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Old 03-24-2008
chris07XL's Avatar
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Speedo and GPS

My truck came stock with 235/75/15s (approximately a 29x9.5)

I upgraded to a 31x10.5 and the calculator I used on the BFgoodrich site says when travelling at 60mph displayed on the speedo, I'm actually moving at 64mph, Give or take.

I understand that after-market GPS systems aren't at all incredibly accurate, but the GarminNuvi360 I have mounted on the dash tells you the speed you are traveling at and It seems to be the exact same as my speedometer, with the exception that it does not react as fast to the speed changes.

I was just curious what you guys thought about this...

Could it be that my speedo isn't actually that innacurate since it was only a small jump in size...based upon what the GPS is indicating? And that with a larger tire size i would actually see the difference?
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Old 03-24-2008
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The change in speed happens exponentially. If you are travelling at lower speeds the change will be less. At higher speeds it will be more. I know for my truck I am travelling 1.113 times faster than what my speedo displays.

At 10mph that is 11.13mph a difference of 1.13mph.
At 60mph that is 66.78mph a difference of 6.78mph.

So the slower you go the less noticeable it will be.
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Old 03-24-2008
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The stock speedometer with stock tires from the factory reads a few MPH faster than you actually traveling. I've found the difference is usually 2-4mph.

So, by adding those bigger tires, you've essentially removed that factory "error" so your speedometer is showing a more accurate speed. I know it sounds stupid but thats how it is for some reason.
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Old 03-24-2008
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The way a GPS in your car works is. It constantly measures where you are on the globe in relation to several satellites. Usually 3 or 4. (there are 27 total. 24 in use at any given time, plus 3 back ups.) It then triangulates that info to figure out exactly where you are. (usually within a few feet on the good ones.) It then uses that information to calculate things like, how far you traveled (odometer, where you are vs where you were) and how long it took you to get there. (speedometer) The GPS you have has gotten good revues. So I'm assuming it's very accurate. It may also only be doing these calculations every few seconds (5 or more let's say) which would explain the lag in updates. With the tire size diff you state, your trucks speedometer should be about 7.5% slow. So at let's say at 70 mph indicated, your true speed would be 75.25. (70 X .075% = 75.25) The only thing I can think of is there was some problem with you trucks computer in the first place. Wrong tire size or rear axel ratio programmed in at the factory. Best way to find out is check your odometer against the mile markers on the side of most roads. If your speedometer is truly off by 7.5% (I'm guessing it isn't) when you have gone 10 miles, your odometer will read, 9.2 or 9.3. (10 X .075 = .75 miles. 7/10 - 8/10)

WOW: all this math is making me dizzy. Off to the pool to recover and work in my tan, LOL
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Old 03-24-2008
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Quote:
Originally Posted by FireRanger View Post
The stock speedometer with stock tires from the factory reads a few MPH faster than you actually traveling. I've found the difference is usually 2-4mph.

So, by adding those bigger tires, you've essentially removed that factory "error" so your speedometer is showing a more accurate speed. I know it sounds stupid but thats how it is for some reason.
my truck from 27s to 30s made it spot on until 65mph
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Old 03-24-2008
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oh...well a little more math than i was interested in reading but just thought i would bring it up.
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