4th gear vs 5th gear 92 vx

[Nchakos]

I have a 92 vx with 146000 miles on it ..I commute 100 miles to and from work (200miles per day) ..I reset the trip meter and check my mileage at each fill up..(ive been doing this for a year or so ..i get 34mpg)
I bought this particular vehicle because it supposed to get 56mpg.
I bought it a year ago with 12000 miles on it

at start,gas mileage 34

I tuned it up
new wires, distrubuter cap , plugs
new o2 sensor
new shocks
valves adjusted

after all that, my gas mileage 34

THEN THIS
I noticed the past week that if i leave the car in 4th gear (never shifting into 5th) that i get better passing performance on the freeway ..may average speed is 80mpg..
since not shiftin into 5th gear...i am getting 46mpg!!

could something be wrong with my 5th gear..?
I dont understand this .
can anyone explain what whould cause this..?

thank u

[BullShifter]

If your try to accelerate at low speeds in 5th gear, then yes the engine is gonna burn way more gas than if were in 4th gear. Just because the RPM's are higher doesn't always mean MPG is suffering. 5th gear is over-drive designed for cruising not accelerating.