The Stovebolt Site Forums The Shop Area Engine Shop Trying to squeeze out a little more gas mileage

57 truck 235 with a T-5 and 3.55 gears in a 37 1/2 ton,it seemed like it done better with the original 3 speed and 4.10 gears so I played with the jet size and installed softer power valve spring and its better. I am thinking those engines were designed to be screaming going down the road and gas mileage was not that big of concern back then so probably a waste of time and money but would like to to do a little better,I was told that the cam was changed in 59 for better low end torque and do have a 60 car 235 so would that be the one to use and work from.

How about some facts and figures to work with instead of all the generalizations? Try documenting every drop of gas used and mile driven over a 3-month period. Then make ONE change and do the same thing. Until you do that, any results you supposedly see are totally worthless. How about if you change a jet size and get an improvement, and offset it with a different power valve spring? Since the carb was calibrated for use with a direct drive transmission, a truck engine, and probably taller tires than you're running now, won't making a change in the camshaft, using an overdrive transmission, and a few other differences mean the carb calibration needs to be calculated from square one?
Jerry

When gas was $4 I decided I was going to make some mileage improvements with my 37 Buick with the old straight eight engine with 16mpg being the goal (never got there). First I put it up on the chassis dyno and dialed in the best possible settings. I have an Oldsmobile V8 HEI distributor in the straight 8 which I matched the distributor curve and rate of the Buick distributor which proved to be wrong and was corrected to give a better reading. The guys at GM hit a home run with the Stromberg 2bbl and trying others gained nothing. Going from a 180 T-stat to a 195 gained a tiny bit as did an electric fan but I've gone back on both. Switched to the high dollar Torqo synthetic fluids and lubricants in engine/trans/differential and after chasing all the leaks down did gain about 1mpg. The biggest gains came from removing the sun visor, outside mirrors, Breezies, and anything that caught air. The 8.25/15 bias ply tires to 2.25/75/15 radials inflated to 38psi resulted in the largest gain---a bit over 2mpg. As Jerry advised; you have to make several trips spread out over at least a couple months time to really show whether your efforts worked. Mine took the old girl from a 12mpg glutton at pump to a not so embarrassing 15.8 at 70mph highway steady cruising. I think peoples belief that there is magic in camshafts and carburetors often hides the woods behind the trees.

I may not be the one to suggest this with the issues I have had with mpg but I added E3 spark plugs and increased my mpg by about 15% for 10 years. Then other issues decreased it.

I know the gas we have is partially to blame.

Try some Beano to help with that little problem!
Jerry

I can tell you after driving my '37 for 18 years now, wind and speed hurt the mileage more then you think. Drive at 55 and it will go up, hit a head wind, it drops a lot. The design of the truck looks good, but aerodynamically it's terrible. The straight up windshield and big fenders really hurt it.

Now for the engine, if the overdrive brings the rpm down under 2400 or so at highway speeds, you mileage will suffer. Even my " late " model 250 likes 2500 -2700 rpm out on the road. I would guess you are running well below the optimal torque of the engine. Run around in 4 gear for a while and see what the mileage does. I run a 3.08 gear with 28" tall tires and automatic trans, 1 to 1 final drive, at 65 mph the engine runs 2600 rpm, this will get me about 21 mpg if the wind is not to strong. As Jeff mentioned, if you fuel has more then 10 to 15% ethanol your mileage will suffer.

One of the best gas mileage improvers I can think of is a simple vacuum gauge. Mount it in the driver's line of vision out of the windshield, and practice moderating the throttle position to keep the vacuum as high as possible as often as possible. Just a tiny bit of heavy right foot will cause the power valve to kick in and cut the gas mileage in half, or worse. If it's just impossible to keep the vacuum up in the cruising range, it's time to downshift - - - - -which is something the OD transmission fanatics just can't seem to process. There's a reason these old rigs were produced with a rear end ratio beginning in "4". Stacking a T-5 transmission in front of a 3.08 rear end ratio and pulling them with a low-torque engine, and expecting good fuel economy is an exercise in stupidity.
Jerry

What are the 5 gear ratios in the T5? Tire size? As Jerry said, more facts are needed. How often is it driven?

I've spent about 8 or 9 weeks studying the Rochester B and BC carburetors and roughly 5 weeks studying the Carter YF models. Through it all I've remained as objective as possible and I promise this was not simply drinking beer, looking at the carburetors and scratching my head. I was as thorough as possible...read as much data as I could find, measured everything I could, compared this/that, talked to several knowledgeable people, built jigs, straightened castings, etc, etc. And at the end of the day: There are aspects of the Carter design that make it a better scheme if running leaner, more smoothly and with better gas mileage are important things to you. But there are aspects of the Rochester that might be appealing also. If you are driving the pickup very infrequently, the gas in the float bowl will evaporate and this will not be good for the Carter accelerator pump. Getting wet with ethanol gas and drying out will cause it to crack and go bad. If that's the way you're driving, I'd say you'd have less to worry about with a Rochester.

And Joe is 100% right. Example: If we drive our Subaru Forester at 57 mph on a relatively flat road with no wind, our gas mileage will be as much as 42 mpg. If we drive at 65 in a crosswind we will get 32 or so. If we're driving into a headwind at 65 mph we can see it go to 27. Big swings.

Now for more about gasoline. Ethanol gas (evidently no matter where it is stored) will undergo changes over time. And these are not good changes. I believe they would affect your gas mileage, spark plugs, valves, performance, etc. Take a look if you're curious:

http://i1154.photobucket.com/albums/p535/JonGoodman/MVC-727F.jpg

The vial on the left is gas right out the pump. The vial in the middle is gas removed from the gas tank of my pickup (vented cap, naturally). It had been in there for 6 weeks or so and the tank is new...a galvanized tank made in Canada. The vial on the right is some gas I poured from the 2 gallon plastic gas container I use with the lawn mower. It had been in there about 2 months or less. The left one is pretty clear. The others look more like strong urine. If you take these darker ones, pour a tablespoon or so into the divot in the bottom of a soda can and light it, you'll see a fire burning with heavy black smoke and a lot of soot. A nasty fire like a wellhead fire. I should have taken a picture of it. It also burns more slowly and the residue left is dark brown and sticky...almost caramelized looking. If you do the same with the fresh gas, it will burn with less smoke and it seems to burn more a bit more rapidly. There will be notably less residue left. Naturally you might think the darker ones are evidence of evaporation causing a concentration of the crap that is now in the gas, but the lawn mower jug isn't vented, it is plastic and there's not much there to react with gas. So I can't say if the air in the jug (and gas tank) exacerbate this change or not but the change definitely occurs and the old gas is not good.

A few years ago a former student called me up to ask for some advice on getting an engine started, one he had just rebuilt and reinstalled in a friend's car. The engine had been out of the car for a little over two months, as he was doing the rebuild as side work to his day job in a dealer shop. After several phone conversations with no luck, I met him at the job site and participated in the startup effort- - - - -with no success whatsoever! Gas, spark, valve and ignition timing, compression, etc. all checked out OK. Still, we've got a "crank- - - -no start" situation. Double check everything- - - -and again- - - -and again! "Hmmmmm- - - - -the gas smells a little funky" Pour a little out on the ground- - - -toss a march- - - -it puts the cotton-pickin' match out! In about two months, the gasoline had gotten stale enough it would not burn! We rigged a gravity flow fuel supply to the carburetor and the engine fired up on the first try!
Jerry

I've read that you should get the best fuel mileage if you can gear your vehicle to cruise at the rpm where the engine gets max torque. For the 235 the torque curve is pretty flat between 1600 and 2400. Assuming you're cruising at 55, your engine speed will be near the bottom of that range in 5th gear and near the top in 4th gear. I have a 292 in my 68 with 3.73 rear and A833 od transmission. It cruises at about 1800 to 1900 rpms, a little above peak torque speed. I get around 17 to 17.5 mpg. I have gm electronic ignition and I used a vacuum gauge to set the timing. It's quite a bit more advanced than what the manual calls for. I also have a Holly 390 cfm 4 bbl carb. I haven't adjusted it yet; I'm not a carb guru; but I do plan to adjust the idle mixture when I have time. I'll use the vacuum gauge again for that. Based on sound and vibration, my engine feels like it's happiest at 1600 to 1700 but it's not too bad at 2000. It has plenty of torque to take the rolling hills around here without downshifting. I've driven it at 70 mph but both the engine and I feel better at 55-60. I try to get ethanol free when I can and it seems to like it better. Some people say it's a waste of time to try to get better mileage out of these old trucks but to me it's just another performance aspect and I'd rather spend time and money trying to improve efficiency than trying to go faster or sit lower. I'm also a tight wad so I like to save anywhere I can.

Often, more fuel economy may be gained AWAY from the engine:

http://www.thecarburetorshop.com/Troubleshooting.htm#Fueleconomy

The two largest fuel mileage increases I acquired on my current shop truck were: (1) replace the dogmatic transmission with manual - 26 %, and (2) remove the bug screen - 21 %.

Changing from a Holley 2-barrel to a Carter 4-barrel picked up about 15 percent, and replacing the Carter 4-barrel with TWO Carter 4-barrels another 6 percent. (No, it isn't a stovebolt 6).

Jon

I need to put a tach in to see how fast the engine is spinning,since I noticed it started drinking gas after the gear and trans change I know the engine is not running at its sweet spot now so might be time to go back to 4.10 gears now that it has o/d. Might be getting too used to the late model engines running around 1800 on the highway and getting decent gas mileage where the 235 is not designed for that.

I have improved my mpg by a few small adjustments .
increased tyre pressures a little.
Slight advance on the timing .
Fenton headers.
3.9:1 diff
31" tall tyres.
easy on the speed .
Cruise at 2100 rpm.

I did get the speedo to read correct but had not been able to drive it to see just what gas mileage it gets,too busy on other projects and the weather,was wanting to drive it this weekend to a car club event but there is rain in the forecast.

Now that I got the speedo very close to where it should be the gas mileage is looking much better and can live with it,I think its a 15 gallon and used 3/4 of a tank to go 189 miles so I am figuring its at 16.8 and much better then the little over 10 I was thinking it was getting. Going to refill and get a more accurate figure this week,gas gauge does not work so I got a stick marked off in quarters and the gas is right at the 3/4 used.

Filled it up today and it got 14.6,was hoping for better but not as bad as I thought it would be.

cruise at the rpm where the engine gets max torque
Only works with mild cams, V8 engines have peak torque above 3,000 RPM and definitely get worse mileage spun that fast.
My comments w/r/t mileage on my site here: http://victorylibrary.com/mopar/mileage.htm

Filled it up again tonight and its getting 15.7 around town and has not been put in 5th even at speeds around 55 so I think it is happy spinning a little,probably could use at least 3.73 gears but since the speedo is accurate I will not use 5th under 65.

Took it on the longest highway drive since I got it on the road and it definately needs 3.73 or 4.11 gears when using o/d,I have a vacumn gauge on it and even at 70 and 75 it was hard to keep the vacumn above 10 so it needs to spin a little faster. I wish I would have been a little smarter when rebuilding the engine as I would have had a custom grind cam made so it would work better with taller gears.

Took mine out for a drive after cleaning the carburetors. Local highway, slight up and down, no major hills but not flat either.

250 inline 6, dual W-1 Carters, 3.08 gear, T350 auto trans with 1:1 output, 28" tall tire, 3300 lbs, 18.5 vacuum at idle (600 rpm)

At 65 mph, on the level, 11 - 12 on the vacuum gauge, 16.5:1 on the A/F ratio, 2450 rpm.
At 65 mph, up hill, 9 on vacuum gauge, 15:1 A/F ratio, 2400 rpm
At 70 mph, down hill, 14 on vacuum gauge, 17:1 A/F ratio, 2450 rpm.

This is the same it ran last year, got 21 mpg on a 300 mile trip.

Good gas mileage- - - - -but what's the piston mileage on one running that lean? You're right on the ragged edge of destructive detonation, especially if the ignition timing is advanced just a tiny bit above optimum. Gasoline is cheaper than engine parts!
Jerry

I can run a fair bit more timing and lean due to the 8.0:1 compression. On average, the A/F ratio is much lower, these numbers are just when I looked at the gauge. It would need to be graphed out to see the true average.