Speed calc too high?

[Kegan36604]

I work as a police officer. My TomTom (300) shows my speed to be one mile less than my radar unit shows as my vehicle speed. I?ve checked this under a number of different speeds, dates, weather conditions, etc. FYI, my radar unit is calibrated daily. Any thoughts on this?

 

[tharvey]

the radar is right, as it is calculated by default on the low side i.e. 59.95 mph will read as 59 mph.

Gps is accurate to within 4 meters thus could be plus or minus 13 feet, thus 59.95 mph would proably register as 60 mph.

The gps also has a time delay in its reading and this could also be the reason.

Now it's funny you mention the 1 mph out as I installed exact size and dia. tires and recalculated the speedo chip to be dead on, as calculated on a dyno and verified by radar to the exact speed.

However when I got my 920 it shows exactly 1 mph on the low side which I have convinced myself it is the time delay evey though I used a flat road for over 10 miles to conduct this test.

 

[SShadow]

My XL-S says that I am actually travelling 4-6km/h slower than my dash tells me.

 

[tharvey]

This is quite common for most cars to be reading faster then they are actually travelling. When the car is new and if the tire dia. is exact then they will reader slower as the tires wear down. An average tire has between 11-13/16th tread on them.

Most cars are also calculated on the high side, one manaf. in question is presently being sued over incorrect speedo's and this import car maker had on average between 3-5% speedo error always on the low side.

Thus when you thought you were travelling at 60 mph you were actually travelling between 57-58 mph.

This resulted in your odometer registering approximately 4-6% more mileage then you had actually travelled. Now taking warranty and used car trade in to consideration this amounts up to considerable expense to the owners of their vehicles.

Other then certified police vehicles I do not know of any vehicle where the speedo is exact and the majority if not all, are always on the low side, thus tomtom will read the relative correct speed.

 

[dhn]

This is quite common for most cars to be reading faster then they are actually travelling. When the car is new and if the tire dia. is exact then they will reader slower as the tires wear down. An average tire has between 11-13/16th tread on them.

Most cars are also calculated on the high side, one manaf. in question is presently being sued over incorrect speedo's and this import car maker had on average between 3-5% speedo error always on the low side.

Thus when you thought you were travelling at 60 mph you were actually travelling between 57-58 mph.

This resulted in your odometer registering approximately 4-6% more mileage then you had actually travelled. Now taking warranty and used car trade in to consideration this amounts up to considerable expense to the owners of their vehicles.

Other then certified police vehicles I do not know of any vehicle where the speedo is exact and the majority if not all, are always on the low side, thus tomtom will read the relative correct speed.

deja view ...... all over again!

If you people want to read a (long) comprehensive thread on this very topic discussed recently on a British forum, go here:

http://www.pocketgpsworld.com/modules.php?name=Forums&file=viewtopic&t=63445

 

[tharvey]

deja view ...... all over again!

If you people want to read a (long) comprehensive thread on this very topic discussed recently on a British forum, go here:

http://www.pocketgpsworld.com/modules.php?name=Forums&file=viewtopic&t=63445

Ok

sorry I brought it up, and I thought I was long winded