Perhaps I should clarify what I meant by "legally" - I can find nothing that says it's actually illegal to run a car tyre on a bike in the UK. The bike will fail an MoT with one fitted, so it's been addressed there, but despite looking I couldn't find anything relating to the prohibition of car tyres on motorcycles in the C&U Regs.
http://www.legislation.gov.uk/uksi/1986/1078/contents/made
Perhaps a closer scrutiny is in order. I'm open to correction on this, and even though it doesn't affect me directly, it would be good know for sure. There is no shortage of hand-wavers and outside-toilet lawyers, and even some legalists who ought to know have often been found to spout total BS on other matters.
Over here, the issue is avoided by there simply not being an MoT for bikes, but if I were to fit a car tyre I'd be careful about it, and not ask too much of it, unless and until I was dead sure of its characteristics.
Anyway, my dead-sure recollection of the 60s and 70s is seeing Avon car tyres of 5.00 x 16 size on the back of various chops and they never caused the slightest problem as far as I know. These chops were the fore-runners and design inspiration of the very same Specials we all know here - and guess what size tyre will fit straight on the back of a Special?
At the time, there was no mention in the MoT of car tyre use on motorcycles, and even though it was a bit of a non-issue, there was pressure to legislate about it from control freaks, so it was sneaked into the MoT, under the guise of 'safety', which as far as I can see is nothing but a load of old cobblers.
Unfortunately, there is no easily-available or cheap radial car tyre in that size, so I'm wondering if I may emulate the old chopper riders and simply fit a vintage-sized car tyre of crossply structure - probably no worse off for general grip and roadholding, but it remains to be seen if the same benefits of long mileage can be got from a crossply car rear as from a radial. I kind of doubt the mileage benefits, and if there's none, I'd be inclined to try it out and not bother repeating it. Otoh, if the mileage benefit is there, I'd stick with it.
Here's a UK example of an old-style Avon (or similar)
http://www.therevcounter.co.uk/thre...-the-Dark-Side?p=419986&viewfull=1#post419986
Fwiw, my other ride, a GS850, will shortly be fitted with a 135R15 car rear, courtesy of a Suzuki Cavalcade rear wheel, once I sort out the bearing / axle match-up.