Yeah we were using TomTom in a '93 merc c-class and a brand new rented Passat and both speedos overread by 5mph constantly according to the GPS. Makes me wonder id speedos are made to over-read to compensate for people creepping just over the limit. Or then again it could just be under-inflated tyres.
Yeah we were using TomTom in a '93 merc c-class and a brand new rented Passat and both speedos overread by 5mph constantly according to the GPS. Makes me wonder id speedos are made to over-read to compensate for people creepping just over the limit.
as both of your cars are german cars, he the solution:
In germany it is illegal to use a car with a speedo displaying even 0.001 km/h less than you really drive, but it is legal that the speedo shows up to 5% higher figures.
So the carmanufacterer design their speedos to display higher results.
i never drink water cos fish ***** in it. (w.c. fields)
I thought it is illegal for the speedo to under read in most countries!
I also heard that if your car is factory standard you can take legal action against a car manufacturer if you get pulled speeding and your speedo says you weren't. That is once the police have checked that it is in error! Hence why they over read.
But you'd be amazed how much of an affect the tread depth has on speedo accuracy...
Now I don't need this picking to death but just quick and rough here's why...
Based on a tread depth of 12mm a 205/55 R16 tyre's OD can change by about an inch from new to worn, that would make a difference of 3.14" (about 80mm) per wheel revolution.
OD = 613.9mm
Circumference = 1984.1mm
60mph = 96km/h
At 96km/h wheels turn @ 806rpm = 48360rph
so in an hour the distance you've travelled can vary by...
48360rph x 0.080m (lets say 80mm is the amount the circumference can legally vary by)
so call that +/- 4km/h (2.5mph) variation, and thats just tyre wear!!!
There will also probably be at least a 2% tolerance in tyre diameter when they are manufactured. I'll bet that gives you the rest of the error.