Anyone know what the proper tyre width is for a 9" wide wheel? According to the Nankang catalogue it's 255.
I had a 255 on them, but Bob Janes put 235 on, and the guy swears they're fine and won't throw the speedometer out.

I agree with 255 or even 265 on a 9" wide rim. Are they really 9" wide though, on the rear of the wagon are they? The width measurement is between the flanges, where the bead sits, it does not include the flanges.

235 is good for 8" wide rims. A 235 on a 9" wide rim would be pretty stretched, leaving the rim and sidewall very vulnerable to damage. There would also be little 'give', for example, the car would snap into a slide giving little notice, which can be especially dangerous in the wet.

There Simmons FR18's AFAIK they are 8.5" on the front and 9" on the rear. Bob's reckons you can run 235 on up to 9.5" - seems extreme to me.

ideal max tyre sizes are..... not sure on minimum sorry

6".... 225
7".....235
8".....245
9".....255

As the rim gets 25mm wider, you only allow the tyre to get 10mm wider. If lower profile tyres are fitted as the rims get wider this would be true, but if not it would be false.

I've seen some pretty extreme stuff in the catalogues. A quick check reveals a 235/40R18 as having a rim range of 8.0 to 9.5, so Bob is likely right.

It comes down to you as the user. Do you want snap handling, vulnerable sidewalls, very poor dirt road performance, and vulnerable rims? Over time as people have seen rims getting damaged they aren't going for such narrow tyres anymore. Ford for example put a 205 on 7" rims on the EB, but to my knowledge has never done so again, using 215's on 7" rims instead. And 235's are almost universally used on 8" wide rims, to put them on 9" wide rims is asking for trouble. How much are the rims worth? How much do you want to hate gutters?

Mate, fitting a 235 to a 9" or 9.5" wide wheel will not promote snap oversteer, it stiffens the sidewalls, allowing less tyre roll and therefore more predictable handling, and twr7cx, both rollin and BI6TIM has 235/45R17's fitted to 17" x 9.5" Koya Drifteks on ED GLi/Futura's in a +18 offset

Tyre roll gives a slower transition from grip to slide, giving more time for the driver to respond. The ultimate adhesion level is lower, but the adhesion curve is softer.

A 9" wide rim is actually about 9.5" when you take into account the flanges. That translated to mm equals 241.3mm, which is wider than the outermost sidewalls of a 235 tyre, and wider again than the tread. Any side contact with a gutter will thus be the rim.

The tyre will also flex less over undulations in the road. Rippled bitumen and road reflectors for example may result in only a portion of the tyre contacting the road, meaning less grip.

Just because someone has done it doesn't mean it's a good choice, but again, it depends on the usage. If the car is mainly a track car for example maybe that would be a good choice, maybe not though. Maybe better to have a second set of slicks?