265 65 17
The first number is the width of the tire in millimeters: 265mm across. The 2nd number 65 is the height of your sidewall (how tall your tire is). 65 means that the height of your sidewall is 65% of the width of your tire. That means that your sidewall is 265mm x .65 = 172.5mm. A second number of 70 would mean your sidewall is 185.5 mm. The last number is obviously the size of your wheel.
As you can see that since the sidewall number is a percentage, if you increase your width from 265 to 275, you will also increase the height of your tire, so to stay close to stock height, you would drop your sidewall ratio from 65 to 60.
Also remember, that the larger the sidewall, the slower your speedometer will be (you will be traveling faster than the gauge states), and your mpg meter will show a worse mpg than you are actually getting. The good news is that your odometer will be slightly slower as well, so you can put a few more miles on the car, off the record.
i hope this helps.