Yep - I've changed from the stock 255/35/20 front and 295/30/20 rear on the XKR 75 to 265/35/20 front and 305/30/20 rear, i.e. a 1cm width increase on both. I did the fronts first, then the rears a few months later - no errors or warnings while they were 'mismatched'. There is a small increase in rolling radius, so my speedo is now slightly more accurate than standard.

 

CurrentNewDifference
Sidewall Height:3.6''4.2''0.6''
Section Width:12.01''12.01''0''
Tire Height:27.2''28.4''1.2''
Tire Circum:85.45''89.22''+3.77''
Rev. / Mile:741.49710.15-31.34
If your speedometer reads 65 mph, you are going about 67.9 mph

 

This site has a useful calculator that will show you roughly what the tyres will look like on your wheels, and also give you the change in speedo reading:

Visual Tyre Size Calculator