Here's how I'd build a super-duper mileage old mopar w/v8 (let's face it...a slant would be a better endeavor for mileage...but should you want a slant, apply much of what is here to it):

Start with the lightest available platform with easy access to parts...for a v8, the 64 A-body comes to mind.

Weight is CRUCIAL, especially with city mileage, and important out on the hwy...grades, minor acceleration, rolling ressistance are all heavily influenced by the weight of the plat-form you are using.

Another reason size is important, is that it is the second most important aspect when determining WIND RESISTANCE (speed is most important).

But going this way, you are minimizing the two most important things with regards to fuel mileage.

Face it, cruising at a steady speed, you need to apply as much hp as there is ressistance (more and you accelerate, less and you lose speed). When HP=rolling resistance+Wind resistance, you have steady state cruise. Increase speed, Wind resistance goes up exponentially, HP demands go up too. Increase weight, and you effect rolling resistance, AND you increase the amount of work needed to accelerate to speed, and to ascend hills either slight or great.

And HP will take a relative amount of gas and air...since really, that is the "work" that HP is calculating.

Given it's a v8, I'd go with the 318. In the ideal world, a 273 WILL have greater operational efficiency at a given speed/rpm/HP demand, but I think that this is more than off-set by parts availability and price.

Given this, and in keeping weight in mind: I'd build a roller 318 with eddy AL heads.
Give me 10.5:1 compression
Eddy AL heads (reduce pumping losses, relatively small ports will maintain intake velocity, wt loss)
Performer or LD4B intake (wt is the key here)
Eddy 500 carb ( tuned to idle as lean as possible, cruise on a slight hill with as little lean surge as possible...I'd aim for roughly 15-15.5:1 AFR).
HYD roller cam specing ~192-196 @ .050, roller 1.6:1 ratio rockers. Ideally lift in the .480 range.
1 5/8th headers
2.5 inch dual exhaust with an X-pipe.
Electronic ignition running 14 degrees initial, probably 34 total, in by 2200rpm...and as much vacuum advance without pinging.

For the tranny, I'd source an AL OD 833 (light weight and more efficient). For the rear end, I'd source a 7 1/4 rear with 2.94 gears for a 2.2 final drive ratio (lighter rear/gears/axles reduce losses/rolling resistance on several fronts...the closer to 1:1 a gear is, the more efficient it is with regard to frictional losses.)

I'd probably get an underdrive pulley set, have manual steering and no AC. I'd use a clutch fan too. A slant six radiator would have to do the job. Run a 205 degree thermostat. Heck, I'd even run a small battery to reduce weight.

Make sure the wheel/axle bearings are in good working order and adjusted correctly. I'd probably WANT 4 wheel drum brakes to reduce any drag on the wheels too.

For tires, I'd run what the insight runs...bridgestone potenza re92 (iirc)...very low rolling resistance. Inflate to 50psi (it's safe, however not recommended).

See where else you could lose weight. Run thin 5-30W synthetic oil too.

The gears/OD combo I selected because of the reasons I had posted elsewhere:

Quote:

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by Prince Valiant

While it's true, you shouldn't go with the lowest gear available/possible, you shouldn't try to target the torque peak either.

You are correct that the torque peak represents the point of greatest volumetric efficiency...this is, however, on a dyno. Cruising presents a different dynamic. You want to generate greater volumetric efficiency, and that is through gearing.

Here's an example of what I'm talking about:

Let's say I've got a car, and I'm driving roughly 60mph. I've got a tire with an outside diameter of 25.6 inches (245/60R14). My car has a 440, that at 100% VE makes 500ft-lbs of torque (these are just given, any number or representation will illustrate what I'm trying to demonstrate)

Now, for arguments sake, let's say it takes approximately 30HP to cruise at 60MPH, once you account for wind ressistance, rolling ressistance (we are, of course, negating other parasitic loses that are either equal b/w the two examples, or at this point unimportant for this demonstration).

Now, say I've got 3.55 gears, and a 1:1 final drive ratio. According to the math, I should be roughly traveling at 2800rpm at 60mph.

Let's do the math on my VE then: IF it takes 30HP to maintain 60mph, and 60 mph translates to 2800rpm then:

(30hp/2800rpm)x5252=56.2ft-lbs of torque that my engine is generating to maintain this cruise speed.

If at 100%VE my engine makes 500ft-lbs of torque, then at this speed, I can calculate that my engine is operating at 56.2/500=11.2% VE.

Now, say I swapped in 3.23 gears AND put a OD in the thing with a 0.69 ratio like the a-518. This brings my final drive ratio to 2.28:1

Same speed as above, it'll take the same HP to go down the road....30HP. But, now my engine lugs along at a relatively low 1800 rpm's.

Now again, the math:

(30hp/1800rpm)x5252=87.5 ft-lbs of torque.

To generate that same HP at a lower rpm, I've now got to generate MORE torque. And, since I generate more torque...I'm operating at a higher VE! In this case, I'm now going along at a 87.5/500= 17.5% VE vs the 11.2% earlier.

This higher VE also comes with higher cranking compression, so you are getting the most out of the fuel you put in there too (since it requires more air and fuel/PER ENGINE REVOLUTION to generate higher torque...but this is offset by the significantly less engine revolutions per mile)

This is why cruising is different. Sure...if your engines peak torque is 4500+rpm, you won't do yourself many favors by going with uber low ratios...but, targeting torque peak isn't the way to go either.

On top of the greater VE, you also have to account for the fact that lower rpm's per mile means less frictional losses (approximately 36% less in the example above) as the pistons don't travel as far, the bearings don't cover as much ground, and the engine accessories (water pump, power steering, altenator, AC if applicable) are turning fewer revolutions/mile. AND you get less frictional/reciporacating losses because not only is the engine turning less, but many of the pieces of the tranny is turning less, if a low rear gear is used, the driveshaft is turning less too. All these things require work, and you make less of it per mile with higher gears.

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The above, are the same reasons why bigger engines don't do as well (less operational VE...but the difference b/w a 273 and 318 are small, and more than offset by the ease of piston/head/roller cam availability of the 318 over the 273).

The car I specify above would easily, tuned well, imo get over 30mpg.