Speedo Out

[ravenhard]

Well I was driving down to Canberra on the weekend and I passed those speedo check signs on the side of the road. The ones that start at 0km and go to 5kms. I was about 80meters less as each kilometer passed as a rough guess. Done the test 3 times and was roughly the same each time.

I changed my rims from the stock 17's with 235/45/17 to BA GT 18's with 245/40/18 but I wouldnt have though that would have made a difference.

So how much is my speedo out? When Im doing 110km/h what is my real speed?

I dont have a GPS or access to one either so cant check that way

[Maximus]

It all depends on the rolling diameter of the tyres.

Bald 18's might have a smaller rolling diameter than new 17's.

Different brands have different diameters so its too hard to work out accurately.

Just drive past a speed camera 10k's over the limit and see what speed they pick you up :D

Then you can see what the error is.

[Maximus]

Have a look here for what I mean.

http://www.toyo.com.au/PCRPDFs/Proxes%204.pdf

It is specific for TOYO's but get a tape measure out on your wheels and you could work it out to suit what tyres you have on the 18's.

[TURBO4LT]

So when you get to the 1km mark are you reading 920m or 1080m?

[ravenhard]

its reading 920

[TURBO4LT]

I'd say 119.5km/h when your speedo reads 110km/h provided your speedo is out by the same amount as your odometer.

[seventytwo]

Try this clicky

[winstor]

235/45//17 -> 245/40/18

Difference in circumference - 31.11mm (1.54%)

So when your speedo reads 100 km/h, you're actually travelling at 101.54 km/h

[tab]

Given that the speedo is always over-stated on these things, changing the tyre size should make it just about right!

Have you got a GPS (even on your phone) that you can get a true reading from?

[X-WING]

My XR at 100km on odo is about 96km on gps latest garmin 110km on odo about 106km on garmin my guess is that all odo are set standard from xt set ups?