Speedometer Accuracy Check

Just bought one of these for the wife's new car as it is easy to get carried away.

http://www.bogaard.com.au/digital-speedometers/about-ds/

The FAQ on speed on steep hills says:

"The slope of the ground (or roads) on which a vehicle is traveling gives no practical difference to either the correct speed or the speed displayed on the HillSpeed HUD 09. Even on a steep slope of 1 in 8 (12.5% slope), the difference between the horizontal distance and the distance measured on the slope is less than 8 metres in each kilometre (0.778%)."


Changing tyre sizes on my 4wd each 1 inch increase gives about 3% difference at the speedo. ie. from 31 inch to 32 inch.
 
updraught said:
Just bought one of these for the wife's new car as it is easy to get carried away.

http://www.bogaard.com.au/digital-speedometers/about-ds/

The FAQ on speed on steep hills says:

"The slope of the ground (or roads) on which a vehicle is traveling gives no practical difference to either the correct speed or the speed displayed on the HillSpeed HUD 09. Even on a steep slope of 1 in 8 (12.5% slope), the difference between the horizontal distance and the distance measured on the slope is less than 8 metres in each kilometre (0.778%)."


Changing tyre sizes on my 4wd each 1 inch increase gives about 3% difference at the speedo. ie. from 31 inch to 32 inch.
Click to expand...

Interesting - since a 3 mph difference at 45 mph represents a 6.6% error and my rough calculations suggested a difference in tire diameters of 2".
 
If you do a search of "tire size calculator" many and varied calculators should come up.

I have 265/70 16 and 265/75 16 (which should be around 31'' and 32'') and for these it is about 3%.
 
Cracker, I did the same thing and for years thought I was only 5 mph off. so at 40 I thought I was going 45. I took the GPS and at 40, the GPS was at 50. all these years when I was driving 45 I was really going 55 (wasn't bad with 3:54 gears). I wrote an article for our club paper encouraging members to do the same thing and get a real handle on how close their speedo was. It was a big surprise to me.
 
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