1994 TOYOTA COROLLA, gas mileage

Greetings;
Car is a 1994 Toyota, Toyota Corolla, a/c 1.6l engine, automatic.
Recently I have noticed that the car seems to be straining to maintain highway speed and the mpg appears to be dropping. On startup the car sounds loud, almost like there is a exhaust system crack somewhere, but I can't seem to isolate it. This also gives a high noise volume when at highway speeds, but not the roar you would associate with broken system. My problem is that I have nothing to compare the car to! I don't know anyone with this type of car. They all have SUV's. ( for now anyway!)
Question is will a obstucted \ broken exhaust system affect the performance of this type of car? I know on some other cars I had owned that a certain blowback pressure was desireable for proper engine performance.

 

Yes it can, aspecially if it broke before the O2 sensors.

 

Thanks for the reply. That is what i tought but I wasn't sure. The nearest isolation I can make is that it is either at the gasket of the sensor or before it. Actually the system is original and flakes alot of rust when I searched for the leak (engine running, but only just after starting. Don't need to burn my hands! Been there done that! Accidently of course).

 

Look around the manifold (the piece that attaches to the side of the engine). There can usually be a crack there. Of course, make sure it's cool