C30 with 250/SM420/4.10 gears MPG/fuel economy results

[66Submarine]

Just tallied up my fuel economy results again today.

As some here probably already know, my current DD truck is a '65 C30 (one ton) powered by a '71 250 six with points and a Monojet carb (initial timing increased, but otherwise nothing done), a SM420 4 speed ("granny") manual trans, 32" tall 235/85R16 remolded mud tires on the back, and 4.10 rear gears; it weighs something like 4,500lbs empty (without me or my crap in it).

I tried to avoid excessive stop-and-go/in-town type driving, but I wasn't going 45MPH on a rural Florida highway and never stopping or anything; 55MPH or so around town and 55-65MPH on the highway, with a reasonable amount (IMO) of stoplights, frequent hills, and some traffic and in-town driving. I don't drive like a total moron (I think?), but I know I was/am too quick to go WOT to get maximum fuel economy (GM told me to avoid WOT starts to get all the economy from my Chevrolet). I'll try harder to avoid WOT during the next testing session...

Anyway, I drove a corrected 355 miles (odometer reads 10% fast) and averaged 14MPG. Not too bad IMO, all things considered.

[Palf70Step]

Sounds about right. Our LEAD BRICKS for trucks just don't flow the air by well. Low MPG is a small price for having a real vehicle!

[66Submarine]

My father actually gets fuel mileage in the upper teens with his half ton (I've never really measured it precisely), and my half ton averaged about 18MPG when he was driving it (I did measure that one).

This one ton truck sits up higher than the half tons (more drag), has deep gears (runs 2,900RPM @ 65MPH), and @ 4,500lbs it's pretty much 1,000lbs pounds heavier than a half ton. I'm sure I can/will get a little bit better mileage out of it just like it is if I try (less fast driving/traffic/WOT), and I'm expecting that adding an overdrive at some point will make a very noticeable difference under most driving conditions; even with the little 250, I could pull another gear a lot of the time when I'm currently stuck in 1:1 with the engine wound up.

I do have a '68 Impala for an aerodynamic/lightweight "economy car" () to go with the one ton, though; expecting/hoping to get over 20MPG from that one.

(I was going to use a half ton suburban or a truck with a camper top, but I figured the actual car would probably be better suited as a car...)

[TJ's Chevy]

That's not bad considering...a 292 would probably get as bad or a whopping 1 mpg better only cause its a little bigger...less engine stress...dump an ol 5 speed GM trans...oh what was it...Spicer 5 speed? That'd give you another gear. lol Something like .76 OD I think...not sure..it was on that GM heritage site. Not sure what the 292 in my C-10 gets...Wonder if the mileage increased a tad with the 4 barrel, headers, an HEI...they say it should get around 16-18 with those mods...but who knows. lol! Put a turbo on that 250..then when you WOT it from a stop light you be up to speed so fast you will have hardly used up any fuel.

[argonaut]

4500 lbs seems incredibly light to me for a 1 ton long bed. Im also impressed that you get 14mpg with no overdrive and 4:11 gears. If I remember correctly my 64 half ton short bed weighed around 4100 lbs empty at the scrap yard and got about 14mpg with a 350, th400, 3.73 gears and 31x10.5" tires.

[66Submarine]

Originally Posted by T.J. MCCAULEY

That's not bad considering...a 292 would probably get as bad or a whopping 1 mpg better only cause its a little bigger...less engine stress...dump an ol 5 speed GM trans...oh what was it...Spicer 5 speed? That'd give you another gear. lol Something like .76 OD I think...not sure..it was on that GM heritage site. Not sure what the 292 in my C-10 gets...Wonder if the mileage increased a tad with the 4 barrel, headers, an HEI...they say it should get around 16-18 with those mods...but who knows. lol! Put a turbo on that 250..then when you WOT it from a stop light you be up to speed so fast you will have hardly used up any fuel.

Yeah, I'm interested to see how the 292 does in comparison; should have it in there before too long.

The old Clark and Spicer 5 speeds were mostly 1:1 in high gear, and they are giant medium-duty (like a C60) transmissions anyway; AFAIK they all (or most of them, anyway) also used an SAE bellhousing with all the bigger clutch stuff.

I almost bought a NV4500 the other day, but I think I'd like the SM420 with an auxiliary OD box better; the extra in-between gears would be very nice to have sometimes (towing, etc).

The mild turbo thing has crossed my mind many times; still might have to do it at some point.

I'm not really sure on the modded vs. stock mileage thing; that's next on the list. I have a little Weber progressive 2bbl carb and a GM HEI that I'm going to stick on there at some point soon, but I suspect that the gains from changing over from a setup like mine that's in good working order will be less than commonly advertised; it'll be interesting to see how it goes.

[66Submarine]

Originally Posted by argonaut

4500 lbs seems incredibly light to me for a 1 ton long bed. Im also impressed that you get 14mpg with no overdrive and 4:11 gears. If I remember correctly my 64 half ton short bed weighed around 4100 lbs empty at the scrap yard and got about 14mpg with a 350, th400, 3.73 gears and 31x10.5" tires.

Yeah, I was surprised, too; it weighed in @ only 4,940lbs with me and my extra crap at the scrapyard scales back when it had a 283 in it. GM said the basic truck (minus rear bumper and with the lighter six) weighed 4,050lbs, which goes along with that. I think I figured it weighs about 4,500lbs empty right now; I'll have to drop some scrap off and get another actual number soon for reference. It does also seem to get decent mileage for what it is (IMO); pretty happy with it overall. More improvements/updates to come...

[TJ's Chevy]

Originally Posted by 66Submarine

Yeah, I'm interested to see how the 292 does in comparison; should have it in there before too long.

The old Clark and Spicer 5 speeds were mostly 1:1 in high gear, and they are giant medium-duty (like a C60) transmissions anyway; AFAIK they all (or most of them, anyway) also used an SAE bellhousing with all the bigger clutch stuff.

I almost bought a NV4500 the other day, but I think I'd like the SM420 with an auxiliary OD box better; the extra in-between gears would be very nice to have sometimes (towing, etc).

The mild turbo thing has crossed my mind many times; still might have to do it at some point.

I'm not really sure on the modded vs. stock mileage thing; that's next on the list. I have a little Weber progressive 2bbl carb and a GM HEI that I'm going to stick on there at some point soon, but I suspect that the gains from changing over from a setup like mine that's in good working order will be less than commonly advertised; it'll be interesting to see how it goes.

An HEI, 2 barrel and headers will get you better mileage and a little better pulling power in the top end...engine can breathe a bit better and let some more hot air out. lol

[66Submarine]

Originally Posted by T.J. MCCAULEY

An HEI, 2 barrel and headers will get you better mileage and a little better pulling power in the top end...engine can breathe a bit better and let some more hot air out. lol

The better breathing means you can fit more air/fuel in and make more power, but doesn't really mean you'll improve fuel consumption as a result.

The little Weber progressive 2bbl will (likely) be a little better because of the smaller primary circuit and all that, but I'm not really sold on the idea of other 2bbl or 4bbl carbs for better fuel economy; I barley have the little Monojet cracked open while I'm cruising down the road, and all the other 2/4bbl carbs (besides other progressive 2bbls or some freak 4 cylinder 4bbl I don't know about) are going to be the same CFM or more on the primary circuit. They could (potentially) be more efficient to some degree, but that's really the only way they'd be better.

I'm also skeptical of big gains from a HEI; not to say there might not be a slight improvement, but several MPG just seems really optimistic to me.

Not to say that small improvements aren't nice, but I think it would be unrealistic to expect any giant gains under my circumstances; most guys that report big gains seem to have gone from what I'd consider poor/horrific to normal (IME).

At any rate, I look forward to more testing on the subject.

[66Submarine]

BTW, I know I've posted this a lot recently, but here's a plug out of this truck; it's obviously running very well as it is, which is what leaves me skeptical of giant improvements.

[argonaut]

I agree that a bigger carb and headers offer little opportunity for improving fuel mileage. I think the HEI might help slightly. More spark energy might get you a more complete combustion and allow you to get equivalent power with ever so slightly less fuel and air.

[TJ's Chevy]

My plug actually burns a bit cleaner then that...and I'm running a 600 cfm 4 barrel and 65k coil(lol high energy alright) and headers.....barely...I mean Barely have the pedal cracked at 60 mph. The neat thing about the 4 barrel and headers an hei is it supplies enough fuel through the rpm to somewhat "change" the duration of the cam.....I had my 292 at 4000 RPM in 4th gear and it was still climbing. lol And that was with 95 pounds of compression..no not down hill. lol

[66Submarine]

Originally Posted by argonaut

I agree that a bigger carb and headers offer little opportunity for improving fuel mileage. I think the HEI might help slightly. More spark energy might get you a more complete combustion and allow you to get equivalent power with ever so slightly less fuel and air.

Yeah, that's what I'm thinking; probably helps a little, just not the several MPG that I see some claiming. Power from optimized spark/ignition is free though, and any extra free power is good!

[66Submarine]

Originally Posted by T.J. MCCAULEY

My plug actually burns a bit cleaner then that...and I'm running a 600 cfm 4 barrel and 65k coil(lol high energy alright) and headers.....barely...I mean Barely have the pedal cracked at 60 mph. The neat thing about the 4 barrel and headers an hei is it supplies enough fuel through the rpm to somewhat "change" the duration of the cam.....I had my 292 at 4000 RPM in 4th gear and it was still climbing. lol And that was with 95 pounds of compression..no not down hill. lol

Not sure how much cleaner a plug could be; that picture actually makes it look darker than they really are. I could try a slightly leaner jetting if I wanted to, but I don't think it would do any good.

I'm sure you barely have the pedal cracked with an Edelbrock AVS/AFB copy; that's a lot of carb for a 170HP engine! Not that it won't work, but the larger primary circuit means that the carb has less of a vacuum signal to work with; it won't work as well as a more appropriately sized carb could. The better breathing can free up some top-end power, but too much doesn't do any good (exhaust or carb).

BTW, I've also had mine to nearly 4,000RPM in 4th ("professional driver on a closed course" )...isn't that about 90MPH? I don't need to tell Barney on you, do I?

Here are a couple better pictures with a real camera; these came in the engine when I bought it, and they've been in there for the couple thousand miles I've driven it since. The dark color on the shell also doesn't wipe off on your finger (loose soot), it's just that color.

[argonaut]

Nice plugs

[66Submarine]

I needed to snug the exhaust back up today, and I took the opportunity to drop the pipe and see how clean it was; pretty clean! Last picture shows a paper towel I wiped in the manifold; came out with red rust on it instead of soot.

I also replaced the fan with a thermal clutch type; seems like a great upgrade so far.

[TJ's Chevy]

Looks good!