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]

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?

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]

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?

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]

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.

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]

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]

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].

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]

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?

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]

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.

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

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]

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.

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

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!

"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.