How many RPM's you're doing at a certain speed depends on the gear ratio in the transmission and the rear end final drive ratio, not the engine itself.

If nothing has been done other than having the transmission serviced, but now you have a difference in RPM at a given speed (400 RPM difference is significant), I'd suspect you may have a low fluid level or a slipping transmission or even the wrong fluid in the gearbox. Start with the easy stuff, go get the transmission hot and check the fluid level with the engine running.

15mpg is terrible fuel economy unless it's all in-town driving stop/go stuff.