Speedo Calibration

[wayz]

Hi guys, long time reader... (great forum!)

Ive just had 17" wheels put on the car (from the stock 14") and i think its put the speedo out of whack, anyone know where i can get the speedometer re-calibrated?

Dont want to stand there arguing with the boys in blue about how i WASN'T speeding

Cheers guys

[syncro]

Hi guys, long time reader... (great forum!)

Ive just had 17" wheels put on the car (from the stock 14") and i think its put the speedo out of whack, anyone know where i can get the speedometer re-calibrated?

Dont want to stand there arguing with the boys in blue about how i WASN'T speeding

Cheers guys

Check it first as it may be within limits.
They normally read about 5-10% fast.

[wayz]

How would i go about checking it? (forgive me ignorance )

i normally do about 550 nearly 600km a full tank and now im doing only 450 or so a tank? This happened immediately after i had changed the wheels. Would anything else be wrong?

[golf3]

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

[wayz]

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

[golf3]

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]

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]

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]

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]

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]

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%.

[peedman]

Hehehe my bad. I really wouldnt worry about it, you would think that if u were doing 61kmph in a 60 zone the coppers wouldnt pull you over.

[silverMK1]

im getting new tires put on my car and there 2cm smaller profile and the mechanic said my speedo would be out... sounds to me it wont be more than 10% so sould i get it checked out or dont worry about it

[rayray086]

Try playing around with this program:

http://members.optusnet.com.au/~sthchan/tiresizer.zip

It's similar to the link that golf3 posted.

Another tyre sizer program thingy can be found at http://www.1010tires.com/TireSizeCal...?action=submit

All three are very useful, and should give you an idea of what size tyres you should really be using to keep the same overall diameter (or something close enough to it).

[syncro]

Legally, replacement tyres should be no larger than 15mm more than any optional fitment.

A Dunlop 205/55x16 may not be the same diameter as a Toyo 205/55x16.