tyre size question?

BFD83 · 23-09-2006, 04:56 PM

i am gonna be buying 18"s off my mate, he brought one of the wheels over and leant it against my existing tyre and said the size looks bigger then normal and not sure if it will fit and put my speedo out, the tyres i got atm are: BF GOODRICH 225/50 XR16 and the tyres im gonna be putting on are FALKEN 245/402 ZR18. does anyone know if it would put the speedo out or would it make no difference?

ENVI XR6 · 23-09-2006, 05:21 PM

Nah they will be fine mate i had a set of 245/40/r18's on my au, doesnt set the spedo out at all

plext · 23-09-2006, 05:30 PM

Circumference of 225/50 16 is 1983.6, while 245/40 18 is 2052.1

This roughly represents a 3.5% change, which will show with the speedometer reading slightly under true speed. Not enough to get in a twist over though.

EDIT: Unless you happen to visit Victoria of course.

RG · 23-09-2006, 05:35 PM

I've got 18's on mine (originals are 17's) and my speedo is out by 10k at 100.

ORESUMSR · 23-09-2006, 07:26 PM

Quote:

Originally Posted by Brent83

i am gonna be buying 18"s off my mate, he brought one of the wheels over and leant it against my existing tyre and said the size looks bigger then normal and not sure if it will fit and put my speedo out, the tyres i got atm are: BF GOODRICH 225/50 XR16 and the tyres im gonna be putting on are FALKEN 245/402 ZR18. does anyone know if it would put the speedo out or would it make no difference?

All the correct information that you require, can be found in the following link. Just scroll down (2/3?) till you get to a section called "Why would I want to change my rims and tyres anyway?" There you will find "A Tyre Size Calculator". You just have to insert the tyre sizes that you want to compare and it will automatically work out the difference in circumferences and speedo error, if any.

http://www.carbibles.com/tyre_bible.html

Luck and happy reading..

BFD83 · 24-09-2006, 04:30 PM

does anyone else know?

lukey · 24-09-2006, 04:49 PM

http://www.fordmods.com/forums/documents.php?doc=30

JC · 24-09-2006, 05:40 PM

Note sure why the correct info above is not enough, so here's some more:

Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/km Difference
225/50-16 112mm 316mm 631mm 1984mm 504 0.0%
245/40-18 98mm 327mm 653mm 2052mm 487 3.5%

When your speedo says 60, you will be travelling at 62.1 (but given that from factory it has between 5 and 10% error anyway, it more likely you will be travelling at 59 indstead of 57).

Even the factory option wheels end up being out by a little bit:

Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/km Difference
225/50-16 112mm 316mm 631mm 1984mm 504 0.0%
235/40-18 94mm 323mm 645mm 2027mm 493 2.2%

Speedo showing 60 is really travelling at 61.3.

5% is the acceptable difference, so either of these tyre choices are well within limits without needing to change your speedo gear.

When either the 235 or 245/40/18s are compared to 235/45/17, the difference is at most 1.5% (max 0.9km/h at 60 difference).