Miles per gallon

[delco1946]

the truck has the aerodynamic qualities of a medium-sized Sears garden shed.

These are all excellent points and thoughts. Makes me feel better about where my trucks stands in terms of MPG - prolly should nick-name 'er "Gas Pig"

 

[Gary Lewis]

... since the truck has the aerodynamic qualities of a medium-sized Sears garden shed.

I respectfully disagree. I think the garden shed is more aerodynamic. Our trucks have recesses, like around the headlights and turn signals, that catch air. Garden sheds don't. Nor do barn doors.

Compare our trucks to the later trucks and you'll see that those recesses are filled in. So, while I consider the later trucks .... less than pretty, they are more aerodynamic.

 

[Rusty_S85]

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

Winter choke was a little slow for me so I got 12 mpg city due to my less than 5 mile drive to work.

Summer I averaged 14 mpg city.

Highway I average 17 mpg.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

[grumpin]

the truck has the aerodynamic qualities of a medium-sized Sears garden shed.

These are all excellent points and thoughts. Makes me feel better about where my trucks stands in terms of MPG - prolly should nick-name 'er "Gas Pig"

I think it has a shift kit because the shifts are firm. The torque converter I’m thinking may slip less than stock. About the best you can do without a lockup converter.

My Bronco is pretty consistent at 15 MPG on the highway. My Xterra does a little better, but it doesn’t have the grunt!

[mat in tn]

I think it has a shift kit because the shifts are firm. The torque converter I’m thinking may slip less than stock. About the best you can do without a lockup converter.

My Bronco is pretty consistent at 15 MPG on the highway. My Xterra does a little better, but it doesn’t have the grunt!

I feel like there must be more efficiency to be had although i have not proven it yet. However the benchmarks i have are many and not reassuring.

E150 5.0/aode and 3.31 gear 13 in town, 16 hwy

E350 5.8/ e4od and 3.50 gear 10/13

Expedition 5.4/4r75w 4wd 3.73 gear 12/15

Excursion 6.8/4r100 4wd with 3.73 12/17

These are some from the last five years and the excursion surprised me . About as much as the expedition disapoints .

[Pete Whitstone]

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Switching from a (numerically) lower 2.75:1 to a higher 3.25:1 ratio will hurt gas mileage by making the engine spin more RPM's per mile driven, won't it?

Still on my first cup of coffee this morning so I might be wrong, but I think that's the way that works.

[85lebaront2]

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Switching from a (numerically) lower 2.75:1 to a higher 3.25:1 ratio will hurt gas mileage by making the engine spin more RPM's per mile driven, won't it?

Still on my first cup of coffee this morning so I might be wrong, but I think that's the way that works.

Pete, a bit of that depends on several factors, the basic item is what is called the "sweet spot" where the engine is at it's most efficient rpm. Back when Nixon put in the national 55 mph speed limit, he cost me several miles per gallon on two of my cars I owned at the time.

My 1966 Shelby with the Cobra 289 and sporting dual Holley 4 barrels with a 3.89 gear went from 20 mpg down to 16-17. Sweet spot on that engine was 3500 rpm, almost all of the mechanical advance (no vacuum) was in and I would be running on the primary barrels of the front carburetor (max 232.5 cfm). This was right at 70 mph. Drop to 55, rpm went down to 2750, about 3/4 advance, so there went my efficiency.

The other was my 1963 Oldsmobile Jetfire, it dropped from 24 down to 18, it was a turbocharged 215 ci aluminum V8 and I had removed the enrichment spring from the Rochester RC sidedraft carb. 3.35 gear and the only one of the BOP compacts with 14" wheels as standard equipment. At 70 the manifold gauge would sit just below the center (0 boost or vacuum) maybe 1-2" of vacuum, so the Garret turbo was doing most of the work on filling the cylinders, but mixture was still fairly lean. Drop to 55, vacuum went to about 1/2 scale, and probably less spark advance. Funny thing, the Jetfire distributor had a vacuum advance, so in theory it should have done better with more vacuum.

[Pete Whitstone]

Pete, a bit of that depends on several factors, the basic item is what is called the "sweet spot" where the engine is at it's most efficient rpm. Back when Nixon put in the national 55 mph speed limit, he cost me several miles per gallon on two of my cars I owned at the time.

My 1966 Shelby with the Cobra 289 and sporting dual Holley 4 barrels with a 3.89 gear went from 20 mpg down to 16-17. Sweet spot on that engine was 3500 rpm, almost all of the mechanical advance (no vacuum) was in and I would be running on the primary barrels of the front carburetor (max 232.5 cfm). This was right at 70 mph. Drop to 55, rpm went down to 2750, about 3/4 advance, so there went my efficiency.

The other was my 1963 Oldsmobile Jetfire, it dropped from 24 down to 18, it was a turbocharged 215 ci aluminum V8 and I had removed the enrichment spring from the Rochester RC sidedraft carb. 3.35 gear and the only one of the BOP compacts with 14" wheels as standard equipment. At 70 the manifold gauge would sit just below the center (0 boost or vacuum) maybe 1-2" of vacuum, so the Garret turbo was doing most of the work on filling the cylinders, but mixture was still fairly lean. Drop to 55, vacuum went to about 1/2 scale, and probably less spark advance. Funny thing, the Jetfire distributor had a vacuum advance, so in theory it should have done better with more vacuum.

True, I had not considered that. I wonder if there is a solid correlation between max MPG and VE. I know there is a strong one between VE and torque production, but I don't know it that also means max MPG. Interesting.

Still, I would have to think that C6 is slipping awfully bad to make up a .5:1 swing in axle ratio and still come out better, even with the sweet spot taken into account.

I will say that regardless of the exact set-up, 24 mpg in a non-aerodynamic truck, presumably carbureted, 2 valves per cylinder, no VVT, variable intake tract trickery, etc. is a tall ask from a 5 liter engine. I mean, 2021 Ford truck factory mileage is in the 26-26 range, and that's WITH all the stuff I just mentioned. I realize they are also probably a lot heavier, but still.

I hope Rusty can hit 24, but I'm skeptical.

 

[Rusty_S85]

302 C6 all original with a 2.75:1 9" axle with taller than stock 31x10.50-15 KO2 tires.

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Switching from a (numerically) lower 2.75:1 to a higher 3.25:1 ratio will hurt gas mileage by making the engine spin more RPM's per mile driven, won't it?

Still on my first cup of coffee this morning so I might be wrong, but I think that's the way that works.

Yes and no. If nothing else is changed it will hurt gas milage by increasing engine rpm at a given speed.

In my case it wont because I went from a 28" tall tire to a 31x10.50-15 tire which is supposed to be 31" tall but is really 29" tall. That extra inch in heigth puts my effective gear ratio from 2.75:1 closer to 2.50:1 - 2.55:1 which is lugging my engine down. Like wise my C6 has excessive slip right now cause I should be turning around 2,400 rpm at 75 mph but right now I turn 2,750 rpm at 75 mph which increases fuel consumption as well. Also the engine I am building the truckmax roller cam I have has a listed cruise rpm of 2,400 - 3,000 rpm for ideal highway speed.

The math I did shows with 3.25:1 axle ratio would put me around 2.90:1 effective ratio when accounting for taller tires. The C6 I am going to have built by Broader Performance should be around 3% - 5% slip when coupled with the Hughs XTM I believe it is tow master converter that multiplys torque more, flows more fluid, and has less slip than a standard converter. With this taken into account if I am in the 3% - 5% slip range with a 3.25:1 axle ratio it will put my 75 mph speed right at 2,750 rpm and 65 mph right at 2,500 rpm. If my slip is more which I wont know till I do real world testing with the transmission I figured a 3.00:1 would put my effective axle ratio around 2.70:1 and between 5% and 10% slip I should have 75 mph at 2,750 rpm.

When making as many changes as I am to my truck you have to do it in steps and make it work together. Why I might have to get Adam to redo the tune on my sniper once I get the transmission and axle ratio change done as it may want a different fuel table than what he comes up with me with my current setup. But my goal is to try and keep 65 and 75 mph with in the narrow 2,400 - 3,000 rpm cruise range that is listed for my camshaft as the roads I drive are 60 - 75 mph but I typically go 65 and 75 mph.

[Rembrant]

With my new engine, replacing my excessive slippage C6 for a aftermarket C6 and switching over to a 3.25:1 axle ratio I hope to be in the area of 18 mpg city and 24 mpg highway.

Rusty, I wish you all the luck in the world, but I have a hard time imagining a 302 running 3000 RPM with a C6 and 31" tires hitting 24 mpg.

Even if you consider the 1987-1996 trucks, with speed density or later on MAF, with 5spd's and 3.08 gears, most of those trucks could barely even hit 20mpg on the highway. They surely wouldn't do in 4th gear at 1:1 ratio like a C6. I can see the Holley Sniper being better technology...sort of...but it's still throttle body EFI, and not multiport EFI.

To throw my old truck into the MPG mix for the OP, it was an '84 2wd Flareside with a mild built 302, a 1991 M5OD 5spd trans, and original 3.08 rear diff. No AC, and no other options really. Truck weighed 3500 lbs. I drove it on the highway quite a bit, and the best I ever got with it was around 18.5mpg...maybe it kissed 19mpg one time, but I'm not sure...my memory is getting fuzzy.

That was at around 2000 RPM in OD, running between 60-65 mph. My AFR was usually a little over 14:1 when I was cruising easily. I did try some highway trips at faster speeds and higher RPM's, and depending on the day it never really hurt my mpg too much...sometimes 1 or 2mpg less, but I think the wind drag negated any efficiency I might have gained with the higher RPM's.

I never did try running the truck up in the 2500-3000 RPM range in 4th gear on the highway to see if the MPG would improve. Maybe that's the answer to getting better mileage...higher RPM's and lower gears...I've never thought to try it.