weird stuff with my zj

[97zjnoob]

have a 97 zj with the 5.2. Ive had around 6 months and love it. When i got it it had about 210k miles on it and no issues that i knew of. I consistently got 20 mpg. I changed the plugs, wires, cap and rotor, oil and transfer case fluid with in the first month i had it and my mpgs went down. I figured it was from my driving habits. I recently changed from 16" to 15" wheels and went from 225 65 16 to 235 75 15 and noticed a large decrease in mpgs. From 18 to 16.5 mpgs. I changed my diff fluid to 75-90w synthetic and lost another mpg. Why would these changes cause such a dramatic decrease in mpgs?

Also my fuel guage will randomly read empty after i start the truck. Thanks in advance for any advice or helpful tips!

 

[69jeepcj]

If you take your 225 16 and 235 15 and place them side by side and roll them one revolution you will see that the 16" tire will travel a farthur distance than the 15" tire, which will help MPG on the highway. Droping down to 15" reduces the distance traveled on each revolution, so mpg decreases.
Dont know why changing oil, plugs, wires, cap and rotor would change things unless using the synthetic made a differance.

 

[97zjnoob]

I did use full syn on all my fluids. I know my rear diff was a little low on fluid but my front seemed full. But i dont think its ever been changed before i did it. I did have my wife pace me in her truck and my speedo is a little off too with the tire change. About 3 mph slower than what my speedo says. If i increase the tire size without changing my wheels can i gain back that 3mph?