Back in the late 50's and early 60's we still had a couple of GMC 3/4 tons with 300 gallon gas delivery tanks on them. We ran 7.50x17 tires on them with the 3/4 ton ratio of 4.56 with 248 motors. They averaged 14 to 15 out on the highway running 60. This should still be attainable. Pulling a trailer pulled them down below 10. The pickups originally came with 228's but in those days it was very common to put in new GMC 248 short blocks. We originally installed the 17" tires for weight carrying and the 228's struggled. The 248's handled it fine and the fuel mileage actually went up.