Tyre size, radius help! Want to go fat!

[Liddo]

Can anyone help.

I have a C70 T5, which USED to have 17" Canistos with i THINK 215/40/17 tyres on. Now, I have 18" Nebs on with 215/35/18 tyres on and I want to go to 215/45/18 tyres, as the rubber bands I have on at the moment just dont give the comfort i want.

Will 45 depth tyres throw my speedo out by miles???

Cheers

Paul

[BruceT]

My old C70 T5 with canistos had 225/45/17 as standard

As for the rolling radius and speedo difference, ill wait for someone to post one of this links with details

[nathT5]

this is what i use :B_thumb:
www.alloywheels.com/tyrecalc.asp

[Redbrick]

Will 45 depth tyres throw my speedo out by miles???

Yes!!

http://www.miata.net/garage/tirecalcold.html

[Liddo]

Brilliant! Thanks for the replies chaps, that's just perfect.

If i go for 215/40/18 tyres, my speedo will read a little faster than what Im doing, so at least I wont get caught speeding ;) If i go for the 45's, which Id like to, my speedo will read 2.6% slower, so at 60, the car will be doing 61.2.

Hmmmmm, well at least i know

;)

[CONDYBOY]

Yes mate, but in general a 10% error is allowed for variants in manufacture of speedo's so get your speedo checked, it could be showing slower anyways!
My GPS for hillwalking shows 2mph below what the speedo shows in my car with standard tyres on. So when I'm doing 70mph I'm actually only doing 68mph.

[wegal]

My S60 came from volvo with 235/45 r18 fitted so cant effect the speedo that much or they wouldnt sell them like that.