Bad gas mileage

My 1990 Corolla LE 5spd has had a fresh tuneup including wires, regular oil changes, new tires, good brakes that don't drag, and idles like a champ.........smooth, but my mileage is down to 32-34mpg from 45mpg. It seems to me that the engine is working harder at 55mph than it used to, and I say this because the exhaust note sounds deeper to me, but at the front of the vehicle.....not out the exhaust pipe. I thought that my catalytic converter might be plugging up so I took it to a muffler shop. The mechanic had me pull in and rev the engine while he checked the amount of exhaust flow exiting the pipe, and said that I had plenty of flow. Is this a valid way to check the converter? I also had a friend recently tell me that my timing belt was probably stretched and that was causing the decrease in mileage. Is there any truth to that? The car has 173k miles on it and the timing belt was last changed at 62k miles. Any suggestions would be greatly appreciated as gas won't be staying around $2/gallon much longer with summer driving season right around the corner. Thank you.