HI folks,
Am I being stupid or is there a glitch where tyre sizes on cars are often restricted to silly sizes? For example I designed a hothatch type model the other day but was stuck with 155mm width tyres on the front and 185mm maximum on the rear!
Any know why?
Steve 
First off, the front tyre width is restricted by your flare width. Go back to the section where you stretch out the body and extend the flares so they fit the tyres.
Check if the flares limit the rear tyres as well. If that is not the case, tell me what year you are building your car in.
Not only that, MacPherson struts tend to limit tire width as well (in the game). Also, check the tire diameter - you need to increase that in order to allow for wider tires.