Wheels v Speedo

[Louie Lungbubble 0]

Does anyone know exactly how much (if at all) running 16" wheels with 205 40 16 rubber round them affects your speedo?

 

I suspected they might do, so got a mate to run down the motorway in a standard focus, with standard wheels and give me signal when he hit 70mph.

My speedo read 65mph :!:

 

I know speedo's lie more the faster you go, but assuming the one in his nice new shiny focus was calibrated to be correct up to the legal speed limit, my 5mph reading deficit at 70mph could catch me out one day with those gatso cameras we all know and love.

 

Has anyone running 16's with the same size rubber had their readings checked?

 

Ta.

[dinkus 10]

Most cars are calibrated to show too fast, even new ones. IIRC the standard setup on the C with 15" wheels reads about 10% too fast. I've heard people saying that with 16" wheels on, that the speedo is more accurate (tested with GPS). But I dunno what tyre sizes they've had on...

[Eug 0]

The recommended tyre size for a 16" wheel is 205/45/16.

 

If you want to keep the width standard you add an inch to the wheel diameter, and subtract 5% from the profile.

 

17" wheel would be with 205/40/17 tyres.

18" wheel would be 205/35/18 tyres and so on.

 

Hope this helps.

[Eug 0]

Basically your speedo would be out -2.66% because your rolling radius would be slightly less than standard.

 

So when you're speedo reads 70, you're actually travelling at 68.14.

 

Taken from The Wheel and Tyre Bible

[pablo_vr6 0]

my speedo is bang on - according to nice policemans laser gun anyway :(

[Louie Lungbubble 0]

Aplogies.... My tyres are indeed 205 45 16 (typing error :oops: )

 

So..... This `Plus One' setup I've got has increased the overall rolling diameter of the standard 15" wheels then?

As (if I understand properly) you're not losing the full inch gained by the rim diameter in the reduced tyre height.

 

So here we go... Increased rolling circumfrence = less wheel revolutions to cover the same distance = lowered speedo readout.

(God my head hurts so much today :lol: )

 

But if as dinkus, suggests, the speedo gets more accurate with this setup cos cars are caklibrated to over-read, then that's fair enough. I'm happy!

 

8)

[Skimask 0]

Hey Louie,

Just by chance I compared my original 15" wheels (205*50*15) with my 16" (205*45*16)

this morning.

The tyres are the same make -and the 16's are about 7mm (radius) larger, give or take a couple of mm for tread wear on the old 15's.

I dont know iffitting 40's would make the total wheel size the same!

[Louie Lungbubble 0]

I compared my original 15" wheels (205*50*15)

Cheers mate. That's just answered my next question.

 

I may sit myself down in a darkened room and try to work out the mph difference between the two. :|

 

 

Who am I kidding? I'm goin the pub! :) :)

[Joe M 0]

Why not let this work it out for you?

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