View Full Version : Tyre profile and radius.

Wednesday 11th May 2005, 14:01
Right guys and gals, Big dilemma here for me as usual, I have on my Tritons 225/40/18 tyres, My speedo is out by 7%,
Now i am wanting to fit 215/35 tyres to bring my speedo back into line and i know i will lose traction with the smaller tyre and ride comfort with the lower profile,
But will my car be 7% quicker 0-60 or standing 1/4 because of correct tyres being on her? My best time at the Pod was 14.9 with the wrong tyres and crap fuel and a air leak lol, Excuses huh :slap: So would i be able to get a quicker time with these 215/35 tyres if i could keep the same traction?
Or is it just not worth the lose and keep the 225/40s on her.

Gary :insane:

Wednesday 11th May 2005, 14:03
Forgot to say what would the difference be if i fitted 215/35s to the front and kept 225/40s at the rear speed wise? :jaw:

Wednesday 11th May 2005, 14:09
In theory, you get less tyre drag so it should be faster, but I would imagine it would take a much more accurate device than a speedo to notice lol.

As for reducing the rolling radius and going 7% faster, no, it doesnt work like this. It will basically reduce the effective gearing of the car, ie your gears will be shorter, what the speedo says wont change, ie you'll still do the same mph on the speedo at the same rpm on the rev counter, but the real speed of the car will fall.

ie I run 195/50 15 on the rover, but they should be 195/55 15, so instead of the car doing a real 24.5mph per 1000rpm in 5th, it does 23.4 mph per 1000 rpm in 5th, though the speedo will say the same thing at 1000rpm in 5th on both sets of tyres, hope that made sense.

Will it make your car faster, it should help it accelerate quicker, but will reduce its top speed, provided you could hit the limiter in 5th before, if not it wont affect the top speed as it will still be limited by wind resistance. Being really anal you could say it might do a fraction of an mph more cause the tyres are creating less friction lol.

How do you know your speedo is 7% out? chances are with a larger rolling radious than its supposed to have its more accurate than most peopls speedos, as all cars come as standard with a slight over read.


Wednesday 11th May 2005, 18:31
Thanks for your reply Ross,
I was running at Avon Park as i crossed the line the Guy said i was doing 96.7 mph my speedo was showing 90mph,I looked twice and both times speedo was 6-7 mph out.
I then went onto a American web site that shows radius's and profiles so you can work out your standard wheel/tyre combo mine being a 205/50/16 against the wheel/tyre i had on there at the Avon Park 235/40/18 and now 225/40/18s.showed i was 6-7% too big.
I have a ex plod car,So unless the clock electronics have a fault,Or the calibration is way out now,
The car should be a lot quicker being a manual Rica,But i would swap top end for acceleration anyday,
As i live on the edge of London we have so many speed cams up here,It's so risky with my wheels being out so far.


Wednesday 11th May 2005, 18:41
Sounds like a good idea then, either that or take the cheap option and remember how much out it is ;) lol

Anyway, yeah theres no prob, if you've used the site to work out what size will get it reading spot on again your on a winner, you'll find you notice no difference, perhaps you may notice it goes through the gears marginally faster, and you might even go faster on a drag strip due to the gearing, though chances are it will take a drag strips timing gear to pick up the difference.

Good luck getting yourself sorted, an under reading speedo is definately more licence risking than an over reading one lol.