Fuel economy is not a major concern with interference type engines. Actually, the main issue with these engines is that they use rubber belts instead of a timing chains, and when the belt fails, the engine will continue to run and a piston might impact an open intake valve. Needless to say, this will result in damage to one or more valves and possibly the piston as well.

Also, rubber belts are rated for about 80,000 to 100,000 km. But, failure can occur at anytime and there is no easy way to conduct regular inspection. Timing chains may also fail, but they're stronger than belts and have a longer service life. In fact, unless something seriously goes wrong inside the engine, a timing chain will last for the entire lifetime of the vehicle.