Tire width on 8" rim

Ifyou peruse the catalog you will find the the tire manufacture lists a checking rim of 70% which was used to achieve the branded size. But the branded size can be rounded up or down to the nearest 5mm on the 5s.
For example a 275 can actually be anywhere from 270.1 to 279.9 both round to 275, Because there are no 280s nor 270s.
The checking rim for a 275 is; (70%x275)/25.4=7.58 Which rounds down to a 7.5 rim. But good luck finding one. If they used an 8" then the sidewall cross-section will grow by about one-half the increase so .2 inch/5mm in this case. So, your 275, if it ever was a 275 will now be a 280. Or in reality anywhere from 275.1 to 284.9.. Which maths out to in the range of 10.8 to 11.2

I highly recommend, for street work not to install 275s on a 7.5 rim; you will not be able to run a low enough air-pressure to not burn the centers off. In my experience 8.5s are about right; and for sporty driving 9s would be better. These will allow you to bump up the pressure and better control sidewall roll-over in the corners.