mpg on a '75 Clamshell-What should I expect?

Negatory on that. The downsized models are a bunch more fuel efficient and shouldn't be compared. OP's mileage sounds typical.
The Rochester 2gc on the 400 is a pig, ditch it for a Quadrajet or other "spread bore" carb.
Also worth investigating would be an "RV" cam for the engine. These were a very mild grind designed for low end torque which is what it takes to get one of these barges going several hundred times a day in city traffic. I put one in a big Dodge wagon years ago and it was an improvement in every way. The engine could be made to smoothly idle down to about 400 rpm, low enough to cause the oil pressure light to flicker. It gave the engine more grunt down low which meant less throttle opening for the same acceleration.

 

Just for the sake of comparison, my 94 Roadmaster estate is 5040 lbs with me in it and a full tank of gas. I routinely get 16 city and 21 highway with a 3.42 rear axle.

 

I wonder if anyone on the planet has swapped an LS drivetrain into a clamshell? If so, I'd be curious about mpg numbers.

 

I have a brother-in-law who's side business is implanting newer VW turbo-diesel drivetrains into older VW's. Since the newer VW's have a tendency to be way heavier than their older counterparts, the mpg's go way up. His late-60's Fastback gets about 70 mpg highway with a newer turbo-diesel drivetrain. AND, it goes way faster way quicker. Now,,, since it's a FWD thing,,, put that VW turbo-diesel drivetrain in a mid-60's Eldorado or Toronado, it might not do so good. Wear and tear and all... But, as I've come to understand, that's a problem with the big V8 FWD drivetrains from GM anyway. SO, what could it really hurt. The Eldo or Toro might not be lightning quick or blazing fast,,, BUT, it probably will get better mpg's. Maybe not MUCH better, but hey, any improvement can be appreciated...

 

With a few minor mods, a friend was doing 8.9L/100km on the highway with his Tahoe (according to the dash read out), and that is about 26 mpg using the American gallon. That was in a 7,000 pound truck. Put that engine and transmission into a clamshell, and you should be approaching 30 mpg U.S. in one of those big wagons that has the same cargo capacity.

 

Sounds plenty good. And the clamshell is way more stylish. Have also been thinking of turning mid-70's Ford and Mercury wagons into bio-diesel vehicles, as well. Nothing too powerfull. A turbo-6, if Ford trucks have such things. I know they have diesel Ford trucks that allready get double the 8 or 9 city and 12 or 13 highway mpg's those big old wagons get. So, getting better mpg's out of them shouldn't be too difficult.

 

@Taranau: There are two easy things to do that will net you quite reasonable fuel economy out of one of the very large Ford wagons: Start with a good dual exhaust system, including balance tube or X cross over. Dynaflow or FlowMaster DeltaFlow mufflers work really well. That is normally good for about 5 mpg. Swapping out the C-6 transmission and installing a mechanical AOD will add about 10 mpg to the total. Putting in an Edelbrock Performer intake and 4 bbl carb that is jetted right and with the correct metering rods, will improve things further yet. It should be possible to get one of these big old wagons well into the mid 20s.

 

Those Detroit Boys

It's become painfully obvious, that in the past, those Detroit Boys did precious little to create good gas mileage. It's only been during the last ten/fifteen years that they've begun to be truly earnest in their attempts in that arena. But, even then, they're not moving too far beyond anomolic entries in the field, that they made around thirty years ago now. The last year of Pinto/Bobcat had sub-models capable of 28 city and 42 highway. And those levels of efficiency were passed on to the Escort/Lynx lines. So, the factory tech has been in that area for a while now, they just haven't been concentrating on it as much as they have been recently. And then, they haven't moved too far beyond that. Only with hybrids, as far as I can tell. Although, if the latest generation of small Fords are heavier than their ancestors, transplanting a brand new drivetrain into a way older/lighter vehicle would probably get us those even better mpg's we've been looking for. And I'm pretty sure the newer drivetrains are more powerfull than the 75 and 90 bhp drivetrains of the earlier eras. So, weeeheeeee!

 

You really have to watch many of the new drive trains. They have a LOT of electronics that are going to be a real problem down the road. Also, since it is cheaper to engineer for horsepower than it is for torque, and they are engineering for fuel mileage on the test rather than the road, you find that the real world economy is just not there with many of the new cars, including the hybrids. The biggest difference between one of those 70s cars and the newer, 80s full sized cars is the transmission. The C-6 or FMX in your big wagon is strong, but there is no torque converter by-pass, and there is no overdrive. Swap out the 3 speed for a build up mechanical AOD and watch the fuel economy jump, big time. Keep in mind, back in the 70s, the engineers were focused on 2 big things at the same time: Emissions and occupant protection. They were doing all of this engineering the hard way, with very rudimentary computers to help them. Keep in mind, too, that the on board computer was quite a ways off, too. Mind you, I don't think the computers help all that much. Not when I'm doing 10L/100k (28 miles to the Imperial gallon) in a 78 Thunderbird, with the original FMX 3 speed transmission.

 

more of that mpg stuff

You're quite right about all that computerization. My 77 Dodge Aspenwagon had a "Ignition Control Module", back in the infancy era of on-board computers, and it was nothing more than a solid-state chip in a block of resin in a brass box. I'm sure the on-board computers are more advanced now, but like you, I'm not seeing them helping all that much. If 1980 Pintos and Bobcats were getting 28 and 42, and new Fords are getting about the same, the computers really aren't doing very much. / Just a couple years ago, Toyota had to change the advertising for Prius, because they were never getting the mpg's, they claimed they were, here in America. BECAUSE, Priuses were tuned for Japan, where the average city speed limits are 30 mph and the freeway speed limits are 55 mph. The Priuses stopped being "all electric" at 32.5 mph. Which meant, even if you were a law abiding goodie-goodie, doing the average 35 mph city speed limit in America, you were never getting that "all electric" mpg rating. / Have often thought about putting a 4-speed automatic in an older car that it'd fit into. If a 3-speed auto from a Granada mated to 302, a 4-speed meant for a 5.0(the same damn thing)should bolt right on. It might be a little longer though. But they've got adjustable drive shafts for that.

 

1973 Olds custom Cruiser

Just completed a 4,000 mile trip and averaged 13MPG 455 engine. Big and comfortable but it likes gas. Pings on regular fuel

 

I'd say most of those MPG are average. The members who claim higher must be driving down hill all day!
If you want a better gas mileage buy a Prius or a SmartCar.

 

Given my low daily driving mileage, I only fill up every 2 weeks at 10.5 MPG. If I got much better MPG, the gas would go stale in my tank before I used it all.

 

I've had my new used car over four weeks and driven 100 miles. I'll live with stale gas!

 

My 75 around town was probably 12-13. On the highway I would guess closer to 20 maybe more. Never really checked to be honest but cruising on the highway it was really good for the size. I did however have a 71 455 poncho that didn't work anywhere near as hard to move the car as the 75 Buick 455 did .