buy your self an[8'x 1/8 "] tape to measure the tires . circle track tricks.... inflate the smaller tire to 70-80 lbs and let it stretch in the sun. it will be larger than you need for a little while but will stabilize to a smaller size. you can measure the tire with the extra air press. and learn what amount of extra time and pressures it takes for your type of tire. this works best with bias ply tires, however ,it will work to some degree on all tires. also , you can add air to one tire and subtract from the other to adjust the steering from the rear tires