[ShaneO]

I have just ordered my i30 CRDi hatch with standard 15" wheels and I notice that some of us on this site have 15", 16", 17" and even 18" diameter wheels fitted on theirs.
Question: Would not this effect the accuracy of the speedometer readings and make some of us more vulnerable to innocently receiving a speeding ticket?
The way I am seeing it is that with 15" wheels running the speedometer would be calibrated correctly.  However, if any larger sized wheels are fitted wouldn't the bigger circumference of the wheels mean that they would not turn as often as the 15" ones therefore would under-report the actual speed of the vehicle, therefore making us open to a speeding ticket?
Anyone else's thoughts ?

[IMCRZY]

Pick the right tyes size and you will have nearly no speedo issues.....

Here is a link to help http://www.miata.net/garage/tirecalc.html

[Dazzler]

Welcome to our great forum ShaneO..

Larger wheels usually run lower profile (sidewall height) and this offsets the bigger wheel..as per the tire calculator link..

[Surferdude]

Shane the above information is correct. If you think about it, using your calculations, by the time you got into 17 or 18" tyres, they'd be fouling suspension and bodywork everywhere and you'd never be able to turn your steering.
I can't open that link so if you have a problem with it also, let me know and I'll arrange a copy of the little Excel spreadsheet they use at Goodyear stores. You can enter the existing tyres size and the size you want and it'll tell you what difference in height you will have (minimal if you use the correct +1 or +2 conversion). It'll also tell you what % variation is speed you'll generate.

[L30N]

What I found with 15" wheels 185/65/15 was that the actual speed (as given by satnav and electronic road signs) was lower than the speedo reading by about 5%. Now I have 225/45/17 with a 2% bigger overall radius and the speedo seems more accurate. So there seems to be an allowance for variance in wheel size.

[IMCRZY]

All speedos are about 3 to 5 kms out for eg.... If it says 100 your doin 97 or so if u get my drift....

The standard 195/65/15 on the i30 has the exact same roling diameter as the standard 225/45/17 on the SR and i think if you were to go a 225/35/19 iit will only be about 0.9% or so out so it will just bring the speedo closer to spot on....

[Lakes]

well the 15",16" & 17" are all the wheel rim diam, when you see tyre size, the first number is tyre width, the second number is tyre profile, the number you see as profile is the % age of the tyre width the profile is, the last number is just wheel diam size tyre fits, what you look at is the distance each size tyre rotates in one turn.

[Myowni30]

I have just ordered my i30 CRDi hatch with standard 15" wheels and I notice that some of us on this site have 15", 16", 17" and even 18" diameter wheels fitted on theirs.
Question: Would not this effect the accuracy of the speedometer readings and make some of us more vulnerable to innocently receiving a speeding ticket?

Have a look down THIS page for 'A tyre size calculator'

It will tell you your speed with the different tyre combinations 

[2i30s]

an excellent source of information.I'm running a 225/45/17 tyre and a 17x7.5 rim and the calculator in the link tells me I'm going slower than my speedometer says,which is correct.my scangauge says 67kmh when my speedometer says 70kmh.my factory rims and tyres were 195/65/15 and a 15x6.5 rim.

[accim]

Which size will you have on your i30? Will it be 185/65/15 or 195/65/15? You'll probably know when you get it. I have winter tires 185/65 and I must tell you that that size isn't the best size if you want accuracy  - the speedometer is showing more than you are actually driving (maybe it isn't as bad as it sounds) 

I had 205/55/16 for summer tires and the readings were much more accurate - the readings are similar (more accurate) also with 195/65/15 or 225/45/17..

[2i30s]

are snow tyres speed limited ?

[accim]

No. They are "regular" tires..some people even use them in summer. Well they are not actually real snow tires, they are kind of winter/summer tires, even though they are terrible when the temps are above 5°C. You can normally drive with them 160, 180.. kph. Mine have speed limit "T" (that's 190kph).

[snowcherry]

i've seen those tyre link pages before on other threads
i don't understand them, i got more confused reading them than before i had read them
i guess some things you get and some things you don't 

[Surferdude]

i've seen those tyre link pages before on other threads
i don't understand them, i got more confused reading them than before i had read them
i guess some things you get and some things you don't 

Don't worry about it SC. I spent many years training tyre fitters and mechanics on tyre knowledge. A significant number of them didn't get it either.
Mature mechanics were the worst because they'd learnt about tyres at college then not had anything to do with them for 20 or so years until they got a job at a tyre place. They would often be set in their ways which were desperately inaccurate.

[snowcherry]

^heh cheers mate, that does make me feel somewhat better