"Graeme" wrote in message
...

Interesting numbers, how old is your car? My two year old Laguna measures
a
lot closer to the limits than that, 73mph indicated gives just about 70 on
the Sat Nav.

It's probably less to do with the age of the car and a lot more to do with
what wheels/tyres are fitted vs the range of options for the car. My car
(also two years old) has options for 17", 18" or 19" wheels. I have the the
17" wheels and the speedo reads about 10% over actual speed. If I were to
fit 18 or 19" wheels the speedo would still read more than actual speed, but
by a lot less as the rolling radius of wheel/tyre combination gets larger.
The car it replaced was 10 years old and it had (for all intents and
purposes) an identical arrangement.

--
DAS

"David A Stocks" gurgled happily, sounding much like
they were saying:

My car (also two years old) has options for 17", 18" or 19" wheels. I
have the the 17" wheels and the speedo reads about 10% over actual
speed. If I were to fit 18 or 19" wheels the speedo would still read
more than actual speed, but by a lot less as the rolling radius of
wheel/tyre combination gets larger.

I strongly suspect you'll find that there's very little difference in
rolling radius between the standard-fit tyre sizes. As the rim diameter
goes up, so the tyre sidewall profile comes down, keeping the overall
size around the same.

If there is a marked difference between, then cars with the different
tyre sizes either factory or dealer-fitted should have the speedo and odo
recalibrated.

Adrian gurgled happily, sounding much like they
were saying:

If there is a marked difference between, then cars with the different
tyre sizes either factory or dealer-fitted should have the speedo and
odo recalibrated.

bad form, etc
....which is merely an option in the dealer's software on modern cars.

"Adrian" wrote in message

"David A Stocks" gurgled happily, sounding much
like they were saying:

My car (also two years old) has options for 17", 18" or 19" wheels. I
have the the 17" wheels and the speedo reads about 10% over actual
speed. If I were to fit 18 or 19" wheels the speedo would still read
more than actual speed, but by a lot less as the rolling radius of
wheel/tyre combination gets larger.

I strongly suspect you'll find that there's very little difference in
rolling radius between the standard-fit tyre sizes. As the rim
diameter goes up, so the tyre sidewall profile comes down, keeping
the overall size around the same.

If there is a marked difference between, then cars with the different
tyre sizes either factory or dealer-fitted should have the speedo and
odo recalibrated.

Yes, I'm sure that's correct. However, the large wheel/low profile tyre
combo rolling radius is less likely to be affected by air pressure
variability.