Speedo

[Ad]

Seeing as I have just got done for speeding (92 on the motorway) I wanted to calrify what difference 17inch wheels will make to the reading on the speedo. My clio came with 15inch as standard. I read somewhere that this equates to the speedo reading 0.5 mph faster than the car is actually travelling. So when I was caught my speedo must have been reading roughly 97mph. But I never generally let my speed get this high as I am aware that you get banned after 100mph. I remember the journey and dont remeber going that fast!!!! Am I thinking along the right lines????
Cheers Adam

 

[cse]

if u put bigger wheels o then ure speedo will measure lower that the speed u are doing, read somewhere that if ive got 70 on the speedo, ure actually doing 74, so clocked at 92, feasable that u were on about 87-88 i suppose

 

[Tidy_16v]

The speedo differences should be calculated from the overall diameter of the wheel and tyre - if it is larger than the original wheel/tyres then it will read higher and if it is smaller then it will read lower than what you are actually doing. For instance I have 16s on my 16v but the profile of 40 makes them have a smaller diameter than the original 15s. I think there is a size comparison chart in the tuning section of this site and I once found a Java calculator on an MX5 owners site that lets you work out how far out your speedo actually is according to your diameter sizes.

 

[Tidy_16v]

Dont forget that a profile of 40 on a 215 tyre is not the same as a profile of 40 on a 195 tyre as it is a percentage of the width!

 

[Tidy_16v]

Hang on got it round the wrong way - smaller diameter...wheel turns more in a given time..thus speedo is faster than actual speed....larger diameter...wheel turns less in a given time...speedo reads lower than actual speed. Also dont forget that speedos are usually set optimistic from the factory so a larger diameter may make it more accurate. Confused!!!!??? Hope all this helps.

 

[Fred@BTM]

Ad, (i didnt tell you this), but i thought your uncle in america was driving (ahem) you lent him the car right? (ahem)

 

[Ad]

Know anyone who has tried that Fred??? (Only jokin). Looks like I have now got six points partly due to my speedo being incorrect. I thought when it read 90 i was doing 84/85mph. Obviously i wasnt going as fast as usual becasue I gather that when the speedo reads 90 im doin 95/96!!!!
I thought that they now had to have signs up to say that there were cameras in the area. How can they do this when there on the top of a motorway bridge???

 

[DAVE77]

So what speed would my car be doing when the speedo reads 135mph? Im running 205/40/17s.

 

[ak_uk]

I think you are entering *ban* territory @ 90mph....which is probably why the police dont do much up to that speed as you only get points....

In dont even think its to do with the 90mph - more to do with the % above the speedlimit...

Can anyone else confirm/deny this.....?

ak

 

[Ad]

No not up for a ban. Ban automatic at 100mph+. Think its gonna be 3points £60.

 

[KTR-CLIO]

As well as effecting the speedo, does this effect the trip computer (MPG,etc)and the digital mileometer

Thanks

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