The Stovebolt Site Forums The Shop Area Engine Shop Better gas mileage

Hi folks! I've had my 71 chevy c20 for about 3 yrs now and really love it. Keeping it all original and still using it. The only thing I don't like is the 10 mile/gal. gas mileage. At today's prices for gas I can't run it for very long. Can anyone out there help me with any suggestions for getting better gas mileage? I've heard that changing out the orignal transmission and/or rear end would help. Please be specific with your suggestions because I'm not a mechanic but I do follow directions very well-----just ask my wife. Thanks.

put in a transmission with overdrive.

do you know what your rear gear is now? putting a taller one it will help lower rpms while cruising.


oh yeah, and drive down hill :p

Think about how many gallons of gas you can buy with the money you might spend on parts + the headache. Just something to think about. I'm not as smart or as talented as Wrenchbender - so I go with "if it ain't broke - don't break it!"

Two cents from the peanut gallery.

Love my overdrive! 16+ mpg in a 64 w/350 & 4 bbl. Brian

Is it a 350, 2bbl? If so putting a 4bbl on it will help as long as you don't open the secondaries to often. Headers and a free flowing exhaust and HEI will help too. But the simplest thing that you can do is change your driving habits. If you see a red light, do not keep giving it gas until you have to hit the brakes. Anticipate a stop and get off the gas long before you have to brake. Remember, brakes just convert all of that gas derived momentum to heat on your drums. The hotter they get the more energy you waste. No jack-rabbit starts either. Each time you stab the gas you actuate the accelerator pump which shoots a stream of gas into the carb to keep the engine from bogging.

Tab,

Im an old man and I have been driving a long time. You really got it right with your advise on how to drive and save gas. I think most of us gear heads have gone through the red light in the rear mirror phase, and seen how time changed our driving style. I just checked my 1950 Chevy 235 engine, with just rebuilt carbuerator, and found I was getting 12 miles to the gallon. Is this right for a Rochester B???

Overdrive is the way to go. Not just for gas mileage, but also

1. Higher speed top end (with your truck being a 1971 model, you already have front disc brakes from the factory so stopping won't be an issue).

2. Lower engine RPMs (means less wear and longer engine life).

I have an overdrive behind a mildly built 292 in a 66 short stepside p/u. Gas mileage is still not good at all, even with sensible driving - I'm sure it's the big @ss 4 barrel I have on it! But I love it. I wouldn't trade being able to cruise 80 mph on the interstate at only 2600 rpms for nothing!

-ftyler

ftyler,

> Lower engine RPMs (means less wear and longer engine life).

If one were to (by any means) lower their rpm's and maintain the same speed, I doubt there would necessisarilly be any reduction in engine wear. The engine would still have to put out the same effort. I believe the laws of physics says that you'll just expend that energy in a different place.

Rance

PS: I could be wrong. I'm not a physics major, not even on TV.

Thanks everyone for all the suggestions. I tend to agree with 64fleetside and ftyler about the overdrive----just seems to make since to me. I also agree with ftyler about the reduced engine wear with lower RPMS. Like Rance, I'm not a physics major, but it seems the energy will be diverted somewhere else like the reduction gearing. What does a "taller" rear end mean? ----a higher gear ratio? Not sure what my 71 has, only that it's the original. Can anyone give me examples of the "taller" rear ends I need to be looking for?

I'm not a physicist either, but I did spend the night at a Holliday Inn.
That is correct. Obviously correct.
The "same output" is also correct. It just wouldn't have to give up as much energy to friction and heat to achieve that same output.
I changed my rear from 4.56 to 3.54 gears and the truck took on a whole new life. It feels like it isn't working as hard and still doing more.

I think your choice of what to change is largely dependant on what engine, trans and rear gears you now have. This is where you need to be specific.

From Kimberly's lead (and an earlier post)

Let C = Cost of the conversion in dollars
G = Price of gas per gallon in dollars
P = Present Miles Per Gallon
N = New Miles per Gallon
and
M = Miles driven to break even

Then

M=C/[G*(1/P - 1/N)]

For instance if I spend $1000 (C) to boost my mileage from 17MPG(P) to 20MPG(N) and gas is $2.25 per gallon (G)

1000/[2.25*(1/17-1/20)] = 50370 miles to payback

but if I went from a paltry 3MPG to 6MPG (same 3MPG difference)

1000/[2.25*(1/3-1/6)] = 2667 miles to payback

buy a cavaler? to drive to work and use your truck for pleasure.

Gentlemen,
(I went back & reread the original post & realized I didn't answer his question.)
Dan,
Changing the fuel economy of your truck requires re-engineering. You can do an OD tranny swap (best choice); add OD like Gear Vendors, swap rear gears which will reduce towing/hauling capacities; or upgrade to modern motor, ignition, & fuel system.

(Disclaimer: some #'s will be wrong, just used for example, & some factors will be left out like weight & wind resistance.)
Whenever someone is worried about fuel mileage, we are probably talking about cruising speed. Cruising speed is all about power vs. resistance (leverage). Ask yourself how much HP/TQ is required to maintain a certain speed. Cruising speed requires only a fraction of an engines max (advertised) HP. That is why cruising RPM uses much less power than max RPM. This is why we have OD transmissions today. Engineers designed our beloved old trucks with high ratio gears (ex. 4.11:1 vs. 3.42:1) because they didn't have a lot of power. They also didn't drive very fast in 1950. I assume pulling 8,000 lbs @70mph with a 3/4ton truck wasn't even heard of in 1950. It's all about leverage. Today's vehicles have more power, which means higher speeds & towing capacities with lower RPM's.
This brings me to the engine wear issue. Less RPM means fewer times each piston goes up & down(among other things). Let's say a 235 can produce 20HP @2000RPM & that is all that's needed to propel your truck @ 60MPH. But, your 235 is spinning at 4000RPM @ 60MPH producing 150HP. That is more RPM/HP than you need, right? Then you lower the gear ratio to produce a certain speed at a certain RPM. The only problem is you lose acceleration. It's all about leverage. You have to find a balance between power(acceleration & pulling power) vs. result (RPM & economy).
Now, I know some of you are going to blast me, but it's all about helping someone maximize the potential of his truck while realizing the limitations. These are not new trucks.

What engine/trans combination do you have ?

Intenal cobustion engines work at 20% only. 80% of the gas you buy is converted to heat.

So there is plenty room for improovement.

Shell Oil bought lots of carb patents since the 1920es and as far as I know Shell never sold carbs.

Think about it. It`s a fact.

The main problem is that a carb only sprays the fuel in the intake. You want the fuel in a gaseous state for best economy.

TBI and other injection systems are a step forward, but come with computers and sensors and all that stuff stovebolters don`t want to see under the hood (an usualy can`t fix whan broke).

So one method is boiling the fuel till it vapours and feed the engine with this refined gas.

The other way is to feed extra stuff to optimize the combustion. We all know Nitrooxyd from saltlakes but that`s expensive, works for short time periods only and so it is nothing for the daily driven truck.

Hydro Oxygen Boosters, are sold from some Companys or even homemade.

You use plain tapwater with an electrolyt plus some current from your battery to split the water in oxygen and hydrogen. Then you feed it to the intake.

What oxygen does to a combustion is well known, hydrogen is also an well tested high energy fuel by itself.

Don´t get me wrong, I´m not talking on driving on tab water allone. The battery would never have enough power to produce all the gas you need.

There is one Company that makes such a setup and guaranties you double the milage or you get it for free.

Here in germany we have horrible gas prices, (right now we pay $ 3,78 for a gallon pump gas!)that`s why we really think about milage.

Here is some more Information if someone cares:

http://better-mileage.com/supercarb.html

http://www.hydrogen-boost.com/tech-info.html
Himac Research

http://www.agsint.com/cond.htm

http://www.wyominginstruments.com/

http://www.geocities.com/davevandrunen/ind3.html

http://www.hydrogen-boost.com/tech-info.html

Don't forget the size tires you run will effect your speed/rpm figures. depending on the tire size you now have,taller tires could lower your rpm's considerably at the same speed. i.e.going from a 235/70-15 to a 235/85-16could give about the same effect to your rpm's as changing from a 4.11 rear end ratio to a 3.73 ratio,and might cost quite a lot less to do.You might even borrow a set of taller tires for a while and see if they improve your fuel economy enough to be worth buying a set that size.
Just some possibilities to consider.
Speed

Wow. Thanks again folks for all the great advice. Uncle Scoot you make me feel like I've gone to college again and I'm listening to my professor. Your response is an education in itself. And v6Jimmy , at the expense of sounding like a total retard, I don't know the eng./tranny combo. on my 71 chevy c20. I do know the eng. is the original 350 v8 and the transmission is the original that came with the truck, but the type transmission I'm not sure. The shop manual (which I can't find right now) lists it as a turbo, hydra-matic.
Anyway, thanks for all the help. And any further info. will be much appreciated. It looks like I'll be going for the OD transmission.
Dan

There is one more way to go if gas prices keep going up this idea might save some money depending on what the costs are to make pure alcohol or if its legal or not to produce for internal combustion engine purposes.?

Dump about 5 gallons of pure alcohol in with your fillup of gas or 25%,ratio gas+25% pure alcohol.

What I understand is if pure alcohol is used alone you have to change all your fuel lines and pure alcohol burns up too fast to put it in its simplest terms.

It certainly is legal to set up a still and run off your own alcohol for fuel purposes. You need to get a permit from ATF to do so. The link below has a link to the ATF form.

If you are interested in making alcohol to supplement (or even replace) the gasoline in your vehicle, here's a great place to start: Make Your Own Fuel

KansasRon48,

The 12 MPG is more due to the Powerglide losses than to the Rochester. 16 to 20 MPG with a fresh engine and manual transmission is achievable.

Engine RPM does not equate directly to horsepower, it only indicates the power that could be produced. The fuel usage depends more on the horsepower required to move a given load than on the RPM at which that power is developed. Does anyone think that a stovebolt going up a 10% grade gets the same milage at 3500 RPM as it does going down the same grade at 3500 RPM? Many vehicles have a higher top speed in high than in high over because they do not develop enough power at the lower RPM. Lowering the RPM does more for driver comfort than engine longevity or improved milage. I guess what I'm trying to say is that if the truck is a toy, you'll be happier with numerically lower gearing, but if it pulls heavy loads, leave it alone.

George

i agree with everyone on this board. your truck, a C20, more than likely has a 4.56 or 5.13 rear end ratio if its stock. why did GM do this wonder? so you could start with a heavy load, since this IS a pickup, made for hauling, pulling, grunt work, etc. and this does nothing for your MPG's...makes for a great drag racing start..but not for cruising. lol. soo...yes. if you want to keep your "pulling ability", yet get better mpg's...i'd say a OD tranny is the way to go, for the most radical results.

If your truck is an automatic, it is possible that it has a 4.11 gear ratio. If you decide to change those, contact Oldtmy48. He has been looking for a set for a while.

I agree that overdrive would be best for you situation. You just have to decide if you want an automatic or a standard transmission.

Phil