Get your calculators out [Archive] - SaabCentral Forums

: Get your calculators out

22nd March 2002, 05:36 PM
I know that there is a way to work this out cos have known people to configure traction control settings with the use of some very clever algebra! Well, that was never me at school so I need an answer to this question please :

Q. I have 17" wheels, so what difference does that make to the reading my speedo gives me? Am I actually going faster than what it says or slower?

Answers on a post card, or maybe in here!

22nd March 2002, 06:13 PM
You will be going slower than speedometer reads...

22nd March 2002, 06:52 PM
Not exactly a postcard but...

If you are using a 205/45/17 or a 215/40/17 tyre then the rolling radius is exactly the same as the original 195/60/15 tyres. So no change.

If you're using 205/40/17 (a more common tyre) then the total wheel diameter is slightly smaller and the speedo will over read.

22nd March 2002, 08:27 PM
arent u going faster?
im not using algebra, just logic. ur tires are bigger so they will cover longer with every revolution, so u will be going further than with smaller tires. therefore u will be going faster.
Im pretty sure im right, check the logic out, see if it works in ur head too

22nd March 2002, 08:28 PM
but it matters on the size of the tires as well

Andrew Williams
22nd March 2002, 09:27 PM

23rd March 2002, 03:20 AM
A larger wheel/tyre goes round slower for a given road speed compared to a small tyre. The speedometer reads the rate the wheel is going round.

23rd March 2002, 09:11 PM
Nice one guys, according to the calc my speedo will be reading approx. 3% too fast! Damn, now i have to go even faster just in case!

23rd March 2002, 11:00 PM
who was the first one to call it?