I drive an octy vrs with 205/50/17s conti.Now these are quite expensive

They will be fine on 225/40 and almost exact match.

Circumference Difference:-0.31 in-7.85 mm Leaving a -0.39 % difference on your speedo.

have a check the tube valves, if you're sure you don't have a puncture and is losing air. After pumping the tyres, shake the tube valves, if there's any hissing sound of leaking air, then they are due for change.

but usually, they don't go all at the same time, so then again, it could really be just the tyres.

I'd recommend going up to 225/45/17 tyres. Lots of people here have switched, no problems with sidewall stiffness. They are a bit noisier and I have found they can reduce fuel economy by about 5%. On the other hand they handle better, look better and can often be bought more cheaply as they're a more common size.

225/45/17 are fine. Will give something like 0.03% error on your speedy i.e no error at all really.

I paid

i got 4 x toyo 225-40 -18 for 330 inc....

seemed like a good price....