ok guys, i orderd ba xr copies off ebay 18 " by 8" now its time for the hard part, i need tyres what size must i get to keep rolling dia the same? my 15" steelies are 205/65 am looking at some at the moment are 235/40 can anyone do the maths and tell me if these will be legal or do i need tosave more and get diff ones thanks alot : )

Stock Tire - 205/65R15 >Search Tires Tire 1 - 235/40R18 >Search Tires
Section Width: 8.07 in 205 mm
Section Width: 9.25 in 235 mm

Rim Diameter: 15 in 381 mm
Rim Diameter: 18 in 457.2 mm

Rim Width Range: 5.5 - 7.5 in
Rim Width Range: 8 - 9.5 in

Overall Diameter: 25.49 in 647.44 mm
Overall Diameter: 25.40 in 645.16 mm

Sidewall Height: 5.24 in 133.09 mm
Sidewall Height: 3.70 in 93.98 mm

Radius: 12.74 in 323.59 mm
Radius: 12.70 in 322.58 mm

Circumference: 80.07 in 2033.7 mm
Circumference: 79.79 in 2026.6 mm

Revs per Mile: 816.0
Revs per Mile: 818.8

Actual Speed: 60 mph 100 km/h
Speedometer1: 60.2 mph 100. km/h

Speedometer Difference: - Speedometer Difference: 0.343% too fast
Diameter Difference: - Diameter Difference: 0.36%


this is from this site
http://www.1010tires.com/TireSizeCalcul ... ion=submit

thanks so muchmate, so does this mean it will be legal???

Not sure. some states say you can only go up 2 inches from standard.
it different all over oz. i'm sure someone on here will know.

just reading over them stats and they should be sweet as the rollin circumference is a fraction smaller to try for a spot on possibly 245,40r18

You want your rolling radius to be the same..

ie.
a 205/65/15 tyre calculates as...
(205mm x 65%) + (7.5 x 25.4) = 323.75mm rolling radius

The first bracket is the tyre wall height, the second bracket
is the rim radius in mm, ie 25.4mm = 1".

thus an 18" tyre to suit would be....
a 235/40/18
(235mm x 40%) + (9 x 25.4) = 322.60mm rolling radius

Thats the way the maths is done.

Legality should be checked with the road and transport authority
in your state, but you should be OK when you retain your
original rolling radius figure. However, insurance companies
do have their own regulations as well, so check with them also.