Rippers

Did you enter 2002mm as the circumference?

Mike Rainbird

iTrader: (3)

Different makes have slightly different circumferences, and also how new they are makes quite a difference as well (as much as 1-2%). So a bit of a stupid question, as it would depnd on type of tyre and how many mm of tread it had left .

Rippers

I know that, but it got to be closer then the 2515mm I came up with lol

I think I'm exactly 500mm out through fucking up my sums so 2015mm as its a new tyre sounds right?

Kelv

iTrader: (4)

have i got all this to come

i thought they were idiot proof, but maybe they can defeat this idiot

Rippers

Mate don't worry I'm tired and talking bollocks, its easy really I just need to measure my tyre again

Mike Rainbird

iTrader: (3)

Please tell me HOW you can get confused . Jack the car up so that the tyre is off the ground, put a piece of string around the outside of the tyre until the string touches each other. Mark where this is, remove string, measure circumference. Or use one of those floppy seamstress tape measures to do the same thing. How can you possibly get 500mm extra ?

Martin-Hadland

iTrader: (2)

Different makes (same size) have differing circumferences?! How do you work that one out? Anyhow don't you just multiply the height by 3.142 to get the figure?

Mike Rainbird

iTrader: (3)

Martin,
Surprised at YOU asking that . This is because different manufacturers have different sizes for the same tyre "size" - they round them up or down. For example a 235 tyre might actually be 240mm wide or a 225 might be 220mm wide. Depends on the tyre type, the size given by the manufacture is just a "guide" and not definitive . And because the sidewall height is an percentage of the width, you can see then why they actually vary in daimeter and therefore circumference .

Also, it CLEARY states in the Stack instructions to measure the circumference NOT to calculate it (and that is because of the reason stated).

Martin-Hadland

iTrader: (2)

If you take a look at most tyre manufacturers sites you will see that their overall height stacks up with a calculation done on a calculator using rim size and 2x wall height all added together. The only time I have found tyre sizes to be approx is when we are talking racing rubber.

Mike Rainbird

iTrader: (3)

Stack still tell you to measure for this reason and the fact of tyre wear . Don't forget that the overall wheel height can be affected by as much as 10mm just by tyre wear alone....

Martin-Hadland

iTrader: (2)

I agree with the tyre wear coment but do have a problem with the other! Anyhow I have always found that a little trimming is needed of the numbers entered to get the speedo spot on regardless of how its measured.

Mike Rainbird

iTrader: (3)

When I measured mine with new tyres on (the time we were at Alconbury together (you with BOM) . My Stack read 172mph crossing the line and the time recorded by the Datron was 172mph . Several months later with the tyres almost needing to be changed, the Stack read 183mph and the Datron read 180.6mph - I think that is pretty accpetible from measuring the circumference and not calculating it .

FYI The AVS Yokohamas are amongst the biggest culprits, due to the square shoulder of the tyre, their 235s will catch the arches on the back of a Sapphire (due to them being approx 240mm wide), where others that run a more rounded shoulder are actually 230mm and fit no bother .

gingeRS

string

you just need some big verniers

Mike Rainbird

iTrader: (3)

Please explain to me in detail how VERNIERS can measure CIRCUMFERENCE

Obviously you can work it out from the diameter, but how many people have verniers big enough to take an accurate dimension to do this . However, most people have some string and a tape measure .

Rippers

I used my old dear's seamstress tape measure and it wasn't long enough so had to add to measurements and I've very obviously fucked it up Just wanted to see what everyone else had come up with to see if I was even slightly close as it was reading 12mph over the Tom Tom at 40

gingeRS

there ya go, you answered your own question

Mike Rainbird

iTrader: (3)

Paul,
Will be around 2000, but measure to get it exactly right and speedo should tie in with Tom Tom .

Rippers

Just bought a tape, Measured 1993mm

Mike Rainbird

iTrader: (3)

Told you there is a difference . If you punch in the just the figures it "should" be for a 225/40 x 18 tyre, it comes out as 2002mm .