looking to buy alloy wheels for mk indy. 15" have been suggested rather than 17"but what size of tyre to be able to use standard sierra speedo and standard wheel arches

I've fitted 15" 195 50 Yokohama AVS Sports on 6.5" alloys, see my photos section. The speedo was ok at SVA - it reads about 10% fast against a GPS.

I had a chat with Grant at Luego re Speedo ratios.
They have fitted an electronic Speedo which can be programmed simply by driving for 1 mile and then pressing a button on the Speedo. It then knows how far it has gone and can therefore tell how fast you are going exactly!

For the SVA you could use a donor set of wheels just to be on the safe side.

Then a tidy set could be stuck on?

If you are using a type 9 box and the speed comes from there Caterham can supply you with a different plastic gear to go in the gear box. I dont think it costs much either!

Or you could just work out what rolling radius you need with 15" wheels to keep the same as the original Sierra.

Email me if you want an Excel spreadsheet that'll help you do this. It'll also give you speed and revs for your tyre/wheel choice if you know the various ratios i.e. gearbox and back axle.

The best bet though would be to get a programable speedo so you don't need to worry. Then you can get the wheels/tyres you really want.

Try this handy calculator - you can save the page and use it offline too!

Tyre Size Calculator