What are you all getting for mpg? The vehicle is a '64 Chevy long bed step side. PO put 265/75 x 15 tires which are 30.6" tall on the stock rims. The trans is a SM420 granny 4-spd. The rear axle is 3.73. Engine has new plugs, points, cap, rotor. Maiden voyage last weekend, drove 800 miles to LA and back. Engine used 60 gal. of fuel. Engine used 1/4th quart of oil.Timing set at 4* and dwell set at 32*. New Rochester 1 bbl carb with properly adjusted lean/rich and idle. ONLY got 13 mpg at 72 mph/ 3000 rpm. This truck is my first Chevy six. Is that all I can expect?
I had a 250 in a '66 chev short bed. I got it up to about 22 mpg with a lot of tweaking. I used a heated, vacuum bleed vapor carburetor piped directly into the intake manifold. I kept the speed down around 60 to 65 mph. This was back around 1977.
Is the 800 miles used in the calculation from the odometer? IF so, is it corrected for the tires and gears? With taller tires than stock and uncorrected odometer, you go farther per odometer reading i.e., if the odometer says 800, you have gone further by the error ratio. As I remember from the day, Chevy trucks (or Fords for that matter) werent mileage kings.
Without some modifications, I don't think you'll do much better than that. Some sort of overdrive, a carburetor other than the Rochester B, an H.E.I. from a newer 250.....actually, a well tuned small 4 bbl on an Offenhauser or Clifford intake and a pair of headers would boost both power and mpg.
I get about 18 from my 250/TH350/3.08 combo. I am using HEI and headers, but motor is stock. Are you sure you have 3.73's? Truck would have come with 4.11's from factory... not that the small difference would significantly affect actual mileage, but you may not have been doing 72 since tires are taller than stock and rear gears might not be true.
I had a 65 C10 250 Rochester 2barrel t5 stock 4.11 rear,new ign,fuel pump. filters hole 9 yards. It would get 25+ and I miss it.
jesus 60 gallons of fuel? No bueno for california with 5.00 per gallon, but the bright side is that it didn't burn a ton of oil. The oil seems decent for an 800 mile trip for a 50 year old vehicle with what I am assuming has tens of thousands of miles. I have owned newer cars that burned more than that in that kind of trip. for an 80 mile trip taking my new truck home it had a misadjusted carb, huge vacuum leak, not timed properly, old plugs, plug wires, cap, rotor, etc. I used right around half a tank of gas in it. I can't remember how many gallons the tank holds (15?) Also, I was driving between 55-65 the whole time. I agree with everyone else on the speedo gears
I built one 250 six in my life thinking I would get better mileage than a SB. Big mistake, ended up with not much better since I had to be on the throttle more of the time, no where near 20mpg and no power. Drop in even a 305 and you will get better mileage and more power. Either engine I would put in a 700r4, that cruise RPM of 3000 is a mileage killer, want to be closer to 2000.
Is that a 3/4 ton? If so the mileage is par for the course. Cut your speed to 55 and you will get 50% more MPG, no shit. I had a 71 Chev short box stepside, 250 and 3 on the tree. The lightest, best mileage Chev truck you could get. 21 MPG on a trip if I kept it below 55. A boxy truck body, screaming gears and six cylinder power means going over 55 kills your mileage.
that with 4 speed and 3.73 gears is going to be about as good as it gets. 6cyl were notmilage queens especially in truck tom
That's not very good at all. My 460 with automatic gets 12mpg on the highway. My 1967 F-100 with three speed 240 6-cyl and leaky carb gets 15pmg, my 1953 Chevy sedan with 292 6-cyl and TH-350 got 16mpg on the way to austin. Were you hauling anything? Get a vacumm gauge in the dash, I never noticed how barely backing off the throttle could change the vacumm so much until I got my Galaxie and it had the old "economy" gauge in it. Back off a little... it still maintains speed, but the vacumm "economy" is way higher.
You are fighting aerodynamics, for one. There comes a certain mph where that engine just has to give it all it's got to keep the truck moving. 70+mph is a tough hill to climb with that combo. By maximizing fuel/exhaust flow and ignition efficiency you can gain a lot, maybe have a 20mpg 2-lane blacktop cruiser. But to get 20mpg at 70mph? You would have to do so much work to get that out of that package, it probably would cost a lot more than any gas savings.
I have a nearly identical '64 C10. Mine is a short box fleetside 1/2 ton. You didn't mention if your's is a 3/4 ton, if so dragging those extra heavy parts around will hurt you. I have the 292, but, everything else is the same. I have 30x9.50x15" inch tires, a SM420, and a 3.73 rear gear. My 292 also has a 400cfm Holley that was hastily set up at best. On my 1500 mile trip from Washington to Arizona http://www.jalopyjournal.com/forum/showthread.php?t=707062&page=5 I averaged 14-15mpg. I would expect that my 292 with a 4bbl Holley would be a thirstier setup than your 250, especially considering I had 1500lbs worth of stuff loaded in to it. However, I wasn't pushing my truck anywhere NEAR as hard as 72mph. It feels wound pretty tight at 65mph, so, I personally wouldn't push mine that hard for that long. As far as the rear gears: If it came stock with that SM420, there's a very good chance that the 3.73 rear gear is stock. Trucks didn't all "come with 4.11s". The 3spd setups most commonly came with either 3.07s or 3.08s (can't remember at the moment) according to my service manual, making 3.07/3.08 probably the most common gear setup out there. If your rear end is a posi unit, and you're certain it hasn't been tinkered with, it almost has to be a 3.73, that's the most common gearing GM put the posi units on, again, according to my factory service manual. I'd try just calming down on the highway speeds until you find a more suitable rear gear. These trucks weren't built in an era where 70mph freeway speeds were common. I'm going to be upgrading my truck to this unit: http://www.advanceadapters.com/downloads/Ranger_Overdrive0.pdf Others will tell you to go to a T5 or NV4500, and that is a fine upgrade, BUT, if you want to stick with the SM420 (as I want to) that unit is about the only upgrade available. I expect to get in the 20mpg range once I get my gearing where I want it, and the overdrive in place. With the gearing of that SM420, I think with a 3.55 or even 3.08 and the torque of a 292 I feel I could still "get out of the hole" comfortably in second gear. Right now, I'm through 2nd gear pretty quickly even in city driving. I don't have to tell you how useless 1st is for anything other than pulling stumps, climbing trees, or loading hay.
After reading your post, I decided to pull out some manuals as well. According to GM Heritage Center documentation, the 250 wasn't an offering for 1964... it lists RPOs for 230 (2 flavors, including one labeled "econ"), 292, and 283 ONLY for the C10. There are other offerings for other platforms like C20/30, P10, etc., but none of those list a 250 either. 1966 is the first year where the 250 appears on any of their documentation for the trucks. Do you have some definitive documentation that says something else?
Lots of things you can test out. Do an online search ''add acetone to the fuel. But that truck is not aerodynamic.
try a clutch fan. especially for highway cruzin', the engine wont have to battle against the wind as much. and put a 4 barrel carb on there, the primarys are smaller than the stock 2 barrel. remove the tailgate or get one of those nets if youre doing a lot of highway trips. nice truck, i have a 66' shortbed.
I should have given more info. The truck is a 1/2 ton C10. The 250 engine and trans is from a 1970 truck with 6000 miles since rebuild. The rear end is the original with a "WA" 3.73:1 non-posi. I used Randy's Ring and Pinion tire height calculator and RPM Calculator, and made a chart of what the mph are when driving in 4th gear. I've made this trip to my parent's house in West Covina, LA, over one hundred times in the past 30 years. It is 398 miles to their house. I will agree with some of you that averaging 72 mph really hurt the mpg. Flew over the Grapevine (mountains) at 68 mph. Had about 500 pounds of stuff in the bed. I drove Interstate 5 down where everyone seems to average 80 mph. PO said he got in the high teens when he drove it down Pacific Coast Hwy to San Diego, and back to the SF Bay Area. He probably averaged 55 mph, on the coastal hwy. Guess a vacuum gauge, Holley 4bbl 390 cfm, intake manifold, headers, and HEI are in the equation. Although a 305 V8 is starting to sound good too.
Maybe a T-5 or 2004R transmission swap to get overdrive behind that six would be a thought,some older Chevys had 3 speed overdrives may be hard to track those down.
You could hardly pick a worse vehicle for that kind of driving. Get a 4 cylinder small car, or if you must haul a load, get a minivan. I switched from pickup trucks to minivans 20 years ago. My current one, a 2001 Ford Windstar, would haul the 500 pounds at 70 easily and get double the mileage of your truck.
I had a 1974 C10 with a 250 and three-on-the-tree. One trip to Scottsbluffs NE and back I averaged 17.3 MPG. I had a pipe livestock rack on the box that was just a little higher than the cab. I drove about 60 MPH the whole trip. Later I had a heavy duty C20 with 292 and a turbo 400. At 70 MPH it was just screaming. It got about 10 MPG.
FWIW, my '80, C/10 fleet side long bed, w/ a 250 6-banger has a Sag. tranny in it along w/ a 2.76 reg'd, truck 12-bolt and not all that tall a rear tire combo. (??, 2.25/2.35 front to rear?? I forget)... I don't know what it's gas mileage is, but I suspect w/ the 2.76's, and running like 55 mph on the Hwy, it shouldn't be all that bad. pdq67
The 250 most certainly wasn't available in a '64 C10. The 230, and the 292 were the only sixes available. I was speaking as to the rest of his drivetrain being stock. Chances are with a Sm420 4speed, and 3.73 rear gear combo, it is. In my experience, with that combo, the truck probably came with a 292 originally. The 292/4spd/3.73posi was kind of the "Top Dog" combo in 63 and 64, as far as a heavy duty work truck.
I ran a 250 6cyl with a 700r4 and 3:25 gears in my 48 chevy and the 6 did not like the 1900 rpm at 70 mph when the torque converter was locked up. I had an HEI dist which really gave me good power on hills and I was getting around 19 mpg. After running two years, I pulled the six and installed a 350 truck motor and now I run down the road great but I don't get as good of gas mileage but I am a lot happier with the V8.
Which is what prompted me to go to my source material. When I read the OP's post, I registered the "250" part in my head, and not the '64 part... and 250's came with 4.11's in C10's (hence my original response). When I saw your post, I started scratching my head... and when I actually looked up the '64 C10 info, I saw that, as you indicate, the 3.73 was standard (4.11 was optional) for the inlines (230 and 292). Thanks for the clarification. I shouldn't have shot my "facts" from the hip without double-checking them first.
My recent experience in my NON AREO DYNAMIC 50 SUBURBAN with the 250 , single carb, HEI, 4sp granny low truck trans and 3:08 gears= 12 mpg newly rebuilt 250, cam, milled, bored, clifford, Holley 390, K&N filter, HEI , 10mm wires, langdons cast headers, 700r4 with 4th lock up and a 3:73 from a 68 c-10 and the same 6.00x16 tires=21.4 Both at 60 mph. I ditched the crappy Cokers and went with a Hancook radial from Hurst Racing with a bonded white walls 215/75/15 front and 235/75/15 rear and getting 18-19 at 60 depending on the load I am carrying. I drive it every day at least 75 miles round trip to work and then some after. The "in town" milage sucks because I can't keep my foot out of it and am sure the light -to -light inline 6 times would be better with a bigger carb, but mpg's and mild power is what I was shooting for and couldn't be happier.
You can try lowering the metering rod in the jet a bit at a time until it lean surges, go in small steps.
If you measure the metering rod you may be able to lean out the carb with a larger rod. Jets are available from 092 -114. http://quadrajetparts.com/rochester...ojet-jets-rods-monojet-jet-c-240_253_294.html Also be sure your vacuum advance works, should be about 20 crank degrees @14.5 inches Vacuum. Mechanical about 30 degrees by 3200 RPM.
Yep, without fuel injection or multiple carbs it is about impossible to tune an inline six to get optimum mileage. My old Binder got about 12-14 when I had it and my '61 Sweptline Dodge refuses to do better than 15 and I have Dutra Duals, electronic CD ignition and have spent a lot of time tuning the 2280 Holley. It has a shell on it and the previous owner has lowered it about three inches. (It's getting raised back up when the shocks go out this year.) You have an inefficient fuel-air path that has to run the inside cylinders way too rich and not enough gears, and you are moving a lot of air with the vehicle. Another Sweptline around here runs the V6 and the 4 speed automatic out of a Dakota with all the factory electronics. He gets 18 mpg or so. So I figure the 15 mpg is pretty good.
Like Hoop 98 said, do you have an operating vacuum advance? And have you confirmed the centrifugal advance is working, too? Initial timing is only about 1/10 the story.
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