tyre & wheel sizes

"jscott" gurgled happily, sounding much like they were saying:

The profile is a percentage of the tread width (in mm). There's 25.4mm in an inch.

Go fer it.

Or alternatively, Google for tyre rolling radius, which will lead you here:

I make it 2.241 metres per rev of tyre for the 235/60 17" (17x25.4)+(235/100x60x2) and 2.247 metre per rev of tyre for the 235/55 18"

So, (if correct) an extra 6mm travelled per tyre rev for the later. Its very slight, but if it was larger it would be interesting to see how this can be related to speedo reading or gearing ratio, but unfortunately at this early hour of the day, my brain can not troubled to fathom it just yet.

"Mark" gurgled happily, sounding much like they were saying:

Clue: "Negligable" is a perfectly acceptable answer...

It's also worth thinking about the effect of the difference in tread depth between a brand spankin' and a knackered tyre.

Chris, Adrian, Mark - thanks for your help.

Yes, obviously negligable. As in "very slight". Just was curious to take this further to see what a bigger difference would make on speedo reading, but the website that does it for us automatically has now taken all the fun out of going any further.

Sorry to have spoilt the fun. FSVO fun, obviously...

Perhaps I got it out of my system in the sixties and seventies, when I raced motorcycles for four seasons, then went on to rally a Mini for a couple of years.

I spent hours producing charts of speed versus rpm for all combinations of tyre sizes, final drive ratio, gearbox internals etc. All by pencil and paper, for affordable pocket calculators were still a twinkle in Sir Clive's eye then!

Chris

Nedavno jscott napisa:

Check here:

I do not know why the rest of the guys are attempting some long calculations, here is my 1p [from what I remember from school maths about 25 years ago]. You increase your rim (diameter by one inch = 25.4mm) and reduce the height of your rubber by 5mm x2 = 10mm. Overall increase of wheel diameter is therefore 15.4mm, divide that by original wheel diameter (17 x 25.4mm + 2 x 55mm = 541.8mm) the result is 0.028 - just under 3%. So your speedometer will show your speed to be 3% less than actual (that is just 2mph at 70mph) and the same goes for distance covered...

Increase the wheel diameter by 0.3%, this should be ok.

More info from a classical tyre calculator tool at

Nedavno GArlington napisa:

235 is tire width 55 is tire highth as percentage of its width R18 is rim diameter

so in this case tire highth is 235x0.55=~129mm (not 55mm)

Nedavno GArlington napisa:

235 is tire width 55 is tire height as percentage of its width R18 is rim diameter

so, in this case, tire height is 235x0.55=~129mm (not 55mm)

"GArlington" gurgled happily, sounding much like they were saying:

One whole penny?

It's overpriced.

Nice theory. Shame it's based on a misunderstanding.

...unless you drive one of those ultra-sensitive 4WD cars where worn rears with new fronts would burn out the viscous centre diff - Volvo was it?