Saturday I bought a its a 1983 Silverado 2wd Drive Cab on a 1980 Chevy 4x4 Frame. It has a 350 4bl with a 350 tranny and a 203 case. It also has lock out hubs. The guy I bought it from said it has 2.56 gears so thought this should get pretty decent mileage. I have a few questions. I had the truck in hi with the hubs set to free. I read that if the 203 doesnt have part time kit that without putting it in hi loc or low loc witht he hubs free you wont move? Also even with this truck in full time 4wd and the hubs locked shouldnt it be getting better than 9mpg on the highway. I was expecting around 15. When sitting at a stop light the truck almost wants to die. It never does but it will idle fine then just crash for a bit and then speed back up to normal idle. Based on the mileage and then idle im guessing the carb needs some work. With the take off and the 1st gear top speed i would say he wasnt lying about the gears. Thanks for the replies in advance.  

I have owned a (400/350/NP203) 1976 K-10 Silverado for about 20 years and always got 10 MPG on the highway (still full time case) with stock tires and the stock 3.08(rear)/3.07(front) gears.  When I switched to 31" tires and calculated the correct mileage I got up to 11 MPG.  You will very likely not be able to get 15 MPG with your truck.  If you really put some time and effort into improving mileage you can get up to 14 MPG.  I question the gears that guy claimed it has.  Make a mark on one tire and on the rear drive shaft and roll the truck forward one tire revolution while counting the number of revolutions the drive shaft makes to figure gear ratio.  Also if it is still there, the front differential cover has a tag on one of the bolts with the gear ratio stamped on it.