Clamshell Fuel Mileage

I've worked out the cost of engine/transmission swaps for a number of vehicles I've owned over the years. Paying more than you think you should for gas is a regular thorn in the side, and I cringe most times I fill up.

In one example, for my Harley, I worked out the cost of a 6-speed conversion vs. the potential increase in mileage. It worked out that my breakeven point was like 450000 miles. So not at all worth it unless my stock transmission needed replacement anyway. For the amount of riding I do, it would be like 270 years.

So I was curious about the clamshell example that occupant laid out. The optimistic cost estimate is about $1800. Assuming a highway mpg bump from 13 mpg to 18 mpg and $3.50/gal, the potential breakeven would be about 17000 miles, which actually was a lot better than I assumed it would be.

But I guess it depends on how much driving you do. For my Ford and the driving I do with it, that same equation would take me at least 8-10 years to break even. Who knows if I'll even still have the car then. However, my transmission does need rebuilt (but still hanging in there), so if my engine packs it in, I'd probably go the same route and pull a complete engine/transmission from a wrecked newer car. There are a gazillion Crown Vic, Grand Marqs and Town Cars out there to get the drivetrain from.

 

You know, something just popped into my head and rattled around like a BB in a boxcar--What's the cost of a Gear Vendors overdrive unit? And while thinking about that, what's the cost on each of the Big Three's OD trans swaps without a modern engine swap?

 

GV units are even more expensive, nearly 3k just for the unit. Granted, it will be bulletproof.

GM PG/THM350/400 - https://www.gearvendors.com/hrgm3s.html
Ford C4/C6/FMX (not FX or MX) - https://www.gearvendors.com/hrgm3s.html
Dodge 727/904 - https://www.gearvendors.com/hrdodge3s.html

700R4 is easy for a Chevy. And I think AOD(E) is easy for a SB Ford. But in my case, Ford FE engines and AOD(E) won't bolt up. You need an adapter that is like $800 bucks. And the floor pan may need mods. I think MartyO had to do that for his 351/AOD swap in his Ford wagon. Chevy though, probably not. I had a 69 Camaro that I had a 700R4 put in, and the floor did not need modded, at least nothing more than a sledgehammer mod. It was tight, but fit.

So I've again come to the conclusion at least for my vehicles that the cost of a mod would probably never be returned by the potential gas savings.

 

BTW, I just remembered I'd found this. Here are a couple Popular Science road tests from 1978. I was amazed at the gas mileage figures they achieved with late-70s, non-computer, carburetor equipped, plain old non-lockup PRND21 transmissions. Granted they weren't long on power, and the axle ratios were really high. But 20mpg was doable without an overdrive.

Page 30:
http://books.google.com/books?id=lwAAAAAAMBAJ&pg=PA2&source=gbs_toc&cad=2#v=onepage&q&f=false

Page 32:
http://books.google.com/books?id=tQAAAAAAMBAJ&pg=PA2&source=gbs_toc&cad=2#v=onepage&q&f=false

Of course those were intermediates, and not wagons. Here are the big boys. Fullsize wagon tests from 1971 and 1974, each including a clamshell. Mileage was about what everyone here seems to be reporting:

Page 42:
http://books.google.com/books?id=kwAAAAAAMBAJ&pg=PA5&source=gbs_toc&cad=2#v=onepage&q&f=false

Page 12:
http://books.google.com/books?id=j989VTOlPwMC&pg=PA2&source=gbs_toc&cad=2#v=onepage&q&f=false

Intermediate wagon test from 1975 -

Page 34:
http://books.google.com/books?id=DwEAAAAAMBAJ&pg=PA2&source=gbs_toc&cad=2#v=onepage&q&f=false

 

I know my Ranchero has a 2.47 to 1 ratio, and with the 2bbl 351W and C4, I get 16 on the highway, and that's an intermediate, weighing about 3700 pounds. And that's with the engine not fully dialed in and the carb not rebuilt.