What's more accurate, my Speedo or Tomtom

[beerman]

Was on a long drive yesterday and noticed that no matter what speed I was doing, Tomtom always reported 3-4MPH less than my speedo.

I was wondering whichh one is more accurate, my speedo in my car or Tomtom. I know speedos aren't that accurate but should I trust tomtom?

[reverend]

Anything using GPS is usually more accurate than the speedo as the manufacturers of the cars tend to set speedos at +5-10% higher - that way you can't them with a lawsuit if the speedo says you're doing 40 and you're actually doing say 45mph and get flashed....

Speedos do tend to lose it even more when you hit higher speeds too - plus things like aftermarket alloys and tyre combos with a different rolling radius can also cause even more problems with a cars speedo.

[beerman]

cheers for the reply, it was more at higher speeds I noticed it. No aftermarket alloys on my car so probably ford trying to fool me into thinking my car is faster than it is.

[psibob84]

By law a car speedo can overread the speed by upto 10% but it can not underead it at all

[harryb]

interesting stuff, i thought the speedo would have been better, proved me wrong!

[maltloaf]

my old mondeo used to show almost 90 on the clock when my gps displayed 73. I drove thru all manner of speed traps at this speed without a single conviction. I went thru one (didnt see it) at 86 on the gps and got a NIP stating the speed to be 86.

What happens with car speedos is they tend not to be linear, but the manufacturers make the dials linear for aesthetics. They have a sweet spot, usually around 50-60 where they are spot on, and are out either side of this and worse the further away from the sweet spot.

[ant007]

Used my tomtom in the US earlier this year with two hire cars. Both chryslers and both speedos agreed with tomtom at speeds of 55, 65, 70 and 75.