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Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1281076 Fri Sep 21 2018 08:25 PM
Joined: Aug 2006
Posts: 482
J
jfnar Offline
Shop Shark
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.


'64 swb stepside (gone)
'57 1 ton
'53 phone truck
'59 swb
'46 1 1/2 ton
'68 swb gmc
Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1281085 Fri Sep 21 2018 09:28 PM
Joined: Nov 2002
Posts: 1,882
C
carbking Offline
Carburetion specialist
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


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1281094 Fri Sep 21 2018 10:52 PM
Joined: Feb 2003
Posts: 1,303
J
Jeff Ogden Offline OP
Shop Shark
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.

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1281129 Sat Sep 22 2018 08:02 AM
Joined: Feb 2001
Posts: 1,041
D
Dusty Offline
Shop Shark
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.

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1284890 Fri Oct 26 2018 01:51 AM
Joined: Feb 2003
Posts: 1,303
J
Jeff Ogden Offline OP
Shop Shark
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.

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1286104 Sun Nov 04 2018 09:37 PM
Joined: Feb 2003
Posts: 1,303
J
Jeff Ogden Offline OP
Shop Shark
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.

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1305069 Thu Mar 28 2019 10:50 PM
Joined: Feb 2003
Posts: 1,303
J
Jeff Ogden Offline OP
Shop Shark
Filled it up today and it got 14.6,was hoping for better but not as bad as I thought it would be.

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1305078 Fri Mar 29 2019 12:11 AM
Joined: Dec 2008
Posts: 1,308
P
panic Offline
Shop Shark
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

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1308707 Sat Apr 27 2019 02:13 AM
Joined: Feb 2003
Posts: 1,303
J
Jeff Ogden Offline OP
Shop Shark
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.

Re: Trying to squeeze out a little more gas mileage [Re: Jeff Ogden] #1311087 Sat May 18 2019 08:57 PM
Joined: Feb 2003
Posts: 1,303
J
Jeff Ogden Offline OP
Shop Shark
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.

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