Yea... Something doesn't sound too right, only 8.5 MPG's!!! I really don't know what to tell you... Sorry

I've never trusted the computer when calculating my MPG's..... I use the good 'ol miles driven divided by gallons used, method!

 

I would lose the TB spacer and maybe clean the TB since you will take it off.(Brake cleaner only and be gentle, no carb cleaner).

Try 89 octane and drive 2000rpm or below and see what happens.

 

check your tire pressure. the door jam says one psi pressure setting and the tire says max psi setting. try and get it in between psi. if the door say 35 psi and the tire says 45 max psi get it about 40 psi. this will give it better rolling resistance. also a clogged cat will mess with the mpg.

 

i get about 7 with mine in town but i drive hard i guess on highway i get about 16 i have a CAI and Dumped pipes and mudders so i cant complain