Speedo Calibration

**golf3** · 23-11-2006, 04:59 PM

Heavier wheels = more unsprung weight = more power required to make them/the car move = more petrol = more $$$

**wayz** · 23-11-2006, 05:04 PM

Arhh, yes.. i thought about that, but these wheels are actually lighter and the car feels like its driving with greater ease

**golf3** · 23-11-2006, 05:21 PM

Rolling diameter will be different every time you change tyres, even when still the stock size of 185/60R14. Tyres are all different. However, with 17's you should be running a 205/40R17 or similar to keep diameter to around the same...

**wayz** · 23-11-2006, 05:34 PM

Originally Posted by golf3

However, with 17's you should be running a 205/40R17 or similar to keep diameter to around the same...

I am running 205/40R17's
But im pretty sure they're a larger diameter tyre, and that would make the difference im guessing/hoping, otherwise im baffled and my car is in desperate need of a tune-up

syncro · 23-11-2006, 05:35 PM

Check your odometer on the M5 speedo check.
Drive next to someone and have a look at their speedo.
Drive really fast through a speed camera and wait for the photo.
Use a GPS.

**golf3** · 23-11-2006, 06:19 PM

http://www.miata.net/garage/tirecalc.html

Using stock of 185/60R14 to 205/40R17
Speedo reading with non stock tyre is 3.2% too slow...
You are actually doing 61.9mph when the speedo reads 60

**peedman** · 23-11-2006, 06:28 PM

It cant be out by more than 10kmph which is the legal boundaries by which ur speedo can be incorrect by. That might have changed tho.

syncro · 23-11-2006, 06:52 PM

Originally Posted by peedman

It cant be out by more than 10kmph which is the legal boundaries by which ur speedo can be incorrect by. That might have changed tho.

No it's 10%.

**Eddy** · 24-11-2006, 06:25 PM

Hoyhoy.
For a start legal tolerances are now only 3%.
So if you want to find your true reading, you will have to ring around in Syd. Look in the yellow pages under speedometers. I done this on the Gold Coast & on the third Ph. call found a bloke who would check the speedo with a mobile GPS plotter which is accurate. Got it in writing, cost me $30.00.