Potential MPG of a 300 Galaxie?

[Parker]

I'm going to drop a 300 straight 6 into 1974 Galaxie. The transmission is a 3 speed 3.03 toploader, no overdrive. My rear end is 3.55 if I remember right. (WRONG - the axle tag says it's 2.75. Freakin smog years geez.

What kind of MPG can I expect at 55? 15? I may get a gear vendors to bolt on the back of the transmission eventually. Since the car sits a little lower and sleeker than the f100, I was hoping it would be better thsn stock for the truck. The old 351 that came out of this Galaxie probably liked the gas too.

A cheap solution for better mpg would be to drop in some taller gears, like a 3.05 or 2.73 if the 9" ever came with those options. I woukd think that the torque of the 300 should have no issue propelling a 5,000 lbs car along with such a tall gear. Any thoughts?

EDIT: With my newfound knowledge of the actual axle ratio, I'm getting these following calculations regarding RPMs, with a (assumed) transmission ratio of 2.99 in 1st, 1.72 in 2nd, and 1.1 in 3rd:

Gear | Speed | RPM
1st, 5 mph 478
1st 10 mph 956
1st 20 mph 1912
1st 25 mph 2390

2nd 20 mph 1100
2nd 30 mph 1650
2nd 40 mph 2200
2nd 50 mph 2750

3rd 40 mph 1279
3rd 50 mph 1599
3rd ~55 mph~ 1758 (In torque band?)
3rd 60 mph 1918
3rd 70 mph 2238
3rd 80 mph 2558 - Okay, come on guys it's a 50 year old car with screw ball steering and rear drum brakes, there is no way I am going this fast...


Yet.

 

[Frank]

I'm going to drop a 300 straight 6 into 1974 Galaxie. The transmission is a 3 speed 3.03 toploader, no overdrive. My rear end is 3.55 if I remember right.

What kind of MPG can I expect at 55? 15? I may get a gear vendors to bolt on the back of the transmission eventually. Since the car sits a little lower and sleeker than the f100, I was hoping it would be better thsn stock for the truck. The old 351 that came out of this Galaxie probably liked the gas too.

A cheap solution for better mpg would be to drop in some taller gears, like a 3.05 or 2.73 if the 9" ever came with those options. I woukd think that the torque of the 300 should have no issue propelling a 5,000 lbs car along with such a tall gear. Any thoughts?

My stock 1 barrel '79 f100 gets terrible mileage with a 3.03 and 2.80 gear. 1100 rpm @ 35 mph. Engine never reaches torque band, never sees 2000 rpm. Properly tuned. I'm about to put a 3.50 gear in it with some other mild upgrades.
Keep the rear gear you have.
Factory EFI engines get best mileage cruising at low "lugging" rpm. I've never had a carb engine that got best mileage if it was below the rpm the cam was beginning to breathe. With a carb cruise rpm 1800+ will give better mileage than lugging with low vacuum. A carb needs some air velocity with decent vacuum to meter effectively and stay out of the enrichment zone. You should get 15 mpg or more as it is.
This has been my experience anyway. .

 

[Parker]

My stock 1 barrel '79 f100 gets terrible mileage with a 3.03 and 2.80 gear. 1100 rpm @ 35 mph. Engine never reaches torque band, never sees 2000 rpm. Properly tuned. I'm about to put a 3.50 gear in it with some other mild upgrades.
Keep the rear gear you have.
Factory EFI engines get best mileage cruising at low "lugging" rpm. I've never had a carb engine that got best mileage if it was below the rpm the cam was beginning to breathe. With a carb cruise rpm 1800+ will give better mileage than lugging with low vacuum. A carb needs some air velocity with decent vacuum to meter effectively and stay out of the enrichment zone. You should get 15 mpg or more as it is.
This has been my experience anyway. .

Thanks. I think it was a 3.50 gear after all. I got my 8.8 and 9 mixed up. This will be my first carbureted vehicle, so I have no idea what to expect from it.

Most of my driving is going 45-55 in the mountains where you don't use overdrive much anyway. My only concern is being able to hold 55 or 60 on the highway and still have a little power. I've got 235 75 15 tires,so I've got some calculations to do. It would be nice to cruise below 2,500 rpm.

 

[rocklord]

I had a 1982 Ford Bronco with a carbed 300, 4spd non-OD transmission, and 3.55 gears.
Drove it like I stole it and got 14MPG. If this brick got 14, you should do better.

 

[pmuller9]

Are you going to rebuild the 300 six before you use it?

 

[comet6]

You carb jetting and enrichment strategy will determine a lot. Carbs with power valves need a reasonable main afr or you will not have enough torque and constantly dip into enrichment. A carb with step up rods is better at this. You need to find out where the motor is happy under load. A vacuum gage helps. You can monitor timing and afr optimized by watching vacuum real time. I have one in dash.

 

[Frank]

You carb jetting and enrichment strategy will determine a lot. Carbs with power valves need a reasonable main afr or you will not have enough torque and constantly dip into enrichment. A carb with step up rods is better at this. You need to find out where the motor is happy under load. A vacuum gage helps. You can monitor timing and afr optimized by watching vacuum real time. I have one in dash.

X2^^
Parker if you're in mountains @ 45-50 mph any taller gear will be miserable, engine will be too slow. My 2.80 gear and stock 235/75 R15 tires, only turning 1500 @ 47 mph. If I've got a trailer or pulling a bridge overpass (the only hills around here), the vacuum is 2-4 inches to hold speed, the carb is in the power band and is unnecessarily rich, the main reason this 1 barrel truck gets 11-12mpg.

As an added point, selecting the right rear gear is more critical on the 3.03 trans because the gear ratios are so far apart. Shifting to second is speed-limited since it's a 1.75:1 ratio. Thinking to go to a slower gear and use second at 45 or 50 up a grade will be too extreme with this trans unless you don't mind winding the engine out.

3.50:1 gear and your tires:
3rd gear
35 mph=1490 rpm
40 mph=1700 rpm
45 mph=1912 rpm
50 mph=2125 rpm
55 mph=2335rpm
60 mph=2550 rpm

In second, engine is maxed @ 4000 rpm at 53mph.

 

[Parker]

Are you going to rebuild the 300 six before you use it?

Not unless something is wrong once I run it.
I took off a couple of the main bearing caps to check on the wear. There was a little use present, but certainly no copper color. The valves look due for an adjustment, but all have good, even travel. Everything in the bottom end is tight.
I will, however, replace the plastic timing gear with a steel version. That is my first to-do once I get it running. Theoretically, all I need to do to start this old iron horse is to attach a radiator and fill it, give it a battery and some gasoline, and maybe a ballast resistor.

I have been soaking the rings in Marvel Mystery Oil for two months, since this engine sat in a shop corner for 3 years, according to the owner. I will test compression after it has ran for a bit and cleaned all of the cobwebs out.

I will also upgrade to an HEI self contained distributor with coil after the timing gear is replaced.

Do you have any suggestions?

 

[Parker]

My stock 1 barrel '79 f100 gets terrible mileage with a 3.03 and 2.80 gear. 1100 rpm @ 35 mph. Engine never reaches torque band, never sees 2000 rpm. Properly tuned. I'm about to put a 3.50 gear in it with some other mild upgrades.
Keep the rear gear you have.
Factory EFI engines get best mileage cruising at low "lugging" rpm. I've never had a carb engine that got best mileage if it was below the rpm the cam was beginning to breathe. With a carb cruise rpm 1800+ will give better mileage than lugging with low vacuum. A carb needs some air velocity with decent vacuum to meter effectively and stay out of the enrichment zone. You should get 15 mpg or more as it is.
This has been my experience anyway. .

Welp turns out I was waaaaaaay off. I just pulled the axle tag to get the full story, and get this. It tells me I've got a 2.75 rear end with the 8.7 ring gear size.

oof.

 

[aussie7mains]

LOW. Any 2250kg tank is never going to do anything but chew fuel. Gearing may help a bit, but dont expect much, if it has a 3.5 axle and 15 tyres then I dont think gearing it higher will help, just drive a bit slower.

 

[Bill56]

There are plenty of places where plastic is just fine.
I never felt like a timing gear was one of those places.

Get it together and see how you like it.
If it isn't good enough then swap in a T5 and long tube headers.

 

[48 F3 Joe]

I had a 1981 bronco with 3.50 gears.tried many things to get better fuel mileage. Finally got 16 at sixty mph for 1200 miles on the interstate. good enough for me.

 

[Bill56]

A 2.75 will cause it to be working hard all the time.
Maybe OK if you intend to only be cruising at 80, but still no decent mileage.

 

[Parker]

X2^^
Parker if you're in mountains @ 45-50 mph any taller gear will be miserable, engine will be too slow. My 2.80 gear and stock 235/75 R15 tires, only turning 1500 @ 47 mph. If I've got a trailer or pulling a bridge overpass (the only hills around here), the vacuum is 2-4 inches to hold speed, the carb is in the power band and is unnecessarily rich, the main reason this 1 barrel truck gets 11-12mpg.

As an added point, selecting the right rear gear is more critical on the 3.03 trans because the gear ratios are so far apart. Shifting to second is speed-limited since it's a 1.75:1 ratio. Thinking to go to a slower gear and use second at 45 or 50 up a grade will be too extreme with this trans unless you don't mind winding the engine out.

3.50:1 gear and your tires:
3rd gear
35 mph=1490 rpm
40 mph=1700 rpm
45 mph=1912 rpm
50 mph=2125 rpm
55 mph=2335rpm
60 mph=2550 rpm

In second, engine is maxed @ 4000 rpm at 53mph.

Gear | Speed | RPM
1st, 5 mph 478
1st 10 mph 956
1st 20 mph 1912
1st 25 mph 2390

2nd 20 mph 1100
2nd 30 mph 1650
2nd 40 mph 2200
2nd 50 mph 2750

3rd 40 mph 1279
3rd 50 mph 1599
3rd ~55 mph~ 1758 (In torque band?)
3rd 60 mph 1918
3rd 70 mph 2238
3rd 80 mph 2558 - Okay, come on guys it's a 50 year old car with screw ball steering and rear drum brakes, there is no way I am going this fast...

So apparently, with the 2.75 gear, this barge is going to be an absolute DOG until I hit about 30 mph, am I getting this right? At 55, it's just past 1750 rpm. Is this where the torque is? I've never driven one of these enough to know how the motor likes to behave.
The only thing I know is that it was an EFI, and it hated being lugged under 1700 RPM.

So cruising MPG doesn't look too bad so far as prospects go - am I screwed at anything around town? How well do these engines do on take off from a stand still? I'm mostly worried about stalling on slight hills when starting out. Do you think I'll have to slip the clutch?

I'm up for the challenge just to say I've done it with 2.75 gears. Most of my driving is to commute 18 miles to work, 55 all the way. Eventually, I'll have to drop in something a little shorter.

 

[Parker]

I had a 1981 bronco with 3.50 gears.tried many things to get better fuel mileage. Finally got 16 at sixty mph for 1200 miles on the interstate. good enough for me.

At this point, I'm not worried about fuel economy. I'm worried if it will even be driveable with how tall that gear is. I'll have to put in a 3.50 or something eventually. I'm gonna be taking a trip to Vegas next spring, so maybe I'll do it after all that cruising.

 

[pmuller9]

The stock cam is retarded 4 degrees. Most aftermarket cams are installed 4 degrees advanced.
Depending on the compression ratio, you can advance the stock cam and replace the stock 1.6 ratio rocker arms with 1.75 ratio rocker arms to increase low rpm torque.

The stock exhaust manifold and the stock intake manifold with the one barrel carburetor is also killing a lot of the power.

 

[bubba22349]

I will also upgrade to an HEI self contained distributor with coil after the timing gear is replaced.

Do you have any suggestions?

The HEI Distributor swap your plaining on isn't going to just drop in and work good in a 300 / 4.9 engine especially with an High Swirl EFI Head, its the totally wrong Timing Curve for them and it would take a lot of work to fix it to work correctly. The 300 has a lot of bottom end Torque and can pull a 3.25 gear well on mostly flat ground the good thing is there are lots of gear choices for those 9 Inch rear axles and you could even have a couple Third Members setup for different uses and swap them out quickly. Good luck

 

[Parker]

The HEI Distributor swap your plaining on isn't going to just drop in and work good in a 300 / 4.9 engine especially with an High Swirl EFI Head, its the totally wrong Timing Curve for them and it would take a lot of work to fix it to work correctly. The 300 has a lot of bottom end Torque and can pull a 3.25 gear well on mostly flat ground the good thing is there are lots of gear choices for those 9 Inch rear axles and you could even have a couple Third Members setup for different uses and swap them out quickly. Good luck

What about a drop-in electronic points conversion kit? I have one in the glovebox from the last guy. He was going to install it in the old v8 before it blew. I had never heard of one before.

 

[pmuller9]

If the distributor is not too worn out and the shaft doesn’t have any side to side play, you can keep the points and use an MSD 6A which will give you the best low end power and fuel mileage.

 

[Frank]

Even with the 3:1 first gear, dragging the clutch on take off is required on my truck. It's difficult to back a trailer, or anything requiring slow speed maneuvers, it requires clutch abuse from having to slip it constantly to effect. Where you live, moving that heavy car with a 3.03 and 2.75 gears will be an annoying experience. Pulling away from an up hill stop without stalling will be a challenge. With the clutch out in first, only 1000 rpm at 10mph. You could go about 50mph in reverse.
I'm not big on spending someone else's $ or giving advice- but with the work you're putting into this unique build, a rear-gear swap needs to be included.
Or go to a 4 or 5 speed trans with a very low 1st gear