Fluctuating Gas Mileage

Well I posted about a month ago about bad gas mileage. Since then I replaced my fuel filter and did a fuel system cleaner. I went a few full tanks and got about 240 miles on that tank, most was city driving, I filled up yesterday and I have used a quarter tank all on highway and I have only gone 38 miles, so that seems a little odd to me. I am going to do my spark plugs first and clean my TB, I am waiting for injectors because they are so expensive. Any other ideas?

By the way all I have are 31s, 3" Rustys, I have the K&N filter, ran the system cleaner. Its weird, I have seen some XJs on other sites with 37's that get 16mpg CITY, and city i get about 10.

stock gearing with 31s is a big part of the reason

im assuming the guys with 37s and 16mpg city MUST have regeared to at least 4.88 or better?

Im definately not a gear expert, as Im still 31s with 3.55s, and im currently saving up for 4.56s (to go to 33s ) and front and rear lockers all at once