just got the Rodeo, 2 tanks so far around 18.5 mpg, with about 75% hiway driving, I was hoping for better???

Depends on the heavy-ness of your foot.

I have the 1st gen. which I believe weighs a little more then the 2nd generation, plus I'm running one tire size bigger = I average 17-18 mpg in town, and generally 20-23 open highway.

I must note however, I am not a lead foot by any means, and my personal preference is to not go faster then 70 mph open highway.

Funny thing for me, I've noticed, is I get better gas mileage climbing the mountains then flat open highway.

If you just got the Rodeo, there's a few things I'd check - air filter, tire pressure, spark plugs, etc.

These can drastically effect gas mileage. Another thing that will help as well, is add another 2-3psi to each tire if you're doing a lot of highway driving.

The placard on the inside of your drivers door should give you the specific psi for front/rear tires for your model year, add 2psi to that and see how that goes.

I get about 21-22 but I also have an amigo.

check out my FAQ link below for tips on how to improve your MPG

best way to ensure good mileage is to keep up your maintenance..pcv, fuel filter, air filter, etc., just overall making sure everything is running smoothly and topped off.
if you have a 4x4, then i wouldn't complain too much, if you have a 2wd then you should be getting a little better. i have an 02 2wd and i get around 20 with a pretty light foot on the highway. i've found that my mileage drops DRASTICALLY (more so than with any other car i've ever owned) with a heavier foot. City driving I get around 17.5. I also keep my air pressure around 8 psi higher than recommended (which is around 26 i believe).