If you live in Europe ALL speedometers (should) read faster than you are
actually going ... it's a legal requirement and explains why the needle
on many speedos start above zero. It also explains why when you say to
the nice policeman "but my speedo only showed xx mph" and he's booking
you for xx+5 he doesn't believe you :-)
In the UK you can check the Construction and Use Regulations, but from
memory current requirements are for the speedo to read <6% high; 0% low.
Having non-standard wheels and tyres can effect the accuracy of the
speedo, of course, depending on the rolling circumference as compared to
the stock sizes.
Steve, think you had better read this bit again .....
"(should) read faster than you are actually going .....but my speedo only
showed xx mph
and he's booking you for xx+5 he doesn't believe you"
In this case you'd actually be doing xx - x, no?
It was late at night when I composed that (well late for me anyway).
What I was trying to say is that when the nice policeman stops you for
doing 35 in a 30 zone and you try to plead that your speedo was only
registering 30 then he isn't going to believe you because your speedo is
designed to read higher than you're actually going.
Does it make sense now?
Yes I understand that. I was trying to emphasise that because the speedo
reads high by design (i.e. to comply with legislation) trying to tell
the police that your speedo was reading less than you were actually
travelling won't be accepted as an excuse because he/she (the police
officer, that is) knows that's not true.
I've got a UK car (not a BMW) where the speedo is accurate.
The requirement is that it shouldn't under-read. From the days of
In these days of pulse counting types, the idea of having a speedo which
is 10% out is ludicrous. 1% high or so when new would cope with tyre wear.
It suits the car makers to have speedos that read high - "my car can
easily do 100 mph" or similar, while the actual top speed might be 90.
*There are 3 kinds of people: those who can count & those who can't.
Dave Plowman email@example.com London SW
Not manure. Check the current construction and use regulations. The
wording is such that to comply fully the speedo has to read over even if
it's by as little as 0.1% - which I would consider to be "accurate". I
also have a non-BMW car in which the speedo is (as far as is measurable
without specialist timing devices) about 2% out between 30 and 70mph. I
haven't checked it outside of this range. I never mentioned 10%.
Germany has a law that requires no speedometer be pessimistic in its speed
reading. Sooo, BMW chooses to insure they are in compliance by installing
optimistic speedometers. Typically 5 mph in the 65-75 mph range.
R / John
It should read faster than you are going. This keeps you from going too
If the signs say 45, and your speedo says 45 but you are really doing 40,
then you will never get a ticket if the speedo never goes above the number
on the sign.
You can calibrate your speedo with a stop watch and your Cruise Control. Set
the Cruise to 80, then measure the time it takes to go a mile. It should
take 45 seconds. The miles are marked on the side of the freeways. If you
watch the right shoulder, you will see signs that count down as you go south
or west. Typically a Call Box will be placed where one of these signs would
belong, but not always. When you see a Call Box, there should be a sign in
one half mile because the signs are normally placed at half-mile intervals.
Do not use the odometer to click the miles off because if the speedo is
having an error, the odometer will have an error too.
You can use the signs to check your odometer (trip meter), and with a stop
watch, you can calibrate your speedo. There are 3600 seconds in an hour, so
if you divide 3600 by the time it takes to go a mile -- 360 / 40 = 90 mph,
3600 / 45 = 80 mph, 3600 / 48 = 75 mph, 3600 / 51.43 = 70 mph, 3600 / 60 60 mph. Alternatively, 3600 / 80 mph = 45 seconds. You can set your Cruise,
and divide 3600 by the setting of the cruise to calculate the time it takes
for the mile markers to whiz by, and use the stop watch to see where the
markers should be.
My speedometer is very nearly perfect between 60 and 90, myj guess is that
it is accurate across the entire range -- but this is speculation. You
didn't say what size tires you are running, but if your tires are too small,
then the speedo will be too fast.
That sounds about right. My odo is also dead nuts on, but my speedo is only
off by about 1.5mph at 80. Over my 40+ mile commute, I think my odo is a
little on the high side, but I'd have to keep a log of the mile markers and
I just don't care that much ...
If the OP's speedo is off by the margin he claims, then he should calibrate
the speedo by the mile markers. If he wants to check the odo, and finds that
it is accurate, then he can calibrate the speedo from it as a mind-game
while he wiles away the hours going to work and back.
On Tue, 14 Jun 2005 16:20:35 -0700, "Jeff Strickland"
I checked mine after I replaced the gears (89 325i) in the speedo
and on a 65 mile trip (by the mile markers) the odo was dead on. I
wouldn't go through the hassle on a short trip either but I figured
what the hell, I've got a straight interstate drive, I'll have a look!
Typically they are 7% high. I would suggest that you calibrate both the
speedo and the speed warning device (in the computer) against the 100m
markers posts on a motorway near you.
If you posted from the USA, then I should tuck a picture of that early
exponent of liberty and freedom (Benjamin Franklin) near your driver's
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