To check your gear ratio buy the tire method jack up both sides mark both tires and the driveline turn both tires together one revolution counting how many times the drive shaft turns. The X2 thing is for turning one tire assuming the spider gears are .5 to 1 and that is not always so. The best way is to pull the cover and count the teeth on both the ring gear ant the pinion and divide the number from the ring by the number from the pinion.
The fact that you lost performance doesn’t sound good for your fuel mileage as a motor must be running at its best state of tune for its best mileage. The changes that you made should have boosted performance if they where going to increase mileage as the changes you made where small so you have the same motor. The good thing is you should have much better fuel mileage available from the combo that you are running. My neighbor gets almost 30 MPG with his 2004 Corvette
