Same here
Speedometer accuracy
Thread Starter
Race Director
Joined: Apr 2006
Posts: 12,921
Likes: 777
From: Huntsville AL
Speedometer accuracy
I have checked my other vehicles against my GPS, and they always read 2 or 3 mph high at 60 mph. I have never really checked the accuracy of the GPS, but I always figured the car makers were being a little conservative.
I checked my Corvette the other day a couple of times, and the speed indicated in the HUD was exactly the same as my GPS said, around 60 mph.
I was really surprised. Have any of you verified the accuracy of your speedometer?
I checked my Corvette the other day a couple of times, and the speed indicated in the HUD was exactly the same as my GPS said, around 60 mph.
I was really surprised. Have any of you verified the accuracy of your speedometer?
Burning Brakes
Joined: Nov 2009
Posts: 1,100
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From: 34°0′34″N 95°30′53″W Indian Territory
The same for me - my Garmin GPS and the HUD will have identical readings. What is odd to me is if I set the cruise the DIC will say "Cruise set to 60 mph" (or whatever lingo it uses) but the HUD will be saying 59. If I tap up one mph so that it says "set to 61 mph" the HUD will then say 60 along with my GPS. My other vehicles are typically 2-3 miles off (lower) from the GPS except for my Mazda3 work car - it's bang on but I'm looking at a needle on the speedometer and no digital readout...
Safety Car
Joined: Jan 2009
Posts: 4,681
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From: Mooresville (Race City USA) NC
My speedometer, my cruise control, my Garmin GPS, and roadside police radar speed monitors are always within 1 MPH of full agreement, and I'm sure this is due to rounding-off to the nearest full number.
Advanced
Joined: Aug 2011
Posts: 82
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From: Indiana
I've found that setting my cruise on 60 in the Vette I'll run 2-3 mph faster than my 2005 Grand Prix but if I set the cruise on 76 the Grand Prix runs 2-3 mph faster. I'm guessing it has to do with the speedo gearing maybe? I'll have to have my brother check it out with his patrol car one day....lol
Burning Brakes
Joined: Oct 2003
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From: Resume Speed Tennessee
Team Owner
Joined: Sep 2000
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From: Far NW 'burbs of Chicago
St. Jude Donor '13
My DIC for cruise control, and speedo needle, usually show 1 mph higher than the HUD.
But they are right on with a GPS. Previously I checked it at 30, 60, 90, and 110 mph. Speedo needle was always within 1 mph of GPS. Haven't rechecked recently.
But they are right on with a GPS. Previously I checked it at 30, 60, 90, and 110 mph. Speedo needle was always within 1 mph of GPS. Haven't rechecked recently.
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Drifting
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From: Redmond WA
Melting Slicks
Joined: Feb 2011
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From: Tree City
I have always heard that your speedometer will "lag" when you drive your car. It will- especially on a motorcycle. Because of the uneveness/bumps of the road, the tires will constantly come up off of the road a slight bit causing the wheels to spin freely. On a motorcycle, the tires come up off of the road quite a bit at high speeds. Hence the saying riding down the road on a "pocket of air". Or, at least that is what I have always heard/thought.
Race Director
Joined: Mar 2005
Posts: 10,399
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From: Austin Texas
Any speedometer on any vehicle is only accurate once in the life of the tires.
Tire diameter is the final "gear" in the drivetrain measuring distance/speed and tire diameter decreases as the tires wear.
Tire diameter is the final "gear" in the drivetrain measuring distance/speed and tire diameter decreases as the tires wear.
Team Owner
Joined: Oct 2007
Posts: 26,745
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From: PA
I have always heard that your speedometer will "lag" when you drive your car. It will- especially on a motorcycle. Because of the uneveness/bumps of the road, the tires will constantly come up off of the road a slight bit causing the wheels to spin freely. On a motorcycle, the tires come up off of the road quite a bit at high speeds. Hence the saying riding down the road on a "pocket of air". Or, at least that is what I have always heard/thought.
Its why guys run a speedo-healer so they get actual mph readings. A bike can be off as much as 15-20% at about 150 mph. Most guys saying "I was going 165!" were probably not even at 150 yet. There was a test done on a Z1000 a few years back and the speedo said 150 yet the gun indicates it was going only 133 or 131. I cant remember its been so long.
This.
Thread Starter
Race Director
Joined: Apr 2006
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From: Huntsville AL
If the tire diameter is 25.5", I calculate that 5/32" of wear results in an error of about 1%. Not too bad.
Team Owner
Joined: Oct 2007
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From: PA
Most people dont realize that as a vehicle goes faster and faster the magnification of the error gets bigger and bigger.
Tech Contributor
Joined: Oct 1999
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From: Charlotte, NC (formerly Endicott, NY)
I've found that setting my cruise on 60 in the Vette I'll run 2-3 mph faster than my 2005 Grand Prix but if I set the cruise on 76 the Grand Prix runs 2-3 mph faster. I'm guessing it has to do with the speedo gearing maybe? I'll have to have my brother check it out with his patrol car one day....lol
Bill
Le Mans Master
Joined: Sep 2007
Posts: 5,526
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From: Mechanicsburg Pennsylvania
Close. But it looks like you only calculated it being lost once. You need to double it because you measure outside to outside. Can you please break that statement down into understandable terms for those of us you just lost?
Most people dont realize that as a vehicle goes faster and faster the magnification of the error gets bigger and bigger.
Most people dont realize that as a vehicle goes faster and faster the magnification of the error gets bigger and bigger.
Thread Starter
Race Director
Joined: Apr 2006
Posts: 12,921
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From: Huntsville AL
Not sure what you mean. did you calculate something different?
Team Owner
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From: Southern Utah