[Superjackson]

Assuming there is no slippage, larger diameter tires make your gear ratio taller which basically decreases your acceleration but gives you more top speed at each gear. It's a really small difference, just like the speedo difference. 2% taller tires will make your speedo show a 2% lower number than before. Speed error is nothing to worry about, it will be consistently off by the same percent.

As for the 295/40/19, I don't think there would be any problem fitting that but you'd have to worry about 255/45/19 hitting the front inner swivels from being too tall.