[buzzboy]

As of late I have been thinking about working on my gas mileage situation. I use the car as a DD and for cruising between home and school(~300 miles). So far I have come up with 3 things that cause our old cars to get bad mileage compared to their modern, equal displacement counterparts. We have carburetors, no overdrive and less aerodynamicness(not a word). For that reason this summer I am thinking about getting the old girl some help. I just want to make sure though that these really are the only reasons that we get worse mileage. Is there some other tuning factor to our engines that make them burn more fuel?

The reason is that I want to add FI and maybe a T-5/similar this summer and I am hoping that I can get at least 20mpg+ highway consistently. Now I am getting only about 10 in the city and 18 highway. I know some of this is my carb(was talked into a 600 vac-secondary by somebody) and the rest is not having OD. What are y'all's thoughts.

 

[cindys_sn95]

with those 3.89s an o/d transmission will make a world of difference. :bigthumbsup

 

[Ricardis]

G'day,
I dont know to much about cars yet, but I know some little things that can help... Tyre pressure is important, less rolling resistance, always keep your gas tank half full, it will evaporate much slower when its full, always fill up in the morning or on cold days because the petrol stations dont have gas compensaters on their pumps only the refrinerines do, (evaporation) so you you get less fuel for you dollar, remove any dead weight that you carry to improve power to weight ratio, if you dont use you back seat pull it out =] apart from those inexpensive mods, a nice msd ignition system, will make sure you burn that fuel properly. :yup:

 

[buzzboy]

Thanks guys but not quite the question I was asking.

 

[MustangBradley]

Think of it like this. Your carb gives each cylinder an approximation of what it needs for complete combustion. It is better to be slightly rich (more fuel) than slightly lean. That extra fuel leads directly to less MPG as you obviously can understand. Then you have steep rear gears and a 1:1 fourth gear, which leads to a high rev at cruising speed. So with each intake stroke, not only are you feeding it more gas, you are also running many more intake strokes per mile.

As posted above, the tire pressure has a direct and measurable effect on MPG.

You asked about other factors that contribute to this but you are already aware of the lion's share of the problem. The computer on a modern car, using the O2 sensors can get your fuel mixture to exactly what it needs to be, thus giving you maximum MPG.

Since you currently get 18 on the highway, I bet a switch of rear gears and a 5 speed will get you to the 20 mark that you seek. If you add FI, it will just be gravy on top.

 

[rex1965]

Hands down, the overdrive transmission will make the biggest difference in your gas mileage, other things will help, but not as much, just because and engine is carbed doesn't mean it is doomed to bad gas mileage, a inline 6 is an old mustang used to get 26 mpg without overdrive, 3.89 gears are not good for gas mileage, the overdrive will help compensate. In your carb jetting your mains 1 size smaller might help, You might also think of some ways to keep the air from getting under the car (spoiler) and air dams. Good Luck.

 

[buzzboy]

Okay, so to rephrase my question slightly, if my 289 was in a modern car with FI and OD then it would get near the same mileage as a modern 4.6? If not, those are the reasons that I'm looking for. Thanks.

 

[Jonk67]

Yes, you should be able to hit 4.6L mpg which is basically ~15/22, your '67 should be 500lbs. lighter than a newer GT also.
As mentioned your 3.89 gears and 1:1 final drive are what's hurting you most right now. I think you'll see the biggest improvement with an OD trans. install to lower the gear ratio at hwy. speeds and drop your RPM's probably 1K or more on the interstate lowering the wear on your engine also.

I've never switched to FI so I can't quote but have read generally that it only improves mileage by 1-2mpg hwy. It may not be worth the expense for the savings, you'll have to decide that. Using your300 mi. 18mpg (16.6 gals.) vs. say 20 EFI (15 gals.) or ~1.5 gal @$2.50= $3.75 saved. What would an EFI conversion cost? <$500?

A 600 cfm vac. secondary isn't a hog of a carb. and not a bad match for your 289, as mentioned testing different jets may help or even gettting a tune to see where your air/fuel ratio is.

Look for gas stations near you that sell 100% gas with NO ethanol, 10% ethanol gas will get worse mileage and will eventually dry out the gaskets/seals in your carb.

If you can do the changes one at a time and log the difference for ~3 tanks of gas and average it.

I plan to drive mine a lot long distance also when I'm bacck on the road and will be installing a 22 gal. '70 tank in place of my old 16gal.for almost 40% more fuel capacity and miles between fill ups.
Jon

 

[Veronica]

Hello. It helps to think of it in terms of how many revolutions of the motor does it take to cover a certain distance in a certain amount of time. If your motor is operating at a level of efficiency greater than that of a modern car, that isn't going to help the mpg if your tach is reading 2900 rpms will the newer car's is reading 2200 going at the same speed on the same road. That's where the overdrive gear makes the difference. In your car, each cylinder had to fire an additional 700 times each minute to accomplish the same thing.
Obviously, you need to have the car in a proper state of tune. Spark plugs are not like light bulbs. They don't function flawlessly until the instant they fail entirely. Their performance degrades slowly over time until they finally sputter out, and this process starts the moment you start the car with the brand new plugs.Change the plugs, cap, and rotor at no greater than 5000 mile intervals, no matter what the manufacturer says. A properly tuned carburetor doesn't miss fuel injection levels of efficiency by much, if any. It is important to have the float level and choke set correctly, have the correct jets and power valve for the altitude at which you live, have the accelerator pump adjusted properly, and, make sure that everything is clean in there. A tiny little chunk of crud, nearly microscopic, stuck in the wrong place can knock the bottom out of the efficiency of the motor. Some free breathing headers will help out on the fuel economy, also, especially with a 2bbl carb.

If the ignition system and fuel delivery system are set up correctly on a carbureted car, it will perform at new car levels. You can use a remote O2 sensor and exhaust gas thermometer to get the right jets in the carb. Test what you have, and change them according to to the readings. You want the lowest amount of oxygen coming out the tailpipe at the lowest temperature. Neither reading will be the absolute lowest possible, but, you will notice that as one increases the other decreases. You want to hit that sweet spot at which they are both really low. You will also need to make sure that your cooling system is up to par.
Properly tuned along with an overdrive transmission, your car will do just fine.

 

[nolan_speed]

MPG is no cliche, it can happen.

I didn't realize the new mustangs only got 15/22mpg! my 65 got even better than that before I pulled the engine to hop it up with heads, cam and a bigger carb. I had a 302 with a stock 88HO roller cam, 54cc 289 heads (mildly ported) performer rpm intake, 600cfm Holley 4160, T5 and 3.50 rear gear.

I have all the fuel injection stuff but after talking to guys who have souped up their 5.0 mustangs, their MPG is pretty bad, so I figured that just a performer rpm intake and a carb swap along with upping the compression with 289 heads would gain me some power without costing me much MPG and it did! A bone stock fuel injected 5.0 might get a few MPG but it would be down on power. You'll gain the most MPG by getting an overdrive, then maybe some 3.50 gears, and make sure your carb is tuned properly. Also if you still have points, convert to some type of electronic ignition, the spark is so much better.

 

[Ivy66GT]

If you can now get 18 MPG with a 3.89 rear end that is quite impressive. Better MPG will require different gearing. Keeping the speed down helps as well.

My '66 289, C-code, C4, A/C (those last 2 don't help MPG) coupe has 2.80 gears and since new has consistently given both me and its original owner 20+ on the highway. I have nearly all the data on the car since new and he typically got 20+ on long trips, even as much as 23. We drove it 3800 miles round trip to Tennessee loaded to the hilt with the whole family/luggage and AVERAGED 21. This is all completely stock, especially for the original owner since its was setup how he bought it from Ford: stock ignition, stock carb jets, etc. I added a cruise control but the PO did it all with his own right foot.

In town, dream on. It is now, and always has been consistently in the 12-14 MPG range.

 

[gcroix]

66 coupe, stock 93 5.0, 1990 AOD, 3.0 rear end. I get 23+ consistently. I am positive the mpg is higher, but it has been a while since I've checked it. The last time I checked it, it was ~25+, but that sounds too unbelievable to post here. At highway speeds, I run just under 1600 rpm if I recall correctly. I want to change the rear gear for better performance though.

 

[buzzboy]

Thanks guys, for the replies. I guess the T5 swap comes as number 1

 

[PaulS]

Okay, so to rephrase my question slightly, if my 289 was in a modern car with FI and OD then it would get near the same mileage as a modern 4.6? If not, those are the reasons that I'm looking for. Thanks.

New cars get better mileage because the are run by a computer or a few computers.
The fuel is managed by a computer - the transmission is run by a computer - the acceleration is operated by a computer. Add to that the cars are more aerodynamic and tuned to take every advantage that can be taken.

If you have a carbureted car and you have made mods to gain power and acceleration then you have given up some fuel mileage but the way you drive your car makes more difference than any of the mods. I get 20+ MPG with my 73 Maverick and the engine is tired at 108000 miles. with a good rebuild (no high performance cam) that uses 9.5: compression and 4 bbl carb and manifold and a better (dual) exhaust I am sure I can get even better.

I drive conservatively - keep the air pressure in my tires at 35psi and I keep the engine tuned and the oil changed. I idle the engine at 650 rpm and rarely use my brakes in traffic because I follow at two to three seconds behind the car in front.
Slow acceleration
Slow decelleration
Hard tires
Warm engine (180F degree thermostat)
and get used to being passed and cut in front of.
Plan your trips - start early, stay calm and enjoy the drive.
That is the best way to improve your mileage.