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rjscooke3 wrote: |
This is all subjective i believe for several reasons on unless you have your car speed tested at a recognized testing facility you would not know if it reads right or not. Secondly I doesn't ever remember reading any data from Garmin as to the accuracy of there units, nor do i ever expect too.
Regards Russell |
bjujitsu wrote: |
Speedos have to read slightly faster than the vehicles actual speed to eliminate the chance of speeding imagine the outcry if people where fined for speeding due to the manufactured speedo reading being slow.
Also motor manufactures like the fact that they have to make the speedo read faster because it makes the fuel consumption look better, service intervals are shorter, it gives the impression that everything lasts longer in terms of mileage, and lots of people change their cars when it has done a certain amount of miles... |
schmidklaus wrote: |
I have the same speed discrepancy between my in-car speedometer and the GPS. My GPS reads a bit higher.
Here is the explanation: The car's speedometer is geared from the car's road wheels and most people don't realize what this means. The speedometer installed in your car is designed for the specific tires that the vehicle is originally equipped with. The circumference of the tires affect the speedometer calibration. If you replace/change the tires for a different brand and/or size - I wanted a wider tire for better wet and winter traction - the ratio of road distance travelled to wheel rotation may change slightly. When I wanted to put a better tire on my mini-van, I sat down with my tire dealer and the tire manufacturer's catalogue/specifications and compared the tire diameters. Of the tire series I wanted to put on, there were none whose diameter EXACTLY matched the diameter of the original equipment tires. I replaced P215/65R17 tires with P225/60R17 and ended up with a tire diameter 0.4 inches (1 cm) smaller. This results in a speedometer reading 1.5% lower. The GPS, since its not based on mechanical wheel rotation, may be more accurate than the speedometer - I certainly treat it as such. |
steven.chaffer3 wrote: |
My Garmin Nuvi 1390t consistenlty shows my speed to be 5% lower than the speed the car's speedo is showing.
At 100 km/hr on the car's speedo the 1390t is showing 95 km/hr. Several times when this is happening I have checked the satelite screen to see the accuracy which has always been at 3 metres. By my reckoning, a 3 metre error in a km (1000 m) is 0.3% which should amount to a 'error' of 0.3 km/hr at 100 km/hr, effectively zero. I can't believe my car's speedo is 5 km/hr per out. (However I will check it next chance I get.) Any ideas about what is happening? Cheers Steven |
johmazz2006 wrote: |
Thought the GPS used ALL available satelites, so shouldn't make much difference whether you go up or down hills, recon it would average out. |
nimadude wrote: |
You know your car speedo could be showing more than the real amount.. I know thats the case with my car - every time I pass one of those cameras that tell you the speed ur going (they dont take picture, just warn you), it shows my speed 5-10km/h lower than what my speedometer shows.. how old is ur car? |
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