17" wheels=speedo out?

[e30-320i]

at the moment i'm running 17" alloys with 215/45/17s the tyres came with the wheels and have loads of tread so i'm not gonna go out and spend £300 on tyres until i need to.but on the motorway at 70mph i seem to be overtaking everyone so i have been wondering would running this wheel/tyre combo affect my speedo much any idea how much difference it would make at 70mph and 30

[Brianmoooore]

Quick mental calculation, (which may be miles out), gives your current tyres making the speedo under read by 3%.
The standard speedo under reads by more than this normally, so even with an indicated 70 on these tyres, you're probably still doing less.

[ed325i]

Have a look here >> http://www.clubpolo.co.uk/articles/005.htm

ED

[e30-320i]

cool cheers guys i do live in felixstowe (where the old folk come to die) so it's probably just them driving at 40mph on the motorway

[fuzzy]

i have a gps that says im doing 30 and 70 when my speedo reads 34 and 77 respectively and thats with 17" alpina reps,so your actually going slower than your speedo reading.

[e30-320i]

thanks for the replies i'm just paranoid about speeding with all these bloody cameras about as i only passed my test in 2004 and cant remember how long the period is that i can only get 6 points before i loose my licence (1,2 or 3 years) on the bright side though 2yrs no claims next month insurance comes down to £400 p/a fully comp

[fuzzy]

i think its 2 years with a 6 point limit. your lucky,im 36 and im paying £906 a year f/c with 3 points

[buster]

the correct tyre to keep the right rolling radius is 215 40 17.Its th 45 profile thats knocking it out.

[e30-320i]

the correct tyre to keep the right rolling radius is 215 40 17.Its th 45 profile thats knocking it out.

yeah the rears rub a little when fully loaded too but i aint gonna chuck decent tyres not at £65 a pop they'll have to wait 'til they need replacing

[buster]

i dont blame ya