Abbreviated gas mileage comparison...

[1986F150Six]

Some of you have read previous threads where I have shared gas mileage figures. For years, I have driven what is a repeatable and consistent route during the week. This is to and from work plus weekend driving. For the most part, this is 25, 35 and 45 MPH with numerous stop signs and traffic lights. There is a 6 mile non-stop stretch which is 45 MPH and I drive that 2X a day.

The following is winter driving [recent]:

1984 F150 with 4.9L feedback ignition and carburetor; 4 speed manual O.D. [son's truck] = ~17.5 mpg.

1986 F150 with 4.9L Duraspark conversion and pre-EGR Carter YF; 4 speed manual O.D. = ~17.0 mpg.

2004 Pontiac Aztec with 3.4L and auto. O.D. = ~21 mpg.

The Aztec was my wife's former car. I drove it to the 2017 Garagemahal GTG and Mr. NumberDummy told me that he had seen a survey which indicated the Aztec was the ugliest car ever made and possibly was the reason Pontiac ceased to exist!!!!

[Ford F834]

Interesting line up David! I know 17 vs. 17.5 is a slim margin, but it is interesting that the 3.55 gear truck beat the 3.08 gear truck?

[Gary Lewis]

David - Does the 17.0 MPG avg for your truck include the early days?  If so, can you give us a sense of what it is doing recently?

Also, give us some idea of the difference in driving yours vs your son's?

As for the Aztek, I had one as a rent car once.  The biggest frustration was where the break in the rear window was.  But other than that it was a fine vehicle.  But beauty is in the eye of the beholder.

[85lebaront2]

Probably has to do with the engine in the 3.08 gear having the engine rpm too low, manifold vacuum lower and the YF vacuum enrichment coming in.

Interesting note, I know a number of us are old enough to remember the national 55 mph speed limit. I had, at the time, a 1966 Shelby GT350 and a 1963 Oldsmobile Jetfire. The Shelby with dual Holley 465 cfm 4 barrels would get 18 mpg at 70-75 mph, at 55 it only got 16, the Jetfire got about the same at 55, but climbed to 22 at 80 mph. Later I had a 1977 F150 I put a Camper Special 390 into, it had a 3.25 gear and would deliver 16-17 mpg at 65-70 mph.

Darth gets 10-11 normally, but a good highway run will pull 12.5 or better. It's all in where the engine is set up to run most efficiently, the Shelby turned 3500 rpm at 70, no vacuum advance, strictly mechanical, the mechanical curve had peaked the initial portion by 3000 rpm, and was into the slow advance up to max at 5500 rpm. At 3500 - 3750 the engine was only running on two of the eight barrels and was very likely right at peak efficiency for that system. Jetfire, I cheated, I had removed the spring under the power piston in the Rochester RC carb, no enrichment until the turbo exceeded atmospheric pressure, at 80 mph, the boost gauge (vacuum and pressure) would be just barely under 0 so, no enrichment, stock it would have been open.

[Gary Lewis]

Torque peak and max efficiency usually occur at the same RPM.  "Efficiency", not MPG, since lean or rich jetting can change that.  But with reasonable jetting efficiency and max MPG can occur at the same point.

[1986F150Six]

Gary Lewis wrote

David - Does the 17.0 MPG avg for your truck include the early days?  If so, can you give us a sense of what it is doing recently?

Also, give us some idea of the difference in driving yours vs your son's?

No, the ~17.0 is for this Winter. In the Summer, it approaches 18.0 mpg.

My son's truck has a little more pep. As Jonathan remembered, my truck has 3.08 rear gears and my son's is 3.55. The gearing is actually closer than that due to the fact that I run the stock sized tires [215/75 X 15] and his has 235/75 X 15. The speedometer on his truck was calibrated to the larger tires. My truck is smoother and quieter.

I believe the edge in gas mileage is attributed to both the gearing and the fact that his still is as the Ford engineers designed. Mine does do better on the highway, though. At 70 mph, mine turns 1950 RPMs and his is at ~2200 RPMs.

By the way, on the same driving course, I have tested two times [1 tank each] driving w/o using the O.D. on my truck. The mileage decreased by 2 mpg [pumping loss].

[Gary Lewis]

I had the gearing backwards.  Ok, got it - for now.

As for the OD and MPG, looks like theory doesn't work well in your truck's case.  

By the way, you may have missed the discussion I had with Adam at Core Tuning re the inaccuracies of my AEM wideband.  According to Adam it is clearly off as it showed 17:1 on yours and he says the engine would have been bucking and missing pretty badly at that point.  So, maybe yours isn't as lean as we thought?

[85lebaront2]

That's why I am using the Innovate LM1, I had purchased it previously to use with the TwEECer and Adam informed my it was a good choice.
 here is a screenshot of my hardware tab.

[Ford F834]

1986F150Six wrote

My son's truck has a little more pep. As Jonathan remembered, my truck has 3.08 rear gears and my son's is 3.55. The gearing is actually closer than that due to the fact that I run the stock sized tires [215/75 X 15] and his has 235/75 X 15.

A quick internet search shows your tires are 27.7" in diameter, and your son's to be 29" in diameter. That would produce an effective change in gearing equivalent to having a 3.39 rear axle with stock tires.

1986F150Six wrote

I believe the edge in gas mileage is attributed to both the gearing and the fact that his still is as the Ford engineers designed. Mine does do better on the highway, though. At 70 mph, mine turns 1950 RPMs and his is at ~2200 RPMs.

David, do you know if your son's truck had the fuel economy package? I'm curious what this consisted of, and whether the emissions label to the left of the hood latch is the same as trucks that did not have the package.

When you say that yours does better on the highway, do you mean better mpg or better driving characteristics because of the power band?

I am finding three different 4-speed overdrive transmissions  used in our trucks. There were variations in the 1st and 2nd gear, but all had the same overdrive ratio of 0.78. Does this jive with the research you have done?

http://www.f150hub.com/trans/t170-smod-srod-tod.html

1986F150Six wrote

By the way, on the same driving course, I have tested two times [1 tank each] driving w/o using the O.D. on my truck. The mileage decreased by 2 mpg [pumping loss].

I find this interesting David, since I am weighing the merits of adding overdrive to my truck (the GVOD). I ran the math with my 31.7" tires and gearing so that I could compare my gearing to yours, apples to apples.

Your truck:
3.08 gears x 0.78 overdrive = 2.40 final ratio

Your son's truck:
3.55 gears x 0.78 overdrive x 0.955 tire factor = "2.51" "equivalent"

My truck:
3.00 gears x 1 (no overdrive) x 0.87 tire factor = "2.61" "equivalent" compared to 215/75R15 tires.

I still have not gotten any good empty mileage runs on mine but I'm guessing that a 0.78 overdrive on top of what I have would be too much.

Thanks for sharing your results!

[1986F150Six]

"David, do you know if your son's truck had the fuel economy package? I'm curious what this consisted of, and whether the emissions label to the left of the hood latch is the same as trucks that did not have the package.

When you say that yours does better on the highway, do you mean better mpg or better driving characteristics because of the power band?"

I am not certain regarding the fuel economy package, but I bet it was "born" that way. It originally had 2.47 rear gears and came with the small air dam attached to the front bumper. At one time [prior to gasohol], it would return 26+ mpg [flat highway] @ 65 mph.

His gas mileage now is ~22 mpg on the highway [larger tires, 3.55 rear gears and gasohol]. The front "spoiler" is now missing. I once did a short term comparison using my truck to compare with and without the spoiler. As best I could tell, the spoiler was worth about 1/2 mile per gallon increase. This was on my normal weekly route.

On the highway, the 1984 [son's truck], seems just a little bit "tight" @ 70 mph. Some engine noise, where as with my 1986, the engine is still in its "happy zone". When his is returning ~22 mpg, mine would be @ 23 or so.

[Ford F834]

Thanks David! It sounds like the '84 probably did have the economy package. I seem to recall tall gears, chin spoiler, and the upshift indicator being on the list.

They just recently rebuilt the Maverick gas station down the road from my house, and they now sell alcohol free "clear gas" with its own special blue pump handle. This is the first time I have seen this. It is 87 octane. I wonder if the additional cost of the fuel would be recovered in increased mpg?

[1986F150Six]

The 1984 does have the upshift indicator bezel on the left side of the dash, but was not equipped with a light or associated wiring/vacuum lines.

In my limited testing experience and with what our local vendors charge for non-alcohol gasoline, I do not think one will gain enough mileage to offset the price difference. By the way, Gary uses nothing but the alcohol free gasoline @ Mr. Bass station... https://www.facebook.com/pages/Mister-Bass/134170459968005

[Gary Lewis]

I think the only way to find out what pure gas does in your vehicle and your driving style is to try it.  There are just way too many variables.  This site says "The stoichiometric air-fuel ratio for pure gasoline is 14.7 approximately, and that of the ethanol-gasoline and butanol-gasoline blended fuel should be less than 14.7."  So, to actually get a complete burn and attempt to use all of the energy in the fuel the AFR should be adjusted.

However, it says later that "The oxygen in ethanol and butanol gives an additional assistance to achieve lean burning in the engine".  That would suggest that you can run the AFR leaner on a blended fuel than you can with pure gas, so even w/o adjusting the AFR the engine should still run acceptably, albeit with less power.  But, does the leaner mix offset the loss of power?

In a carb'd engine the air/fuel ratio will vary by each and every vehicle as well as each and every day since the temp and barometric pressure will have changed.  And, how far you are into the enrichment circuit will vary by driver and location - if not his/her mood.  So, it is a very complex equation and can probably only be solved by mere mortals via empirical data.  

[1986F150Six]

During the "gas run" between Blue [at that time, Big Blue] and "Little Red", both trucks consumed the non-ethanol enriched fuel from the same pump.

[Gary Lewis]

David - That is very true for when I'm "local".  But, I have ~17K miles on the 2015, and almost all of the miles have been accrued elsewhere, like the trip the Grand Canyon with the boat, the trip to Florida to get Big Blue, and the vacation a year ago to Colorado.  So I've used a wide variety of fuel in it, from pure gas when I can get it to ethanol-blended fuels when I can't.  And I can't easily tell the difference in MPG.

Having said that, I can tell a big difference when towing in the octane of fuel used.  This fits with what the owner's manual says about fuel, as it suggests using higher octane while towing.  I saw between 10% and 20% better MPG with 91 or 92 octane than with 87 octane, and while the math doesn't say that the increased MPG is a no-brainer with the additional cost, when you are getting ~10 MPG you'll pay the difference not to have to stop that often.

[85lebaront2]

I do know that contrary to what the government claims, the ethanol enriched gas gives about 5% less fuel mileage as non-ethanol gas. Non-ethanol gas also doesn't rot everything it gets near as seen here:

This was inside the tank on the 1967 Mustang the late Pete Medlin built using a 1996 Z-28 for running gear. He finished it either in 2010 or 2011 just before cold weather and put it in a container at his business. It sat there until May 2016 when it was rolled out for his memorial service. When they hooked up the battery to show it off, smoke came out of the dash so it sat until his son asked if I was ready to fix it. It was brought over here in October 2017. This is what the Camaro fuel pump looked like:
 

[1986F150Six]

1986F150Six wrote

Some of you have read previous threads where I have shared gas mileage figures. For years, I have driven what is a repeatable and consistent route during the week. This is to and from work plus weekend driving. For the most part, this is 25, 35 and 45 MPH with numerous stop signs and traffic lights. There is a 6 mile non-stop stretch which is 45 MPH and I drive that 2X a day.

Regarding the "repeatable and consistent route"... I was cleaning out my bill fold and found the last 3 gasoline receipts for the Pontiac Aztec and the 3 tanks returned: 21.17, 20.91 & 21.38 mpg.

[Gary Lewis]

Assuming that all three fillups took the same amount of gas, that's an average of 21.15 MPG, with a deviation of .09%, 1.1%, & 1.1%.  Extremely close!