPS. Get on the interstate and set your cruise at 60. Then when you pass a mile marker start a stopwatch. Drive for at least 3 miles, then check the time. Best to have a friend along to do this. The more miles you drive the more error you will take out of the test.
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Old 12-14-2010, 12:11 PM   #5
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X2 on the tire size making a big difference. It will even make a difference between having a brand new tire and a nearly bald tire. That 1.5 inches of extra diameter will definitely do it.
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Old 12-14-2010, 12:34 PM   #6
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Quote:
Originally Posted by Danthurs View Post
Another option is one I've used many times, well, before GPS. Get on the interstate and set your cruise at 60. Then when you pass a mile marker start a stopwatch. Drive for at least 3 miles, then check the time. Best to have a friend along to do this. The more miles you drive the more error you will take out of the test.
I was thinking the biggest concern was if his odometer is affected by the inaccuracy. This is great for seeing how accurate the speedo is. I don't believe this will tell the accuracy of the odometer however.
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Old 12-14-2010, 12:43 PM   #7
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Yes it will. It'* just a mater of resetting the trip at the mile marker and then driving say 10 miles. when you pass that last mile marker look at the trip.
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Old 12-14-2010, 02:06 PM   #8
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Wow, thanks for the responses guys. I never thought about the tire size, although I believe I am using stock size - wear.

Will definitely test out the mile marker next time I'm on the expressway.
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Old 12-14-2010, 02:13 PM   #9
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I noticed the same on my pickup. Checked some of the forums before this site was all GM and ended up finding out that either GM didn't aim to be as precise or the various tire factors threw us off 2-5mph. I'd think it was in the programming. Most of those folks with tuners adjusted for it.
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Old 12-17-2010, 11:50 AM   #10
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FWIW, your PCM could be functioning perfectly with mathematically correct numbers, but the driver in your instrument cluster could be the cause of inaccuracy.

the way to test for that is to get a scanner, get up to a steady speed and then confirm if it'* reading the same as the GPS. if they match, your cluster is the issue.
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