Like a rock! I know that is the Chevrolet slogan, but this GM family member is all that and a bag of chips. The 1978 ¾ ton single-cab long bed GMC Sierra with a 454 V8 and I believe 3.83 rear-end make for a load pulling fast moving vehicle. (Think of Rizzo from M*A*S*H saying "Veeeehicle"). My family purchased this truck in 1980 and it quickly became a workhorse on the ranch.
Today Chevrolets and GMCs are close to identical with the exclusion of their luxury packages, grills, and other exterior trim details. This wasn't the case in around 78. The drive train on the ¾ ton GMC is the same as the 1 ton Chevy trucks.
Gas mileage: Here is the good news, it doesn't go down too much when you are pulling a load or in city traffic (I don't know why you would ever consider driving this into the city). The bad news is on its best day it only gets 7mpg. No I didn't leave out a digit SEVEN MPG. OUCH!!!
This truck has outlived its life for benefit of anyone on the market, but it has served its purpose well.
My rating was for when it was new and not how it would be on today's market.