Strange Question - Speedometer

Imp

Road_Hog wrote: »

What he/she said, that is the only way to get an accurate GPS measurement and even then it can be flaky.

GPS is wonderfully accurate at determining speed, it does it with far greater accuracy than it determines position. When I was looking a chips for an embedded GPS application, they were all providing instantaneous speed to within +-0.1m/s. This is calculated directly from the satelite signals and is not derived from the distance travelled in a set time.

The disadvantage of GPS speed readings is that they are instantaneous. They update at set intervals, typically every 1 second. Just less than one second later they are still reading the same value. When accelerating or decelerating, this does cause errors in the readings, but under normal driving I doubt the error is any worse than the hysterisis in a normal speedometer.

red_eye

janninew wrote: »

Quick question.

Is there any way of checking if a car's speedometer is correct?

I have a Mazda RX8 and after my Brother drove it for the first time over the weekend he told me that the speedometer doesn't seem to be very accurate. He said when he was driving at 30mph the car was travelling much quicker!

After he said this I've noticed it to. I have another car and you can really feel the difference in speed when they both say 30mph!

Its a 2009 reg so not old and has only done 10,000 miles.

Any ideas?

Thanks.

placebo effect, the seats in the rx8 is closer to the ground, the suspension is firmer and stiffer tyre side walls all add up and this gives the illusion of travelling faster. one way to tell is using satnav and comparing the figures

red_eye

TonyMMM wrote: »

Have the wheels/tyres been changed for a different size ?

Take it to a garage with a rolling road (e.g. an MOT tester) and they can check it for you. Legally a speedo can read high, typically they are over by about 10%, so it says 33 when you are actually doing 30, but cannot read under the true speed.

mot regulation rollers cant calulate the speed as the only used for testing brakes.

red_eye

janninew wrote: »

I've had it from new and they wheels or tyres haven't been changes.

My brother has this app on his IPhone and he said it was reading at 39mph when the car said it was 30mph - I'm not sure that an IPhone app can be considered that exact though!

Surely I would have been done for speeding though as I travel through a lot of speed camera's every day and I'm not one of these people who drop down to 20mph when I pass a camera in a 30 zone!

The garage idea seems the best, will do this tomorrow.

If the speedometer is broken or not accurate - is it a simple job to fix it?

the speed is taken by the abs sensor which I read by the ECU. last time I was looking for an ecu it was about £400

pendulum

Lum wrote: »

Speedos are allowed to overread by 10% + 6 mph, so up to 39 for 30mph or up to 83 for 70mph. They are not allowed to underread.

Where do you get the 10% + 6mph from? I have always been under the impression it is up to 10% maximum.

red_eye

TrickyWicky wrote: »

The only real way to test a cars speedo is on a rolling road. I've yet to find a garage that can actually test a speedo as I had a problem with one a few years back that kept sticking and I had a hell of a job finding anyone that could sort it out for me.

Take the car to the dealer, get them to deal with it. Ignore those telling you to follow others and compare speed readings etc, you've already confirmed you know its not right. What you need is to get it fixed. Go to the dealers, tell them there is a fault and let them sort it.

can not be confirmed until tested. I can say that your not right in the head, I may be wrong or right no way to 'confirm' with out getting you tested:p

kevin0410

what a load of waffle in this thread.
The OP has already said that gps software has confirmed the speedo is grossly under-reading.
The next step is garage / dealer

red_eye

a friends iphone app is not rally confirmation we dont know what app and if the app is calculating correctly. a satnav or being following by a police car on the other hand.

shaun_from_Africa

pendulum wrote: »

Where do you get the 10% + 6mph from? I have always been under the impression it is up to 10% maximum.

It's found in the Motor Vehicles approval regulations. (the actual figure is 10% + 6.25mph)
http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

19. Speedometers
For all true speeds of between 25 mph and 70 mph (or the maximum speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V/10 + 6.25 mph
where V = the true speed of the vehicle in mph

.

anotherbaldrick

shaun_from_Africa wrote: »

It's found in the Motor Vehicles approval regulations. (the actual figure is 10% + 6.25mph)
http://www.legislation.gov.uk/uksi/2001/25/schedule/3/made

.

From the above the permissable error at 30 mph is from 39.25 to 30 mph on the dial ( instrument must not read less than the actual road speed)
At 60 mph it amounts from 72.25 to 60mph.