I have a question about gas mileage for my 1992 Ford Tempo?

I recently bought a used 1992 Ford Tempo that had a redone engine about 70,000 miles ago. I am getting horrible gas mileage on this car. I changed the air filter, fuel filter, and did a fuel injection cleaning. When I drive at 70 MPH the engine revs up to 3500 RPM and I cannot find any overdrive en ANYWHERE in the car. I have had this one friend tell me that the reason my gas mileage is bad is because it is not kicking into overdrive and that I need to take it to a mechanic. I don't want to take it to a mechanic if I don't have to because it will cost a fortune. Can anyone give me advice? Do you think it is the overdrive? Is it normal for a car to rev at 3500 RPM at 70 mph? Any help would be appreciated. Thanks...