I'm getting and average of 18mpg when 95%-98% of my driving is 5th gear @70m/h

shouldn't it be more in the range of 22-24

 

that doesnt seem that low. it could be a cause of many different things.

the way you drive
the condition of the engine
fuel issues (bad filter, semi clogged injectors)
old sprk plugs.
maybe even something like worn out wheel bearings.

either way. 18 isnt horrible.
just ride close to semi's, youll mileage will go way up.

 

lol

 

I have a 90N/A and i Drive 90+ on the highway. i stay around 3000rpms and i get right at 18mpg also. i did a tune up and nothing changed. motor has 50,000miles on rebuild.

but ever since the rebuild i have always got around 16 to 18 to a gallon

 

Do you have the stock catalytic converters? How about the air filter, change that recently? I get 21 highway with my crappy automatic at 75+ mph, but I have the 3.9 rear.

 

yep still have the stock cats but its probably the the weel bearing that i need to replace but don't have the money to because of how much of my money goes to gas