wheel diameter has to do with gearing. the car is geared for 16's at the most. using a 14" rim shortens the gear ratio to the ground, allowing the wheel to spin more per revolution of the trans.

you may be able to fit 245's on the rx-7 rim but tire size has to do with matching the tire to the car not the wheel. putting a 245 on that rim and putting on an eclipse would be begging for wheel well rubbing and chewing your tires up in 2 months. that's why i said 225 50's are the largest tire you can use with those wheels.

225's are the absolute widest you can go on our cars. any higher is going to rub, roll over in a turn or need more air pressure than you can safely put in the tire.