hi there....
anyone will be able to tell me by how much my speedo is out as i changed my tyres at the front from 165 to 135 size....
and assume the reading is now affected....is it more ....or less??????
cheers

what profile are the new tires and what profilea are the old ones?

old ones 165
new ones 135
still in 15...

Those numbers quoted don't appear to be profiles. The numbers on your tyres should be something like 165/65/R15 or 135/50/R15. The middle number is the profile mesurement. It will tell you the distance from the rim to the outer limit of the tread.

The first number is the width of the tyre and the last is the rim size.

The higher the profile the larger the tyre which means the speedo will show a speed slower than true and vice versa.

Hope this makes sense.

With 165s if your speedo was reading 50 mph, you were doing 50 mph, with 135s and the speedo reading 50 mph you would be doing 46.15 mph Roughly 8% less than the speedo reads.

Thats based around them being std 165/85/15 and 135/85/15

did you buy the tyres from a dealer (like goodyear/bob jane) etc?? as they should have informed you if it would effect the speedo reading, especially if you have asked for a specific tyre size and profile. my preivous dealings with tyre people is that they will recalculate the profile if you change the width and have told me if the speedo reading will change, but usually standard tyres are made to compensate for this.

ok...old size was 165-65-15
new one is 135-sr-15
michelin has fitted the new ones at the front as i kept the back ones....mmmm...

so how do you work out the speeds then?

I'm running 195/70/14's at the front and 205/70/14's at the back (if i remember right) and I have absolutely NO idea how fast i'm going

Ive got an excell spreadsheet that graphs gear ratios, rpm vs. speed, its got additional calcs on tyre size, and relative speeds. If anyone wants it, or wants to host it on a web site somewhere email me at debbiehill@ihug.co.nz