Strange Question - Speedometer

janninew

Lum wrote: »

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.

Ahh right! Thanks for the explanation, the car hasn't had any work done, just the usual services. Its due its first MOT later this year.

Premier_2

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.

If you can't lay your hands on a satnav, then is there anywhere locally with a roadside speed indicator?



Not the most precise of devices but if you take both cars on the same day it will indicate if there really is any difference or just your preception.

For more accurate speed indication, you can calculate it from the time it takes to pass between two mile markers on a motorway
(If you try to maintain a constant displayed 60mph, it will make the calculation easier for you)

Road_Hog

Lum wrote: »

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.

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

The alternative is to get a friend with a car and a passenger each and a decent clear road at a quiet time and and have the friend drive behind you at 30 mph. You can then stay in contact by mobile phone and if your car pulls away and gives a wildly different reading, you know something is wrong. It's not scientific, because all cars give different readings, but it would give you an indication and if you don't have a GPS, it is a solution.

Wig

The best thing to do is both drive at indicated 30mph on an empty stretch of road, and who ever is going slowest should then speed up to fiond out what indicated speed is when matching the speed of the quicker car.

Rather than pay a garage for a check, which I doubt would be accurate either.

Also instead of using mobile phone you could use walky talkies....if you have them. Not that you really need to talk to the other driver, I mean all youre doing is driving at an agreed 30mph and who ever is slowest then speeds up you don't need to talk to do that.

The RX8 is a lowride car so the driving perception could be affected by the seat position..... Does it still have a wankel engine?

vax2002

Next time you are on the motorway, pull in behind the HGV's, they generally do exactly 56mph.

mgdavid

.. or find a quiet motorway and check your speedo against the mileposts along the hard shoulder. This will improve your mental arithmetic too :-)

TrickyWicky

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.

forgotmyname

Rolling road or a HGV Tacho calibration centre.

Lum

Wig wrote: »

The RX8 is a lowride car so the driving perception could be affected by the seat position..... Does it still have a wankel engine?

It's a 1.3 rotary engine yes. It's a different design to the old 13B from the RX-7 that supposedly doesn't munch through apex seals quite so often.

It still has a tendency to munch apex seals if you don't give the car a good thrashing from time to time, however.

TrickyWicky wrote: »

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.

I was under the impression that the OP wasn't sure. We're just suggesting some better tests before she potentially wastes time and money at a main dealer for a fault that doesn't exist.

casper_g

Lum wrote: »

We're just suggesting some better tests before she potentially wastes time and money at a main dealer for a fault that doesn't exist.

Assuming the OP can confirm the fault exists, shouldn't it be covered under warranty? The car's less than 3 years old (first MOT test due later this year).