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Discussion Starter · #1 ·
Anyone notice the faster you go it seems like the speedo gets off more and more? Went by one of those "your speed" showing 58 when we we going 61 according to the digital speedo. Tried a GPS app and it showed 73/74 when we were going 80 on the digital speedo. It seems like the digital speedo is off a bit too when compared to the needle on the car. I know in Europe the car can't understate your actual speed. I would think it should be pretty accurate though.
 

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It's pretty normal that speedos are usually not accurate and things like tyre wear can also make it less accurate.

Speedo from my experience of different cars I have had are usually between 3-5% more than actual (according to GPS) which I guess helps to protect you from speed cameras (which I have read are calibrated for 10% margin of speedo error e.g. 77 mph for 70 mph limit). I wonder if that is why manufacturers scalibrate the speedo to be optimistic for the specified tyre size to protect the buyers from speeding without their knowledge (and being sued afterwards)?

vetsvette - as you changed to 215/45/17 tyres (I have done the same) the diameter is slightly larger than OEM 205/45/17 making it closer to being accurate
 

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vetsvette - as you changed to 215/45/17 tyres (I have done the same) the diameter is slightly larger than OEM 205/45/17 making it closer to being accurate
That's correct. I meant to include my tire specs, but it slipped my mind. By the time I remembered and went back to add the info the "edit window" had closed. I only drove it with the original tires for a week and didn't have my GPS installed to compare to the speedo.
 

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Discussion Starter · #6 ·
I think I will do what you did Vet and when I upgrade from the factory tires I will go with a different ratio to help with the speedo being more accurate. I have under 2K miles on the car so I will probably wait till spring to get performance tires since I have winter tires for it already. The run flats are too noisy for my taste.
 

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Discussion Starter · #8 ·
My car is brand new so I would assume it is already calibrated from when it left the assembly line. I know it is off thanks to the speed warning signs along the road in construction areas and from my GPS.
 

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"GPS speed calculations are more accurate than a vehicle's speedometer since it is not affected by the same inaccuracies, including the vehicle's wheel size or drive ratios. It is dependent however on GPS satellite signal quality but with the use of moving average calculations any errors are minimal."

Dec 29, 2011
 
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