Speedo Recalibration

[robpol]

Has anyone that has fitted larger aftermarket wheels had their speedo recalibrated after they have had the wheels fitted?

 

Cheers Rob.

[robpol]

I have just found out that the standard 215/55 r16 wheel and tyre has a rolling radius of 642mm

and the the tyres i would fit to a new set of 18's would be 235/40 r18 which has a rolling radius of 645mm, surely 3mm difference would hardly effect the speedo reading if at all?????

[gregers]

3 points?worst case senario, :)

[Saif Rehman]

Speedometers, generally speaking are normally calibrated to show a slightly faster speed than actually travelling, so a difference of 3mm rolling radius will not make a significant difference in what is shown on the speedometer.

[gregers]

Speedometers, generally speaking are normally calibrated to show a slightly faster speed than actually travelling, so a difference of 3mm rolling radius will not make a significant difference in what is shown on the speedometer.

 

saif,have you tried this with a sat nav,my garmin is only about 1 mph difference from the speedo readout.but i agree the speedo is a guestamation.

[mk2vr6]

I have just found out that the standard 215/55 r16 wheel and tyre has a rolling radius of 642mm

and the the tyres i would fit to a new set of 18's would be 235/40 r18 which has a rolling radius of 645mm, surely 3mm difference would hardly effect the speedo reading if at all?????

 

 

really? where did you get your figures from?

 

215/55/16 = 321.45mm rr

235/40/18 = 322.6mm rr

 

a difference of 7.22mm or 0.36% so at 70 your speedo will be reading 70.25mph

 

 

your stating the diameter of the wheel to have a rr of that size (1284mm) would mean more than just rolling the arches! and dropping the car by 50mm wouldnt make much difference to the handling! I think JCB's run rear tyres the size you quoteing :)

[robpol]

This is the website i got my info from mk2vr6 http://www.performancetyresdirect.co.uk/co.../converter.html

[Saif Rehman]

saif,have you tried this with a sat nav,my garmin is only about 1 mph difference from the speedo readout.but i agree the speedo is a guestamation.

 

gregers, my Tomtom sat-nav would read 70mph when my car speedometer would be showing 67-68mph. I've tried it with other sat-navs and all show about the same readings i.e. car speedo showing 2-3mph less than sat-navs.

 

I assume thats why police cars get a proper speedo calibration done to their cars because normal speedometers have a degree of inaccruacy.

[gregers]

as im reading my speedo which could be wrong to how im sitting behind the wheel?

be intresting to see what a digital speedo reads compared to a sat nav.

[robpol]

was the info i got from that website right, and was i right in thinking it was stating rolling radius?

[Saif Rehman]

robpol and mk2vr6,

 

I've just spent the best part of 10 minutes trailling through websites for "tyre rolling radius" and can see both websites where the robpol and mk2vr6 got there information from and to be honest my brain is frazzelled with figures so i am gonna bail out until someone who can explain in layman terms where these figures come from or how they are calculated. :)

[robpol]

saif, i've just found conflicting websites too, lol, i wonder what i found first was rolling raduis? Sounds as if i could have been wrong?

[Saif Rehman]

saif, i've just found conflicting websites too, lol, i wonder what i found first was rolling raduis? Sounds as if i could have been wrong?

 

I think where we are getting confused with is all to do with terms used on these websites like 'static rolling radius', 'rolling radius', 'diameter', and 'circumference'.

 

You were partially correct with your figure which is the diameter...........anyway, back to your query, the difference will be very slight and not worth worrying about.

[robpol]

215/55/16 = 321.45mm rr

235/40/18 = 322.6mm rr

 

a difference of 7.22mm or 0.36% so at 70 your speedo will be reading 70.25mph

 

mk2 surly thats a difference of 1.15mm or am i getting this all wrong?

Edited December 15, 2009 by robpol

[robpol]

Thanks saif

[mk2vr6]

215/55/16 = 321.45mm rr

235/40/18 = 322.6mm rr

 

a difference of 7.22mm or 0.36% so at 70 your speedo will be reading 70.25mph

 

mk2 surly thats a difference of 1.15mm or am i getting this all wrong?

 

it is 1.22 my bad doing to many things at the same time! cant multi task well add up, type, play travian, drink beer and eat .... something has to go wrong and I aint wasteing food or beer :)

[robpol]

something has to go wrong and I aint wasteing food or beer wink.gif

 

Well said that man!!! :)

[big_kev]

Surely there would be a bigger difference in the rolling radius of brand new tyres and nearly bald ( just legal say ! ).

[robpol]

Yeah i was thinking that too last night Kev....