I have a 90 toyota pickup 22re 5sp 4x4, I bought the truck off my best friend and he was getting 24mpg, with 33 12.50 tires, shorty after i bought it i put 31s on it and i checked my gas mileage and it was only getting 18.5mpg no matter how i drove it hard or easy, i've change the air filter, fuel filter, clean the throttle body out, check the injectors, but my questions was it that normal gas mileage or should i be getting better and what should i check to improve it? I would appericate any help being that gas is going up and up. Thanks

I am experiencing the same issue.

I know it's been a long time since you posted this, but here's what I'm thinking about the whole issue. Hopefully it helps.

I'm assuming since your buddy had 33" tires on that 22re, and was getting mileage like he was, he probably had 4.88 gears or higher. 4.88's are a good gear for 33" tires, but if you have 4.88 gears, and put 31" tires on it, you're probably running the engine at to high of RPM, which may be causing you to have decreased gas mileage. I have the same 90 4wd. Not too long ago, I had the stock 4.10 ratio, with 31" tires. I got around 15-17 mpg. I swapped my 4.10 gears for 4.56, and it jumped up to 21-23 mpg because the engine didn't have to work as hard(Not to mention had more power). There's a fine line between having too much gear, and not enough for the tires you're running. Generally speaking, vehicle manufacturers try to set up a vehicle with a gear ratio that is optimal for power and economy for that tire size. So, your best bet is to stick with a power ratio that closely matches the stock one. In a toyota's case, it happens to be 4.56 gears with 31" tires. So, just an idea to check in to.