My O/T 1-ton dually with a TBI 350 and NV4500 5-speed will do 13 MPG or so at 65-70 on the highway. And it must have a 4.88 or 5.13 in it, you could pull a house off the foundation with the granny low in it. A 2-carb intake would probably help, the most fuel-efficient carb would probably be the little 2bbl they made out of the Rochester quadrajet in the late '70s, if you can find a pre-computer one with the 2bbl casting (I have one here that uses the 4bbl casting, with the secondaries blanked out). That and a gear change to get the RPMs down would probably do it. The rest is just aerodynamics. It sits high and believe it or not the air passing under hurts you some, an air dam or even lowering the front some would help to an extent. That's why land speed cars often run an air dam right down almost to the ground.
Looking at comparables of known vehicles I would say the expected range for your truck @ 70 MPH would be 13-17. A 78 250 1/2 ton 250 was epa rated at 17 Hiway. Here is a simulation of fuel burn for a 70 Blazer with a 250. Some things that can make large MPG differences. Power piston position is the biggest. Even at 70 cruise you should be on the lean step of the metering rod which is like .080 compared to .048 which all use for the power part. On a Monojet full power is controlled by main jet size and cruise by the metering rod size. Being on the rich step at cruise is definitely a couple MPG. Next is to be sure you are around 54 degrees advance at 3200 with vacuum applied to the advance (4 plus 30 mechanical plus 20 vacuum). Those two areas could yield big results. The Monojet power piston is a bit unique as it uses both linkage and a spring operated vacuum controlled piston. In the rebuilding process that spring is often lost, replaced with the wrong spring etc. When we got the lean off idle in the 70s I found I could destroy mileage by making the q-jet power piston move at 6 inches instead of 12. Guys would say just make it run right, then come back in and say I had killed the mileage by a couple MPG. And at 80 cents a gallon they couldn't afford that. Hopefully one or some of both of these can get you in the 15-17 range. After that it gets harder as many have attested. If you could find a clifford intake and a small two barrel (Ford 2100 or a Dual Jet) properly jetted you might get more power and economy. A 292 exhaust manifold from a 40-50 series and 2 1/2 exhaust wouldn't hurt.. A 194 head with the lump port mod would be the piece de resistance, 9.5 compression and much higher velocity 20 MPG here we come!!! OD it and could it be, 23
70mph=higher rolling resistance,aerodynamic drag,parasitic driveline losses. 3000 rpm is not a good rpm for economy either. The truck has the aerodynamics of a brick. Back when that truck was designed,50-55 mph was a good speed for a truck and there weren't interstates everywhere to run 70 I'd bet if you run it about 60mph-you'd see a big increase in fuel mileage.
We have a 65 LWB C10 250 three speed and 370 gears. It has a newer 1980,s 250 with integrated head and electronic iginition. I gets about 18 MPG but we dont usually drive over 60MPH. the engine is the one bbl version of the integrated head with the single exhaust. Ive never had or seen a 2bbl integrated head that wasnt cracked. We also have a 66 GMC short bed with a 68 250 and factory powerglide. I has hedders offy intake bored .040 , RV cam & electronic iginiyion. Its lucky to get 10 MPG. Old Wolf
I used to shoot at a range where the had old cylinder heads hanging from poles as swinging targets. Every one of them was one of those was those integrated 292 six heads, they got them from UPS as I remember.
From experience with my 48 with a 250 in it with a three speed driving a bit slower and taking a bit longer to get there helps a lot with gas mileage. We had a very similar truck to yours around the farm for a number of years but I never did know what mileage it got. It belonged to mom's companion and he drove it everywhere for a long time and never complained about gas mileage but the man never broke a speed limit in the 80 years he was alive.
Yup. 3:73 = about 13mpg with a 1 to 1 final gear trans is about right. That ratio burns more gas over 60mph. 3.08 gears is what you want for great traveling gas mileage & the 6 to 1 first will still get you out of the hole but can brake driveshaft u-joints when lighting up the rear tires. I'm very happy with 3.73 to 3.08 change in my 4spd 68 Chevelle & recommend. Put a t5 in it. Or a TKO 5speed. A t5 is kind of weak & might break, but you could buy & go through 4 for the price of 1 TKO. Overdrive Auto trans would be cheap & easy.
i drove a 67 camaro with a 250 and powerglide/3.08 gears and it usually got 22-23 hiway and 16 or 17 in town. never cared much for that engine. probably should have put a 283 in it--they were cheap and plentiful back then.
It's lame, but that's why I have a beater Toyota station wagon that I've stripped a bunch of parts off of to save weight. 36 mpg on 91 octane which means that I have more $$ left over to waste getting 11 mpg's when I want to drive my Chevy. I drove old cars every day up until 2 yrs ago, it's just not feasible anymore. One thing to also consider is whether or not you are running ethanol gas, you will get a lot less mileage with this new ethanol bull$hit than with the old conventional gas.
One would think it would be a little better,my 66 F250 that has a 460 got 13 on a recent trip. A little tuning and a electronic ignition will help along with a o/d trans or taller gears.
Back in 81 I picked up a 67 C20 that had a 250 and 4 speed plus 4.10 gears and it got about 16 to 18 on the highway but that was in the days of the 55 mph speed limit and probably much better gas.
Six cylinders have their own charm and they're super dependable, but a lot of guys come to the same conclusion as you have. They just don't get the gas mileage that you would expect from a small engine. You can definitely make some changes in the carb, ignition, and exhaust, but when it's all said and done, you can build a small V8 with similar bolt-ons and get close to the same mileage, it'll make more power, and it'll sound better. If you decide to keep the 6, I would definitely consider an overdrive transmission. Of all the bolt-on changes you can make, the one that will give you the most mpg gain is the addition of another gear (or two).
the 373 isn't bad but I would go with a overdrive trans either 5 spd or 700r4 auto that will get your final drive closer to the 2000 rpm range.
My '65 C10 has 327, 3.73 gears Sm465 and a Ranger III od box. Granny gear and overdrive twin stick setup. It's been a daily driver since 2010.
My son and I had almost twin chevy pickups with 6 bangers. Mine was a 68 stepside with a stock at that time] 292, and a 55 chevy 3 speed overdrive trans. The kid's was a 69 stepside with a 250, 3 speed non-O.D...we both ran 3.73 gears. We took a trip together and left Omaha on I-80, westbound. It was totally flat and we ran together in our pickups at about 65 MPH. I could hear his little 250 buzzing like a bumblebee while my 292 lugged along at about 2200 in overdrive. I was thinking my fuel mileage would be much better than his but when we filled up our tanks, we both put exactly the same amount of fuel in our trucks! We both got a smidgen over 17 MPG.
Only a tach, no speedometer. It's about a 500 rpm drop. on a 1/2 hour trip to work, It seems to have gained an extra round trip on the tank
Have a 65 Chevelle 4 door wagon. 250 with flat tops Lump port head Headers Holley 2 bbl 270 advertised duration cam 5 speed no overdrive 2.56 gears Gets 22 MPG going 80 on flat land @ 2800 RPM And plenty spunky.
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