My part-built car came with a set of Ford RS alloys with the following markings:

6Jx13xCHx19

6 presumably means 6 inches wide
13 is the diameter
What do the other things mean? Is it anything to do with offset?

Also, the tyres are currently 185/70/13 on the front and 205/60/13 on the rear - if my calculations are correct, this means that the diameter is about half an inch bigger at the front

a) am I right in the calculations?

b) would I be better off with the same size tyres back and front?

c) since the rear tyres look about the right size to fill the arches nicely, what would be the effect of have bigger tyres on the front as well?

Thanks

[Edited on 17/2/05 by Humbug]

Your calcs are spot on I get your sizes to give 0.5 bigger dia on front.
Ideal tyres depend on weight distribution I don't know your spec but if we assume it weighs 550kg and carries 30kg more on the rear wheels the rear wheels carry 290 kg (53%) and the fronts 260 (47%)

As not all tyre section size is is contact with the road deduct 50mm from the tyre section size for calcs 205-50 = 155mm
Ideal tyre section width for front = (155x47/53)+50 =187 aprox 185 same as the 185/70x13 already on the car however I would change to 185/60x13 so the tryre characteristics front and rear are better matched. This would give a tyre dia of 21.8" on the front lowering the front by roughly 0.75".

[Edited on 17/2/05 by britishtrident]

The 19 is mm of offset as you correctly imply. Ive found that offset to work well with cortina based front ends.

You are also correct on the diameter and width comments.

When calculating tyre rolling rads dont forget they deform, gut feel tells you that a lower profile tyre, which has a stiffer wall, has a higher springrate and as such will deform less under the same applied loading.

For example i dont know what you calculated for a 185/70/13 but try about 280mm for size.

You may find the difference in front to rear wheel radius is less than you think.

I dont know what power and usage your planning but your better to choose you tyre sizes based on that rather than what looks right.

Once again BT, i would agree, 185/60 front and 205/60 rear is common pair, if youve got the power to usethe 205's

http://www.domsat.co.uk/pages/wheels.htm

look at this link, seems to contain some usefull info

Cheers

Fred WB

[Edited on 17/2/05 by Fred W B]

[Edited on 17/2/05 by Fred W B]

Thanks all for the various responses