math genius required

[charlski]

how much are my 33" tyres going to throw my speedo out, come up from 31's after getting my 4" lift fitted. ive done loads of searches, and the best i seem to get is that my 31's would have been accurate, as the surf are calibrated to run taking into consideration that these were factory options.

cheers for any help
KC

[TonyN]

Popular opinion has it that you'll be going about 5% faster than whats shown on the speedo.

[Forest Pete]

if my speedo reads 5 mph fast as it does if I fitted these size tyres would it then read correct?

[Chillitt]

definately not a jenyous

get some chalk, mark each tyre, roll it along the road, measure the distance for one full turn on each (both unloaded ideally...) divide the diference by the smaller tyre circumference and that is your percentage error. i think....

[Gamedawn]

Going off info from the BFG site http://www.bfgoodrichtires.com/asset...rain_ta_kd.pdf the revs per mile for a 31x10.5x15 is 677 and the revs per mile for a 33x12.5x15 is 636. Working out the difference between these and assuming the 31's gave a correct speedo reading, on the 33's it will read slow by 6.4%.

My speedo read correct on the original 265's and now on 32's it is 10% slow. I've checked this with a gps over a 100 or so miles and the measured result fits in with the calculation so I guess the above is going to be fairly close.

[whippet]

The maths... if you want it -

Circumference = π x Diameter

31” tyres: C = 3.142 x 31 = 97.402"

33” tyres: C = 3.142 x 33 = 103.686"

C of 33” tyres is (103.686 – 97.402) / 97.402 *100 = 6.452% greater than 31” tyres

So if speedo is accurate with 31” tyres and says 30 mph you will actually be doing 31.94 mph with the 33” tyres.

All dependent on both sets having a similar amount of wear.

BTW: 265/70s have an appx calculated circumference of –

π x (((265 x 70%) x 2 / 25.4) + 15) = π x 29.606 = 93.023”

Cheers,
Alan

[Greebo]

The maths... if you want it -

Circumference = π x Diameter

31” tyres: C = 3.142 x 31 = 97.402"

33” tyres: C = 3.142 x 33 = 103.686"

C of 33” tyres is (103.686 – 97.402) / 97.402 *100 = 6.452% greater than 31” tyres

So if speedo is accurate with 31” tyres and says 30 mph you will actually be doing 31.94 mph with the 33” tyres.

All dependent on both sets having a similar amount of wear.

BTW: 265/70s have an appx calculated circumference of –

π x (((265 x 70%) x 2 / 25.4) + 15) = π x 29.606 = 93.023”

Cheers,
Alan

Holy sh*t!!!!

[charlski]

cheers fellas ................... pure genius.

KC