Festiva MPG problem

HI,

I've got an 1989 festiva 5 speed manual that I've been working on. It was only getting 25 MPG instead of the 40+ it is supposed to get. So I replaced the.

Air filter
fuel filter
filled tires,
flushed radiator changed hoses

then discovered the thermostat was missing. It has run for over a year without a thermostat. So I put in a new one.

Now the car gets 31 MPG but still only heats up about 30 percent. I also don't get heat. The car blows air (not AC) strong, but no heater.

Any ideas on what might be ruining my MPG?

Thanks

Cris