Why our cars get bad fuel milage

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This has been a fun thread to read. Now that several folks have had their fun, I have some fairly serious info to share (any surprises there?). We have a few fairly thirsty pony cars. The firs tis our venerable 1969 Shelby GT500 with its very powerful 428CJ engine. It is in pure stock form, to include the A.I.R. injection system with a still functioning A.I.R. pump. Usually, when driving in a sane manner, I can get 8 MPG. If I am cruising at 60 - 65 on an open highway, with no stops other than to refuel, I can get up to 13 MPG. But, if I begin to have some fun with Wide Open Throttle runs here and there, the fuel gauge and tachometer race each other to see which one's needle can reach its opposing extreme position first. I have no doubt that once that begins to happen I am running fractional Miles Per Gallon, and a more practical way of measuring fuel consumption would be Gallons Per Mile. An aside, Ford rated the 69 428CJ at 335 HP, but at only 5,400 RPM or so. When it is run to its "real" maximum HP rating it reportedly puts out closer to 415 HP - a fairly big number back in the day.

As if that is not "bad" enough, we also have a 2020 Shelby GT500. Its SuperCharged 5.2 liter engine is a lot smaller than the 1969 428CJ. But, between the computer controlled fuel and ignition, and the SuperCharger, that engine puts out up to 760 HP. So, more power is good news. The other good news is the ignition and fuel systems are both computer controlled, so there are some inherent efficiencies that engine has. With me driving in a same, calm way I usually get about 13 MPG. I am certain if I drive even more civilly I could get 15 or more MPG. But, when I am driving in a spirited manner I can count on 8 MPG. However, like the 69 428, if I begin to mash on the throttle I have little doubt I am achieving a 1 MPG rate of consumption. I dare not run it at wide open throttle long enough to get an accurate mesaurement however, as by the time the on-board computer calculated the MPG at WOT I will have hit Warp Speed, and may never come back to reality.

So, the two GT500s represent one extreme of both performance and fuel consumption under varying driving conditions. We also have two 1973 Mustangs. The 73 Mach 1 has a transplanted 1994 351W (not C) that replaced the original 302 2v engine. It is built for moderate street/strip performance, and sports (amongst other things) a Holley 735CFM Vacuum secondary carburetor. Many folks would think, and perhaps even say, the 735 CFM carb is too much for the 351W. But, the Vacuum Secondary controlled throttle blades make it a perfect carb for street use. I know when I open it up at road speeds of under 50 MPH, and the tranny in 2nd gear at Wide Open Throttle (WOT) I can both hear and feel when the secondary throttle blades are beginning to open, and given how much harder the engine is pulling it is obvious the secondaries are coming open at an appropriate time as the added volume of air/fuel coming into the intake manifold is not overwhelming the engine when it is at WOT and the engine RPM is climbing to and past 5,000 RPM. Normally, with tame driving the 73 Mach 1 runs at about 15 MPG. When it is opened up it sucks fuel very quickly. Likely not as fast as the 428 CJ, or even the supercharged 5.2L engines. But, it is a fast enough level of consumption to cause me to feel I ought not be behaving like that too often.

Then we have our docile 1973 Mustang Convertible with its 302 2v, C4, engine/tranny quipped vehicle. With its 2.79:1 rear axle ratio gear set there is not much in this particular Mustang that roars "high performance." But, it is an otherwise good performing machine, no complaints. I have not yet tried to measure its fuel economy, but I am guessing, based on prior Mustangs I have owned, it is getting an average of 17 MPG, and can be persuaded to tun at highway speeds at 19 - 20 MPG if I am very judicious in how I am driving.

All that said, the ONLY reason I am even the slightest bit concerned about fuel economy is for driving range considerations. This is not an issue for the 2020 GT500, as it is designed to tun with ethanol mixed gasoline. But, for the vintage pony cars I use only non-ethanol gasoline, despite its low 90 octane rating. Not every community has a non-ethanol fuel pump available. So, I have to plan my trips carefully to make certain I can continue to avoid using ethanol gasoline. In a pinch I know I can use ethanol gasoline, but I prefer to not do so with the older Mustags and Shelby - just because.

There are two considerations with all 4 pony cars that can help me gain higher MPG performance, and one more thing I can do with the vintage pony cars to help ensure the best possible fuel economy. For all 4 pony cars I can keep the tire inflation at the highest rated PSI level. I know a lot of folks will say it is not good to run tires at their maximum rated PSI level, but I do it to reduce rolling resistance, which increases MPG. And after all these years I have not had tires wear out their center tread from running at 32 PSI (or so for the older pony cars).

And, for the older pony cars it is good to be running the ignition timing as far advanced as possible without causing pinging. With me using 90 octane non-ethanol gasoline that means I am running the engines at the factory 6 degrees BTDC. If I had decent octane levels available I could move that to 12 degrees BTDC and have far more efficient and powerful engine performance. Along with the initial ignition timing being set to a level that allows good performance without significant pinging it is important to check the Vacuum Advance Diaphragm on the distributor to make certain it is not leaking or ruptured, and to make certain Ported Vacuum is provided off idle. Having a non-functional Vacuum Advance Diaphragm will cause the engine to run warmer than needed/normal, and will reduce both engine performance and fuel economy.
 
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When I was 16 and had my first car (71 Mach 1) I didn't give a crap about miles per gallon. I would fill up the tank on a Friday (payday) Give the gas attendant, who would check the oil and tires while filling my tank up with regular, a 10.00 dollar bill and he would give me back change. 😁
 
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I just have to say your Mustangs are not design for Gas Mileage not even a Six with 2.79 3rd. The Improvement started with the Mustang II and Four Banger and then the US finally switch over to Over Drive Power train systems. That was in the early 80's then EFI systems mid 80's Now everything is Computer Controlled with a EFI, Variable Timing Cam and Individual Coils to Spark Plugs. And I can go on but Lets get back to the Older First Gen Mustangs. Not sure who the 7734 came up with that Designation Like nothing change until the Mustang II's?
Most Mustangs are or has been modified ether for Speed or Handling which both are not great for Gas Millage. Let's stay with the 1971-73 Mustangs, Transmissions 1:1 final Ratio's, 2.79-3.25 3rd's and Wheels E70/14 (25.9" Diameter/about 8.4" wide) and wheels are different depending on models.
Once we start mods the Gas Numbers goes down.

Here is what you can do to get the numbers up:

Overdrive Transmission - 5-Speed or AOD with Final 0.65:1 gear
Rear 3rd - 2.79-3.00 (Lower is better)
Taller Tires - about 28" overall Diameter (Don't really care about rim diameter).
- about 9-10" Width (225-255mm Smaller is better)
Better Ignition System - Spark about 42-50KV
Last Better Fuel System - First - Dual Plane intake with greater Length Runners
- Second - EFI - Computer Control 2/4 Injectors
- Third - EFI - Computer Control Individual Injectors
If you Want Speed - Turbo or SuperCharger -
- NOS -
Lowering or strengthening suspension the Car really won't have any effect to Gas mileage but when you start putting on Wide tires and 25" Diameter..... Plus increasing Tire Pressure you are sacrificing the ware and cost of replacement more frequently.

With the mods 302 or 351 about 15-18 City and 24-26MPG on Hi-way.
Here is a comparison of 18" and 20" Rims on a 1971 Europa
 

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You all really check your fuel mileage? I don't know if I've ever checked it in the 45-years I've owned my 72. Then again, I wasn't thinking about fuel efficiency when I bought it.
Yep, same here. When the Stang needs gas I put it in...
 
Definitely could be!
I have a 73 Q code with a 670 cfm Proform carb. I'm getting around 8mpg in the City but 15 on the highway. The factory carb got me around 11 mpg in the City and about 15mpg on the highway.
 
You all really check your fuel mileage? I don't know if I've ever checked it in the 45-years I've owned my 72. Then again, I wasn't thinking about fuel efficiency when I bought it.
I check it against different premiums in my area, but when I get in the mood while she’s running right all bets are off regarding any accurate SPG analysis ;)
 
Somewhere I have a picture of my dad in his ‘71 Boss 351..Pulling a boat and trailer! and If that’s not bad enough, it was early spring in Michigan. He had SNOW TIRES on it! That should have cost about 5 mpg!
if I can find the pic I’ll post it. or maybe none of you could stand to see it. (lol)
Oh, ....I have done that for 33 years with my Mach 1. It really tows well. I get around 10 pretty much all the time, street or highway.
However.......mileage is right at the bottom of my list, as to why I own and drive such a car. I've said it before...If the cost of fueling the car is of major concern, you shouldn't own it. Most musclecars from that era get comparable mileage, I'm sure. Go buy a Chevy Volt, and see just how many people give you a thumbs up, or yell across the street "Nice Car!!!!"
 

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Oh, ....I have done that for 33 years with my Mach 1. It really tows well. I get around 10 pretty much all the time, street or highway.
However.......mileage is right at the bottom of my list, as to why I own and drive such a car. I've said it before...If the cost of fueling the car is of major concern, you shouldn't own it. Most musclecars from that era get comparable mileage, I'm sure. Go buy a Chevy Volt, and see just how many people give you a thumbs up, or yell across the street "Nice Car!!!!"
Absolutely!
 

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