91 celica ST transmission swap?

[mazdamx640]

So I picked up this car almost a month ago and was amazed by the fuel millage as i've never had a car that would beat 30 mpg and the celica gets about 38. I've done a few things to it like new spark plugs, wired, cap and rotor, a/c delete, power steering delete and alignment so I'm hoping it will do better next time I fill it up. But there's still something holding it back the transmission it revs way too high down the highway and that's where I do most my driving and might be able to swap one of the transmissions from a model with higher gearing any ideas ?