Speedo reading below GPS Speed

[Russ35]

I was just coming back from watching the TVR racing @ Oulton Park and noticed that my speedo was under reading when compared to my TOM TOM.

Speedo GPS
80 84/83
70 73
60 62
50 51/52
40 41/42
30 30

Not tried less than 30 yet, but from looking at the above I expect to be correct or over reading.

I'd noticed ages ago that 30 matched my old GPS, so I don't think it's my Tom Tom. I'm going to try against my old GPS tomorrow just to check.

Assuming I am under reading has anyone any suggestions where the problem could be.

Russ

[valve bounce66]

mine is the other way around,speedo shows slightly higher than tom tom,believe sat navs are very accurate.every car i have tried it in the car always reads higher.

[Russ35]

valve bounce66 said:

mine is the other way around,speedo shows slightly higher than tom tom,believe sat navs are very accurate.every car i have tried it in the car always reads higher.

That's the way it should be, the speedo should either be correct or over read against true speed. Assuming my GPS reading is correct I'm currently breaking construction and use rules!!

[Gerald-TVR]

yep Speedo over reads - on my daily driver (Rover 75) 8 -10 mph over at worst

[falcemob]

My Land Cruiser reads 7 to 8mph over the sat nav reading at higher speeds, over 50ish. Have you got larger diameter than standard tyres fitted?

[Russ35]

falcemob said:

Have you got larger diameter than standard tyres fitted?

Nope, standard wheels and tyres. I'll double check the tyre size in the morning

[Roy C]

According to my TomTom, my speedo is inaccurate too. It under-reads at low speeds.
40mph on the car is about 45mph on the TomTom, but at 60mph and above the two agree.
It makes average speed camera areas "interesting".

Was that your S parked behind mine at Oulton?

Roy

[Russ35]

Roy C said:

According to my TomTom, my speedo is inaccurate too. It under-reads at low speeds.
40mph on the car is about 45mph on the TomTom, but at 60mph and above the two agree.
It makes average speed camera areas "interesting".

Was that your S parked behind mine at Oulton?

Roy

Yes, it was me behind you. I did keep an eye out for you but you were never around the cars at the same time as me.