Speedometer/odometer/sat nav

[WiltshireHam]

This is a question that has been going around in my head for many years, ever since I got a sat nav, but I have never been able to find an answer...

I've noticed that the speed shown on my sat nav is lower than that shown on the speedometer. For instance, driving on a motorway, the speedometer shows 70 mph while the sat nav shows 65 mph. Which one is correct?

Also, with regard to the odometer, am I really travelling 70 miles in an hour? or just 65 miles in that hour?

[adrynalyne]

On 21/09/2025 at 08:29, WiltshireHam said:

This is a question that has been going around in my head for many years, ever since I got a sat nav, but I have never been able to find an answer...

I've noticed that the speed shown on my sat nav is lower than that shown on the speedometer. For instance, driving on a motorway, the speedometer shows 70 mph while the sat nav shows 65 mph. Which one is correct?

Also, with regard to the odometer, am I really travelling 70 miles in an hour? or just 65 miles in that hour?

GPS is probably more accurate. It’s my understanding that there is a 5mph range that traffic enforcement in the US allows due to this but that’s really far off. 
 

Mine if off too but only by about 1mph and I think it’s because Ford and possibly other manufacturers do not calibrate their speedometers for wheel size variants offered and mine are 20 inch wheels while base is 18.