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#635109 - Mon Apr 05 2010 08:55 PM Best gas mileage in a 60's Chevy?  
NewAtThis  Offline
New Guy
Joined: Feb 2010
Posts: 87
North Carolina
From one of my other threads, located at

Justhorsenround has posted "Here is something to think about. Gas mileage depends on how hard the engine is working, right? I can almost guarentee that I can get as good or better mileage from a small block V8 and auto transmission geared for highway usage than the average Joe can get from the 6 and stick. The 6 has to work too hard. Just my 2 cents worth but based on 50 years experience."

What do you guys think? My assumption was a 6-cylinder stick would be the best for gas mileage, but I know nothing about vehicles, especially vintage ones.

What combination of engine, transmission, and other factors would result in the most efficient truck?

#635135 - Mon Apr 05 2010 10:49 PM Re: Best gas mileage in a 60's Chevy? [Re: NewAtThis]  
hoggyrubber  Offline
Shop Shark
Joined: Sep 2006
Posts: 1,077
springfield, mo
i'm sure not saying horsenaround is wrong, but that has not been my experience. if that was the case my 292 would get better milage than my 250 because it doesn't have to work as hard. it does not! noice how he said "geared for highway use and updated". that makes a difference. if you are going to be pulling big loads, consider the bigger engine, he's right it won't have to work as hard- everything else being the same. notice also it was him verses the average joe too- he is going to have it set up right where the average joe is not. and how about the miles on either engine, that's a factor too. i bet if he set up both with same gearing and same tranny, if the six is smaller, it would get better milage. you were looking for a 1/2 ton truck right? the six won't be underpowered if you keep the orig rear. my 64 c20 wrecker pulls over 10,000lbs gvw now and then and can do it fine with a 250, but it has the gearing for it. but whenever i am empty i reap the benifits of better milage than a 350. more than 1/2 of my miles are empty, a 350 would be a waste for me. i might be able to rarely get to the top of a hill without having to downshift less than now when i'm fully loaded. for the most part it wouldn't help me.

Last edited by hoggyrubber; Mon Apr 05 2010 10:50 PM.

#635161 - Tue Apr 06 2010 12:55 AM Re: Best gas mileage in a 60's Chevy? [Re: hoggyrubber]  
LGriffin  Offline
Shop Shark
Joined: Nov 2002
Posts: 1,081
Oakdale, CA
I'll concede the average 6 to him. The Austin Healy 3000 is one awesome l6, and the mpg is not bad. Then again you could not shift the Le Mans Healy into 2nd without dropping the engine almost to idle, or break the tires loose. :]. I saw a nice 65 C10 Custom and the owner said he just put a 4bbl on his 250 the mpg went from 11 to 15. It was your average 65 just well taken care of, been in the family since new.

I think what we are talking about is gearing for optimum use.


I don't own a vehicle that isn't old enough to drink.

#635167 - Tue Apr 06 2010 01:13 AM Re: Best gas mileage in a 60's Chevy? [Re: LGriffin]  
NewAtThis  Offline
New Guy
Joined: Feb 2010
Posts: 87
North Carolina
Thanks, guys.

Ok, so let's talk about standard options first.

Hoggyrubber, yes, I'm looking for a 1/2-ton stepside shortbed, 1963-1966. And my main driving would be on a highway with speed limits between 45 and 55 and several stop-lights between here and work.

So, standard options:

1) V8 4 on the floor
2) V8 automatic
3) I6 4 on the floor
4) I6 3 on the tree
5) I6 automatic

Then I guess there are a couple different sizes of V8's and I6's.

I've always heard that a manual will get better mpg than an automatic, so I'd guess that neither of the automatics would be the winner here. I've heard that a 4-speed would be better than a 3-speed if the low gear was really usable, not the "3-speed with a granny" style, which I think is what the I6 4-speed would have been. So maybe the 3 on the tree vs. 4 on the floor would be a wash?

And a good 6-cyl should beat an 8? Would a smaller 6-cyl be more efficient than a bigger 6-cyl?

Am I putting too much thought into this?

Last edited by NewAtThis; Tue Apr 06 2010 01:13 AM.

#635189 - Tue Apr 06 2010 02:05 AM Re: Best gas mileage in a 60's Chevy? [Re: NewAtThis]  
hoggyrubber  Offline
Shop Shark
Joined: Sep 2006
Posts: 1,077
springfield, mo
i don't think the newer automatics are bad, but the older ones had quite a pit of power loss just to drive tranny. the manuals were the most eficent back in the day. if upgrades are not a problem you could run a automatic fine i'm sure. all the originals had 3 speed with granny type or sm420 so no real difference between that and saginaw 3 speed. the final drive ratio is 1:1 i believe unless you install a latter tranny overdrive unit.
i wouldn't say any six would beat a 8 on milage. the style of six for these trucks run 198, 230, 250, and 292. if it's just a empty 1/2 ton it will weigh around 4000lbs. the 230 would be kind of a dog but ok. i think the 250 would be fine. now if you are comparing it to a 283 not a big advantage on milage. seems like everyone wants to drop in a 350 and i would bet the 250 would beat it on milage. now i have not built a 350 and put it in my truck to compare so i can't tell you the exact #'s.
would be interested if anyone has seen a change in milage in swithching from 6 to 8 or back. if both were in comparable shape. i have owened some wore out v8's like a international 352 (i think) that got the milage of a school bus, so you couldn't compare apples to oranges.

#635207 - Tue Apr 06 2010 02:37 AM Re: Best gas mileage in a 60's Chevy? [Re: hoggyrubber]  
Rusty64  Offline
Shop Shark
Joined: Jan 2007
Posts: 263
St. Peters Missouri
I have a 64 with a 350 (stock build), 700R4 and 3:08 Posi. Goodrich all terrain tires (all 4 wheels). About 1800 rpm @ 65 miles/hour. Gets 16 mpg on the highway.
Tried swapping out the 4 barrel for a 2 barrel (stock cast intakes), still 16 mpg on the highway.

#635210 - Tue Apr 06 2010 02:45 AM Re: Best gas mileage in a 60's Chevy? [Re: Rusty64]  
NewAtThis  Offline
New Guy
Joined: Feb 2010
Posts: 87
North Carolina
Ok, forgive my ignorance here... The 350 is a v8? Also, the 283 is also a v8?

Seems to me that 16 mpg with a 350 is not too bad. It would be nice to get closer to 20.


#635218 - Tue Apr 06 2010 03:00 AM Re: Best gas mileage in a 60's Chevy? [Re: NewAtThis]  
Adam61  Offline
Shop Shark
Joined: Jun 2009
Posts: 377
Calgary, Alberta, Canada
the 350 is a V8, so is the 283. I have a 283 in my truck, but I don't know the gas mileage...I don't think its very good though. I have the stock 3:90 rear end and a 4 speed with granny low, so I'm not geared for highway use that well

#635219 - Tue Apr 06 2010 03:01 AM Re: Best gas mileage in a 60's Chevy? [Re: hoggyrubber]  
LGriffin  Offline
Shop Shark
Joined: Nov 2002
Posts: 1,081
Oakdale, CA
Shortbed, not going to haul a lot of weight, but you can still load it with camping gear and an occasional refrigerator. Not many around, nice little truck.

Automatic your Dad was right then, as hoggyrubber says the newer automatics are much better. The trick is to get a locking converter. I'm not sure on the particulars there, just when does the torque converter lockup and how.

3spd there was a transition in the late 60s from the old 3spd (sm318?) to newer Muncie which has a synchro 1st gear. You don't want any ponys in front of the old 3spd. I got very good at changing the slider gear (2nd&3rd) in an old 3spd behind a 300 hp 327. Both transmissions had over drives, but I believe the later with OD is harder to find. The synchro 3spd was available in passenger cars in 64(?).

Rear axle I don't see any way not to change the rear axle ratio, even with an OD. An OD might be a reason not to change the rear axle, your mpg would take a hit. What rear axle ratio depends on engine, 3.08 to 3.55

Engine, trucks don't have much weight over the rear axle, and a short bed has less. Bigger tires are not the answer to good mpg, there is a limit where they will hurt mpg due to weight. There is not much difference in weight between a 6 and 8 cyl, the 6 is heaver. I'd go with a 250 or 283, not the 292 or bigger v8. I would build to 250 to about 220-250 hp. That would be enough cam for a mild lope, for the stop signs. smile

The proverbial 2s


I don't own a vehicle that isn't old enough to drink.

#635296 - Tue Apr 06 2010 07:40 AM Re: Best gas mileage in a 60's Chevy? [Re: LGriffin]  
Markeb  Offline
Shop Shark
Joined: Dec 2008
Posts: 344
Washington state
It really all comes down to a balancing act with many variables to consider. Even a short bed step side weighs about 4,000 pounds empty, with the aerodynamics of a brick, and a built in speed brake above the windshield. 20 mpg isnít unreachable, but it would be easiest to obtain with an automatic overdrive transmission, optimal rear end gearing and electronic fuel injection on whatever engine is selected. The trade off being the cost of the upgrades compared to the savings in better mileage.

Whatís most important is setting up each truck to fit the realistic needs of the owner. Years ago my daily driver was a 1954 Chevy 2 door sedan. With a 235 six it got about 10-12 mpg, and after I swapped in a 283 V8 it jumped up to about 16+. Not sure how much of the improvement was mechanical vs driver input. I was much easier on the V8 because it would climb to speed easily (and in the beginning I was still running the break easy stock transmission and torque tube rear end), without shoving the pedal to the floor all the time like I did with the 235. The 235 in this case was a bad fit for me because I wanted a hot rod and the six was a low rpm torque motor.

My current driver is a 1960 GMC short step with a stock 350, Super T10 4 speed, and 4.10 rear axle. Normally this would be a ridiculous combination for a daily driver, but itís ideal for me because 90% of the time itís driven at 35 mph, and rarely goes above 50. Last time I checked it was getting about 16-17 mpg. If it were driven on the freeway the mileage would probably drop near single digits.

Personally I like both the six and the V8 for different reasons. The sixes are simple and easy to work on, make a lot of torque, can last forever with almost no maintenance, and are popular at rod runs because theyíre not very common anymore. For around town driving they can be ideal, where theoretical mileage may not be all that important if the truck doesnít rack up a lot of annual mileage.

A modern V8 spins higher, faster and is much easier to tune to lean running, because so many combinations of parts are available compared to six. A small block V8 is physically smaller with a nearly unlimited amount of inexpensive speed parts available.

Any combination of V8, six, automatic or manual transmission can produce an ideal arrangement if itís suited to the application. If an automatic transmission is chosen, a lock up converter will lower engine speed. A few years back I swapped in the 4 speed manual replacing a Turbo 350. With no other changes, my engine speed dropped 500 rpm in drive by eliminating the torque converter slippage. An overdrive transmission can also eliminate the need for a rear axle change. A .70 overdrive will change a 3.90 rear end ratio to a 2.73, providing potential savings, but it is important to match the final drive ratio with the power curve of the engine. If the engine is running at a speed below where it makes torque, mileage will suffer.

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