Wheel Change/tech 2

**cosmic** · 25th February 2007, 08:05

Having changed my allys for the SRI ones sometime ago, I noticed a slight drop in fuel consumption (as anticipated, larger wheels than original 16's) question is can the computer be re-adjusted to read 17" alloys without paying the stealers for Tech 2 and who would do this in the Lancashire/Manchester area ?

Currently averaging 44-47mph, prior was close to the 50's, in my 2.2DTI (motorway use mainly, I also use acetone to keep the fuel system clean).

**parapaul** · 25th February 2007, 09:02

How much difference is there in the overall circumference of your new wheels? I don't know what size tyres the 16" wheels take, but if you go from 17" to 18" for example, the lower profile tyre means that any difference in radius is small enough to be negligible.

**cosmic** · 25th February 2007, 10:18

Originally 215/50/16, I am now on 225/45/17 a difference of one inch with a profile reduction of 5mm, therefore 1"=2.54mm means an increase of 2mm or 4/5th of an inch, maths lesson over for today kids ! god that took some working out at this time of day !!!

**FIL 4822** · 25th February 2007, 10:51

Originally Posted by cosmic

Originally 215/50/16, I am now on 225/45/17 a difference of one inch with a profile reduction of 5mm, therefore 1"=2.54mm means an increase of 2mm or 4/5th of an inch, maths lesson over for today kids ! god that took some working out at this time of day !!!

you got the tyre size correct but............................................... ..
1" = 25.4 mm not 2.54mm ! Maths lesson over !!!

**VectraV6CDTi** · 25th February 2007, 11:01

Dont bother with any maths, just use this http://www.miata.net/garage/tirecalcold.html

(You may have to do some subtraction

)

**parapaul** · 25th February 2007, 13:03

From that one....your speedo will read 2.1% too slow...., when your speedo says 60mph, you'll really be doing 61.2mph.

From the POV of consumption, your new tyres are 2% smaller than the circumference of the old ones, so apply that 2% to your mpg....it shouldn't make a noticeable difference, less than 1mpg.

Bet you're driving faster now you've got faster wheels on!!

**stehanson** · 28th February 2007, 19:57

I changed my 215/50/17s to 225/45/17s few months ago and the computer on my SRI was telling me I was only geting 300miles to a tank whereas I was geting 350 on the old ones - (also backed the figures up by some maths myself) then last week it seems to have readjusted back - I remember at the time working out for every 60 miles the car was actually doing 58.6ish - v mysterious how its now doing the same miles per tank!!!

**cosmic** · 13th March 2007, 16:05

Thanks FIL 4822 for the maths lesson, this is not due to numeric incompetence but fat fingers & trying to work at the same time ! excuse over, I used to get in the region of 480/tank (mostly motorway) now in the region of 420/tank which seems excessive, will monitor and advise (just had its 60k service and feels better already due to by excellent mechanic/pal)