Could 17" Alloys affect the speedometer reading??

[quikshine]

Hi,

When I purchased my car there was a set of 17" Wolfrace alloys on it with low profile tyres.. Great I thought, but now I am beginning to believe that the size/weight of the wheels are affecting the reading of the speedometer...

This is slightly worrying as I believe its illegal to have a unaccurate speedometer. (unsure)

I did a simple test, I drove the car at what I thought was 50mph (didnt feel like 50mph) and my mum followed close behind but her Mini which has a digital speedometer and that said she was doing 38mph! Meaning my car was saying I was roughly doing 12mph more than I actually was.

Has anyone ever come accross this problem on their Clio before? and what can I do about it? A few other people I have asked say its the wheels, does anyone else have any suggestions? Can it be fixed?

I really dont want to have to go to the expense of changing them!

Thanks for your help.
Ross

 

[BenCup172]

Its not a particularly accurate way to test it. I mean it could be roughly 12mph behind, but with someone just following you can never be doing exactly the same speed. Try taking it to a garage and have it on the rolling road.

 

[theduck]

Larger, or smaller, than standard alloys do affect speedo reading. Its not illegal to have inaccurate speedo as they are inaccurate out the factory!

 

[quikshine]

Interesting responses... a few things there that I will have to investigate! thanks guys!

If I do find there to be a descrepancy with the actual speed and the speedo reading when I seek professional advice.. Does anyone know how it can be fixed so the correct speed is displayed on the speedo?

Thanks

 

[Da]

Speedo's are generally 10% out, so at 50mph you could have been doing 45-55mph. But for your mum's to be reading 38 (although not accurate) this would suggest they are affecting the reading. Least it's in the right direction though!

Speedos are calibrated on the overall diameter of the standard wheel/tyre combo. Any deviation from that will affect the readout.

Simple solution, get a GPS

 

[Gary-Lew]

175/65/14 185/60/14 195/50/15 185/55/15 195/45/16 205/40/17 are all give or take the same rolling circ

 

[cat171]

Yep, if you have the same rolling radius as the normal size wheel & tyre you won't have a problem with the speedo, as said though in general most are 10% out in.

 

[Jaff]

no bigger wheels would make it work the other way you would be going faster than the speedo, 17s ona car with usually 15s would increase the speedo about 12% (some onling bible i looked at a while ago) meanign if you are at 70 indicated you wuld be going 77 in reality,
what you are describing is symprons of your wheels being smaller than normal,
does you mother have aftermarket alloys one her car, coz they could be throwing her out also.

you could try a gps but they are 5mph +/- easy, iwas doing 20pmh at lights once, i'd just watch the speed cameras.
legaly i'm not sure where you stand, i know cars have to have a functionign speedo, but how accurate it needs to be im not sure.

 

[Kneel]

When I put 17s on mine it made the speedo read accurately to what GPS said I was doing.

 

[MRBILLYUK]

Of course it affects it . Any deviation from the standard wheel / tyre size will give an inaccurate speedometer reading . Check this out :

http://www.alloywheels.com/tyrecalc.asp

 

[iarno]

no bigger wheels would make it work the other way you would be going faster than the speedo, 17s ona car with usually 15s would increase the speedo about 12% (some onling bible i looked at a while ago) meanign if you are at 70 indicated you wuld be going 77 in reality,
what you are describing is symprons of your wheels being smaller than normal,
does you mother have aftermarket alloys one her car, coz they could be throwing her out also.

you could try a gps but they are 5mph +/- easy, iwas doing 20pmh at lights once, i'd just watch the speed cameras.
legaly i'm not sure where you stand, i know cars have to have a functionign speedo, but how accurate it needs to be im not sure.

im sure i know someone who's car passed an mot with a broken speedo.. :S

 

[Gary-Lew]

Of course it affects it . Any deviation from the standard wheel / tyre size will give an inaccurate speedometer reading . Check this out :

http://www.alloywheels.com/tyrecalc.asp

like i said earlier, punch in all these sizes and you will be -1 to 2.5 mph under at 70mph for a 205/40/17
175/65/14 185/60/14 195/50/15 185/55/15 195/45/16 are all give or take the same rolling circ its all in the % profile to width and wheel size they equal each other out

 

[daydotz]

i noticed a diffrence of 5mph between my satnav an speedo is gps more acutrate ?

 

[GordonD]

i noticed a diffrence of 5mph between my satnav an speedo is gps more acutrate ?

If you are travelling at a constant speed the satnav will be very very accurate. It will be within plus or minus 1 mph of your true speed.

If you are accelerating or decelerating or your speed is varying up and down at all the satnav may be giving you an inaccurate reading because it doesn't measure the speed you're doing now like a speedo does. What it does is calculate your average speed over half a second then display it for half a second. So its display is telling you the speed were doing an average of half a second ago. If you are accelerating it will give you a lower reading than your real speed, and if you are accelerating it will give you a higher reading than your real speed.

Also if your speed is varying up or down even the smallest amount there is a delay as you switch from looking at one to the other, during which your speed may have changed.

What you have to do is travel at a very constant speed, compare you speedo reading to your satnav speed reading, then remember that difference as how far out your speedo is at that speed. You'll find your speedo is out by a different amount at different speeds. Mine for example reads high by 6 km/h at 60 km/h, but that only increases to 8 km/h at 100 km/h.

 

[kevinhall]

I believe speedo sensors are usually in the gearbox and the speedo works by knowing that for every revolution you go x distance. If you change the distance x by increasing or decreasing the circumference of the tyre you change the speed but the speedo reading remains the same. Get a GPS and check - they are generally speaking more accurate than the speedo. If under-reading either the circumference has reduced or the speedo is broke!

ps my speedo is out by 10-30 mph dependent on day, weather, stage of the moon - still got through an mot fine. Gotta love british build quality.

 

[daydotz]

see an this is were mobile speed cam vans seem a bit unfair

 

[Max46]

they way i have tested speedo readings is witha road angel they are pretty spot on and in my old car was reading a 5/ 10 mph difference ( 10% ) so thats how i would test it as for fixing it wouldnt worry to much mate.

 

[kully]

technically , yes .

 

[Footerz87]

I'm just about to invest in some 17" rims to replace my Dynamique alloys.

Just wondering how many people have had their speedo's adjusted and roughly how much it costs?

Or have most people simply left it?

Dan

 

[theduck]

Footerz, dont bother having it adjusted even if you can find somewhere that willl do it!

 

[rob192]

I'm just about to invest in some 17" rims to replace my Dynamique alloys.

Just wondering how many people have had their speedo's adjusted and roughly how much it costs?

Or have most people simply left it?

Dan

Stop being such a pussy and buy some Pace

 

[Footerz87]

Alright then..

Whose selling a set of 17" TSW Pace?!

Footerz

 

[C.slicker]

Not sure if its already been said but 17s should bring your speedo more in line.

The speedo should under read in relation to standard.