It's been a while (C5 Adventure)

[Mandrake]

OK the tyres fitted to the car when I bought it were 195/65R15 the current wheels are 205/50 17

http://www.tyresizecalculator.com/tyre- ... dimensions

For the first tyre, rolling circumference 1994mm and 502 revolutions per km, for the second one 2000mm, with 500 revolutions per km. That's a 0.4% difference which is completely negligible, as up to a few percent is allowable.

[Hell Razor5543]

But we do not yet know if these are correct for the car itself. I went down (when elma asked) and checked my C5, and the tyres match what the label says. That gives my tyres a circumference of 2019mm.

[Mandrake]

But we do not yet know if these are correct for the car itself. I went down (when elma asked) and checked my C5, and the tyres match what the label says. That gives mt tyres a circumference of 2019mm.

Well, what tyre size does it say on the door card, Steve ?

[DHallworth]

The tyre size on the V6 I had was 215/55/16 which gives them a rolling circumference of 2019mm, the same as James's. I'd guess that all C5 wheels and tyres would give the same rolling circumference (different sized wheels with different sized tyres give the same total I'd of thought).

Hope that helps!

David.

[elma]

Yes, to do the maths and compare to see if the speedo would be affected the oem tyre size (as on the door label) is needed.

[Trainman]

I'll have a look later, i'm at work now.

[Trainman]

sticker says 205/65 R15 94H

[Hell Razor5543]

And do the tyres match?

[myglaren]

sticker says 205/65 R15 94H

IMG_0172.JPG

That is the same as mine and my speedo is very accurate.

[elma]

Your speedometer error is unlikely to be down to tyres then. You are -1.7% from OEM and your speedometer should over read by about 1 mph all the way up to 100mph.

[Trainman]

Hmmm....... OK, I wonder then if it could be the Satnav that's out? I'm using one from my phone on the odd occasion I need one (and before you ask it's one called WAZE). I'm sure I've seen something about a speed checker somewhere in the apps.......... looks like something to play with this weekend.

Thanks for everything guys

[Mandrake]

I can't see any way that the satnav could be out, but you could try another one just to be sure.

[elma]

Steve the best way to check is to hire a laser speed gun, they cost bugger all to hire these days and are as accurate as you'll get.
Satnav isn't as good at working out speed as people think. It's only accurate in a straight line, on an accurate map and if it has good signal, I find all 3 of mine have good days and bad days as far as accuracy is concerned.
The other thing to do is drive along with a friend with a known good speedo at a set speed and speed match them.

[Mandrake]

Satnav is very accurate if you use it properly - drive on a clear road without any tall buildings or trees around with a good view of the sky, drive in a straight line and drive at a constant speed. Give it a few seconds for the reading to settle, then it will be accurate within 1mph easily.

Speeding up or slowing down, cornering, driving between buildings etc will all cause temporary aberrations in the reading. I would trust satnav used properly over comparing it with another car.

[Trainman]

I've just downloaded a speed app to my phone, I'm heading up the M6 shortly so hopefully I can get a reading along there............