Can you improve my MPG's?

[Kfort]

I'm a new guy to this site and not much of a motorhead so I will greatly appreciate any sound advice. I have a 1972 C20 Custom Camper long bed equipped with a 350, 3 on the tree and an Edelbrock 600cfm carb (saved original carb). I would really love to slide in a camper and see some country.

The Issue: Primarily, my gas mileage stinks. Secondarily, I don't have a Tach but my RPM's @ 65mph are up. I wish I had an overdrive. I like my current gear ratio between 1st, 2nd and 3rd...if I could only shift on more time and cruise @ 70.

The fix: My truck is all original so I'm afraid to start changing parts so whatever I do, it should be done right and not decrease value of truck.
1. Is a 4-spd manual tranny part of the solution and will the rear diff gears then require swapping?
2. Do I have too much carb and if so, what should I use instead?
3. What size tires are optimum for improved mpg's?
4. Am I missing another component for mpg improvement besides "slowing down?"
5. After making all the appropriate modifiations, what are the most highway mpg's that I can expect?

[jeffahart]

What's your MPG? It might be a good MPG for your setup! Check out this thread. See what others are getting so you have kind of a baseline.

http://67-72chevytrucks.com/vboard/s...ad.php?t=51534

Old trucks can eat some gas.

[barry1982]

Hi There, I would agree .... these old girls are not known for good mileage....
My opinion, but a four speed won't help you at all. Both your 3 speed-and the 4 speed are "direct" in high gear. What you need is an overdrive. In the early 80's GM offered a 3 speed standard with overdrive. I believe it was called a A822 but I could be wrong. They came with a "hurst" style floor shifter. These tranny's are not easy to find, but would definitely drop your highway RPM's.

[GASoline71]

To make torque with a SBC 350 you need to make RPM's. If you're pulling or towing with that C20, right near 3,000 RPM's is going to be the "sweet spot". Which is probably where the truck is operating now at 65 MPH.

These engines can run at RPM all day long. They were built for it. Most people are just used to the way modern Fuel Injected engines mated to Computer Controlled O/D transmissions operate at lower RPM's.

Not trying to be a Debbie Downer, but to make power to pull/haul, you gotta put your foot in it.

Gary

[barry1982]

Hi again, Sorry, I gave you some incorrect info..... the O/D trans is a A833.

[LongBox]

Back in the 70s, it was common practice to find a carb from an older engine, say a 66 or 67, and install that in place of the later QJet. Then, you could buy a distributor advance kit at the local auto parts shop (I recall the one I bought was branded "Mr. Gasket"). This allowed more low RPM advance, but limited the total advance. The final step was to put on dual exhaust with low restriction mufflers. My 71 C1500 with a 350/350/3.73 drive train improved from a round 10 in town to around 13 in town.

There were no smog tests then where I lived.

[Stocker]

Welcome to the board! Good replies above.... these trucks are not nearly as fuel-efficient as newer stuff. They have a drinking problem.

Start by calculating what your mileage actually is. Not miles per tank which means nothing, but miles driven divided by gallons used = your mpg. Get more accuracy by tracking a rolling 3-tank average, as many times as possible.

Your driving style has more effect on fuel mileage than almost anything else. Slowing down helps, but most of us are not too good at that.

Determine your gear ratio. Changing it *might* help.

Depending on your gear ratio and how the truck will be used, you might be able to fit taller tires. Resist the urge to go wider, as that typically increases rolling resistance (and to a smaller degree, aero resistance) which hurts mileage. Narrower tires can usually run higher air pressure which decreases rolling resistance.

600 cfm is not too much carb for a stock or mild 350. In fact, it's about right.

If you have a fixed-blade or flex fan, upgrade to a clutch fan. Or electric.

[cdowns]

from the info you provide which is very little i'd think 10-14 mpg wood be good if you have it tuned very good

[TwoFiftyShifter]

I'm getting about 13.5 mpg in my 71. C10 LWB. 350 3 speed 3.73s. some in town and highway at 65ish.

[Kfort]

Originally Posted by Kfort

I'm a new guy to this site and not much of a motorhead so I will greatly appreciate any sound advice. I have a 1972 C20 Custom Camper long bed equipped with a 350, 3 on the tree and an Edelbrock 600cfm carb (saved original carb). I would really love to slide in a camper and see some country.

The Issue: Primarily, my gas mileage stinks. Secondarily, I don't have a Tach but my RPM's @ 65mph are up. I wish I had an overdrive. I like my current gear ratio between 1st, 2nd and 3rd...if I could only shift on more time and cruise @ 70.

The fix: My truck is all original so I'm afraid to start changing parts so whatever I do, it should be done right and not decrease value of truck.
1. Is a 4-spd manual tranny part of the solution and will the rear diff gears then require swapping?
2. Do I have too much carb and if so, what should I use instead?
3. What size tires are optimum for improved mpg's?
4. Am I missing another component for mpg improvement besides "slowing down?"
5. After making all the appropriate modifiations, what are the most highway mpg's that I can expect?

Great input. Thank you. What are your thoughts about going with a Tremec 5-speed and an EFI system?

[LongBox]

The upfront cost of the Tremec and the EFI system will probably take years to recover unless you drive a lot of miles, and I mean a lot of miles.

I don't think anybody buys one of these 67-72s expecting good gas mileage. Back in those days, before the first oil crisis, nobody cared about mileage.

[TwoFiftyShifter]

Axle swap to a 14bolt with a 3.23 ratio. This will let you run about 2300 rpm at 65. 3.42 if you wish for a little more.pulling power. Otherwise, look up that NV4500 swap.

[Myoldtruck]

I was looking at factory 3spd with overdrive unit from the 60s. I found two on craigs list. This would help with your highway driving. I ended up putting in a four speed, i needed the extra low speed pulling for my trailer.