This can mess with your speedo and cruise control apparantly.
However, check out this website though,
the 245/265 set up has a difference in circumferance of
so 27mm or just over 1" in overall wheel circumferance in favour of the fronts.
Despite being wider the rears are actually smaller in the standard set up, and turn more often than the fronts as a result.
if you change it to 245/275
being a difference of 2mm and is a lot closer to the size of the fronts and only 25mm/1" over the 265 .
The difference between the 265 and 275 would be a speedo error of 1.18% so as long as your not getting any cruise errors the difference on your speedo wont even be noticeable, in the next 100,000 miles you'll be out by about 1000 miles.
But, this only applies to new tyres as they wear down the circumferance will reduce, meaning that no speedo will ever give a truly accurate reading of the mileage or speed, on any car.