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hi there

im going to be fitting 15's with a 45 profile to my car which i think will have an overal larger size in diameter. Will this effect my speedo ?

cheers

james

James0121 wrote:hi there

im going to be fitting 15's with a 45 profile to my car which i think will have an overal larger size in diameter. Will this effect my speedo ?

cheers

james

Depends how wide you are going - the 45 is a percentage of the width not an actual measurement 195/45 15 = same (well bloomin close) to std rolling radius so will have no noticable effect on speedo inacuracy

I run my Mk 4 polo on 205/45/17 (with no arch mods - and no scrubbing, but thats another story) ANYWAY,

I was worried about the speedo being affected so took my boyfriends day road angel, speed camera detector thingy and put it in my car. it shows what speed you are doing on screen and was exact same as the speedo.

So i reckon if mine are okay yours will be too.

xXx

oh right, well sounds as if mine should me fine which is always good

k o easy formula if you're prepared to follow it.

rolling circumference of tyre = diameter x pi... as pi is constant you may as well just use the diameter for comparisons

you thereby find the tyre total diameter by.....

wheel size.... plus
tyre width x profile % x 2

and maybe, i'm not sure, you have to add the tread in (again, x2) as well.

confusingly you'll have to do metric to imperial conversions, or the other way round, as the wheels are in inches, but the tyres are in millimetres. choose one or the other and get all your figures into it. the inches give a more "solid" number (e.g. 22.5) but the millimetres are more modern and precise (eg 572)

just take your number for your new set, divide it into your old ones, and you'll have how much it is bigger or smaller as a fraction a little either side of 1.0 (or if you like, 100%) and how much it will influence your speedo.

e.g. new tyres blingin low profile 195/45s on 15s.... 195 x 45% (0.45) x 2... or 195 x 0.9 = 175.5
15 inches = 381.15mm (25.41mm to the inch)
and say 5mm of tread as a suitable average over the life of the tyre
= 562mm all in ...... or for the hell of it, 22.1 inches.

old tyres, stock chubby 155/70s on 13 inch steelies...
155 x 1.4 = 217.0
13 x 25.41 = 330.33
plus 5mm
= 552mm / 21.7 inches

so thats 562mm vs 552mm, or only 10mm difference in 560... 1/56th... i.e. the new rubber (assuming those are both what you had, of the same tread and inflation) is 101.7% the size of the old stuff. Call it a 2% road speed increase for the same indicated speed. Might be worth bothering about if you're currently going thru speed cameras in 50 or 60mph zones right on the upper limit of what they'll allow, based on your speedo reading, but really..... the difference in tread from brand new to the legal limit could cause almost as much variation as that.

whats the actual numbers like?

If you go on www.clubpolo.co.uk there is a section on there that will work it all out for you. All you have to do is enter the size of the original tyers then the new one and it will tell you if there is any diffrence. I think you can find it under Mechanical and Tuning.

I hope this is of some help

on a mk4 195/45/15 are the perfect size as the speedo difference is very small

ie 2%