I would be tempted to have the spare the same WIDTH as the front tyres so as to avoid any steering imbalance - a difference in tyre width on the rear where the wheels are just "trailing" is (IMHO) less likely to unbalance the handling.
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Good point, Scanner. I'd not thought about steering effort with using a rear on the front.
The difference between sizes front/rear is to introduce (AFAIK) some oversteer, the rolling diameter being virtually the same, front to back, on the standard tyres.
I would, however, say that if you suffer a puncture, you should always take care in your driving style when using a spare, if only because it's been hanging under the van for years, condition untested! Use it to get you home/ to the tyre shop!
Must say, there's a lot to be said for running the same size all round
SD
