Strange Question - Speedometer

janninew

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.

Mojisola

Have you compared the speedometer with a sat nav reading?

TonyMMM

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.

janninew

Mojisola wrote: »

Have you compared the speedometer with a sat nav reading?

Nope, I don't have a sat nav! I could borrow one and check, I didn't realise they told you your speed.

janninew

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.

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?

Lum

GPS devices (satnav and mobile phone) can be accurate provided you have a decent view of the sky, and are driving on a straight, flat road at a constant speed.

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.

Bald tyres will cause a slight increase in the speed reading, compared to brand new ones.

colino

Apart from the quick satnav check, has the diff been change at some point? Lots of these are used as drifters which can knacker the diff, and its been changed for the wrong one, or perhaps some previous owner preferred faster acceleration over top speed?

Strider590

Different cars feel different enough to change your perception of speed.
My Westfield feels like it's doing 80mph when it's really doing 50mph.

Engine noise is another one and I know the RX8 uses a rotary engine, fast cars can feel slow if you are hearing something unfamiliar under the hood.

janninew

colino wrote: »

Apart from the quick satnav check, has the diff been change at some point? Lots of these are used as drifters which can knacker the diff, and its been changed for the wrong one, or perhaps some previous owner preferred faster acceleration over top speed?

Sorry I have no idea what you mean when you ask if the diff has been changed?! Also I've had the car since new so no previous owners.

Lum

He means the differential, also known as the "final drive". You'd know if it had been changed due to the gigantic hole in your bank account.

janninew

Strider590 wrote: »

Different cars feel different enough to change your perception of speed.
My Westfield feels like it's doing 80mph when it's really doing 50mph.

Engine noise is another one and I know the RX8 uses a rotary engine, fast cars can feel slow if you are hearing something unfamiliar under the hood.

I agree about different cars change your perception of speed, my brother has a new polo (1.4) so I should imagine my RX8 feels much quicker to him.

Think the best thing to do is to take it to a garage for a check.