A 275/40/19 has a larger rolling radius than a stock tyre not smaller so you will be covering a slightly greater distance which each turn of the wheel (you will be going faster given the same engine RPM).
Speedos generally play it safe and overstate you MPH (so it will tell you your doing 70, but actually your doing 66 for example) with a 275/40/19 i found it slightly under reads, so when the speedo tells me im doing 70, im actually doing 73, whilst not a massive issue it can be corrected very simply using VCDS.