Which size tyres for IVA

[Peter-C]

I'm a bit confused over what size tyre to fit. I'm using the sierra gearbox and speedo and I want to make sure that the speedo readings are within IVA spec.
The sierra original tyres were 185/65 R14 and I am looking at Toyo T1R's. The perfect match are 195/55 R15's at Approx £60 each whereas 195/50 15's are £36 each.
Will the cheaper ones be ok. I assume that the speedo will read less than actual.

Cheers

Peter