Does having bigger/smaller wheels effect your Speedo?

[PhantomCorona]

I was told round about a week ago that if i had bigger wheels that it would put my speedo off?

Even if it did i wouldnt make much of a difference. I would think.

Anyone know if that this is tru?

[bozurolla]

This what i'm told..

as long as the diameter of the tyres are in spec with the originally supplied off the production line you wont have problem..The 'Ronas' i'm pretty sure came out with small tyre sizes.

Even if you did go large on the tyre diameter it would'nt be much over but if you done by the cozzers I wouldn't try arguing with them,I've had some of my mates change the speedo 'Gears' to a lower ratio for your speedo to run true.

ie the cheapest way to see if ur speedo ran true is to run by a mates car on the straight road at the same pace & compare speedo readings at the same time..(well that's how we did it)

[Sweety]

I have gone from stock 205/60/16 to 235/45/17 and my speedo is only 1.4 out.

There is a tyre size calculator at www.miata.net.

[PhantomCorona]

Oh thanks.

I think i had i dunno somefin like 175 i think or around and i changed to 205s.

Without changing the rim size.

[qkslvr]

Yep. The speedo works off revolutions. so if you change the rolling-radius of your wheel, each revolution of the tyre is going to cover a greater distance - you'll be going faster than what your speedo is telling you.

Also, speed errors of this kind go out by percentage. so if you see a 3Km/h different at 60km/h (5% off), then you will be out by 5km/h at 100km/h i.e it will say 100km/h when you're actualy going 105km/h

to find out distance covered by your wheel with one revolution use the following formula:

Pi * Diameter

Note: Pi (pronounced 'Pie') = 3.14

[SuperDave]

to find out distance covered by your wheel with one revolution use the following formula:

Pi * Diameter

Note: Pi (pronounced 'Pie') = 3.14

Even then, for example, a 205 tyre will not always be 205mm wide. The best method is to physically measure the circumfrence. Or if you like navigating Japanese websites, the manufactures give actual data on their tyres.

[PhantomCorona]

Sweet thanks guys.

qkslvr i thought thats how i thought it picked it up.

But i would have thought a bigger tyre would make the speedo under what your actually going.

and smaller tyres would make it faster.

But hey thanks for the feedback.

[Blind Kid Seeks]

Sweet thanks guys.

qkslvr i thought thats how i thought it picked it up.

But i would have thought a bigger tyre would make the speedo under what your actually going.

and smaller tyres would make it faster.

But hey thanks for the feedback.

Yeah you had it right. smaller wheels need to turn faster to roll the same ditance as a larger wheel.

[jaderd]

E=MC2

;)

[PhantomCorona]

So what sort of formula would i use to work out how off my speedo is from the stock wheel size?

Would i go....

STOCK WHEEL SIZE - BIGGER WHEEL SIZE = X

using that Pi X Diameter from qkslvr

But how would it work out how off the speedo is?

[~Dj Dirty]

Hmm.. Went to 31" and made a little difference... When I go to 35" itll be out by miles HAHA

Edited August 8, 2008 by ~Dj Dirty

[STYLSH]

Tyre size calculator

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

[SV21]

This thread is timely for me as I have just purchased some 15" 'Ultima' alloy rims and going to fit tyres soon,my SV21 has the standard 14" rims and I was wondering the same thing.

The thing about factory spec issued items such as this did cross my mind but I was going to go and see a speedo guy I know in Albion (QLD) to check anyways.

Thanks for the info.... :D

[Hiro]

This thread is timely for me as I have just purchased some 15" 'Ultima' alloy rims and going to fit tyres soon,my SV21 has the standard 14" rims and I was wondering the same thing.

The thing about factory spec issued items such as this did cross my mind but I was going to go and see a speedo guy I know in Albion (QLD) to check anyways.

Generally when you have a car which comes with 13" and 14" wheels depending on spec, for instance, the outside rolling diameter is still roughly the same. So fitting 14" wheels from the lux-spec model, as long as the proper profile tyres are used, won't affect things at all.