Biggest tyre that will fit 19" rear wheels

[lotus49]

I have been running 295/35/19 MPS4s for the last year. They have lasted surprisingly well but are looking a bit worn at the rear so I'll need some new ones soon (if I ever get to drive my car further than the shops that is). I'm sure I'd have traction issues no matter what tyre I put on these but I'd like to put 305s on if possible so I have two questions.

What size do I need? I can see that MPS4s come in 305/30/19 or 305/35/19. I have always been a little hazy on using different tyres and neither of these tyres will have the same sidewall size as my existing tyres. Does this mean my speedometer and odometer will be wrong? 295 x 0.35 = 103.25, 305 x 0.30 = 91.5 and 305 x 0.35 = 106.75 - does this matter?