730 and car speedometer

[NicolasG]

Hi,
I have a small concern about the 730. When I'm traveling at 50 km/h (on the car speedometer) my 730 display 48 km/h which is only a 2 km/h difference, not a big problem but when the speed is higher 110 km/h the 730 display 103-104 km/h.

Am I the only one with this problem ?

thanks

 

[dhn]

Hi,
I have a small concern about the 730. When I'm traveling at 50 km/h (on the car speedometer) my 730 display 48 km/h which is only a 2 km/h difference, not a big problem but when the speed is higher 110 km/h the 730 display 103-104 km/h.

Am I the only one with this problem ?

thanks

Without showing the link here (cuz I forget where it is ), trust me when I say that what shows on the GPS will be accurate, NOT what shows on the speedometer.

 

[GAW]

Your TomTom uses changes in position vs. time to calculate your speed. While it may not be that precise, several units I have used has had the speed reasonably close.

When I have had speed discrepancies, it has in the past been a function ot tire size versus original equipment tire size. If you have replaced your tires with a wider cross section profile, it could have a smaller radius by enough to result in your car's speedometer indicating a higher speed that you are actually going.

Even original equipment tires can be off if the car manufacturer used different brands on the car. You can go to a number of websites that give specs for specific tires, and tires from different manufacturers of the same rim size, width, and aspect ration can vary by 5-7 % in radius, which would account for the difference you are seeing.

 

[NicolasG]

Your TomTom uses changes in position vs. time to calculate your speed. While it may not be that precise, several units I have used has had the speed reasonably close.

When I have had speed discrepancies, it has in the past been a function ot tire size versus original equipment tire size. If you have replaced your tires with a wider cross section profile, it could have a smaller radius by enough to result in your car's speedometer indicating a higher speed that you are actually going.

Even original equipment tires can be off if the car manufacturer used different brands on the car. You can go to a number of websites that give specs for specific tires, and tires from different manufacturers of the same rim size, width, and aspect ration can vary by 5-7 % in radius, which would account for the difference you are seeing.

I have changed the tire but it's only the model (goodyear to Nexen) same specs. It's not a real problem but only a thing I notice.

Thanks for the help.

 

[GAW]

Again, with the same specs, actual dimensions between brands vary enough to throw it off.

Well, by indicating a faster speed, maybe it will help protect you from the "bear" (and automated speed traps also).....

 

[kbarter]

I had a similar question when my old Windstar speedometer would indicate speeds of 120 km/h while the TomTom indicated speed was 115 km/h.

Turns out it was the speedometer in the Windstar that was incorrect. The TomTom was accurate.

 

[nicholb]

Also another way to tell if it is tire related is that the difference is larger at higher speeds. Each tire rotation adds a little error. The faster you go the more tire rotations per second and the larger the error.

 

[Steve Boultbee]

The following is a good article from Car & Driver about speedometer accuracy:
http://www.caranddriver.com/reviews/hot_lists/high_performance/features_classic_cars/speedometer_scandal_feature/(page)/1

Basically, they're not as accurate as you might think, and different brands have more accurate speedos than others.

 

[NicolasG]

Thanks for the help.
I'll still use my car speedometer for my actual driving speed, but I'll look a my TomTom to know my "real" speed.

 

[ckacyon]

Car speedometers typically read a bit high. In fact they cannot read low, it's the law. GPS is accepted as one of the most accurate measures of speed, also.