They will. But the interior plastics and (mainly) electronics won’t. They’ll be scrapped (recycled!).
As to fuel mileage, what’d that truck weigh? 4,200-lbs with you and full
gas tank? Today’s 1/2T are well north of 5,200lbs and have the advantage of multi speed transmissions. I know of a man with a 400-HP Chevrolet gets above 20-MPG at 60-mph. Aero & weight count for most. Precise fuel control is the other.
Would “best” have something we can re-engine. Today’s motors but with carburetors and points sounds good. Bureaucracy is what’s in the way. The last of the manual transmissions is around somewhere, the rest scrapped.
As these are trucks they have to be able to work. A 318-3 was stout and long-lasting, but low on power. Trucks ain’t commuters, by design. Honesty to what they are dispels sentiment. Doesn’t work for a living, it needs to go to someone who WILL work it.
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We're talking about two different points here: chassis longevity on one hand, engines on the other. I'll address the chassis first. Ford Econoline vans are a great example to illustrate my point. I'm really not a Ford fan, but I have a lot of experience driving the Econoline in a professional setting, going back 30 years. The Econoline used to be one of the few vehicles Ford didn't experiment with much because it was their bread and butter and they didn't want to lose their standing as the top commercial van seller. But then they went typical Ford on us and threw that away. The vans I'm talking about are third gen Econolines (1975-1991). They had a good quality body on a real frame, while their competition all used unibody construction. E250s had full-floater Dana 60 rear axles; E350s generally stepped up to a Dana 70HD.
Then they started experimenting. Most E250s still got a D60, but now it was a C-clip design with the axle shaft riding directly on the bearing. That meant that when a wheel bearing wore out, it took the shaft with it. Even some E350s got that piece of junk.
Then Ford decided they could do even better by building their own axles. Enter the 9-1/2" corporate axle. It looked really beefy on the outside, but it was a C-clip axle with even more cost-cutting inside. Ford went too far with that one, and a large percentage of them were failing before 100K miles. So Ford made a couple of tweaks and built the 10-1/2" corporate axle; same housing but with a bigger ring gear and a bit better pinion support. With that one they ended up with an axle that would last through the warranty period for most users, which was their goal.
In 1992 Ford introduced the 4th gen Econoline. The frame was pretty much the same, but they made the sheet metal lighter and used more of their typical cost cutting measures. They made those from 1992 through 2014, at which time they discontinued the standard cargo van in favor of a completely new design, the Transit. I've driven a few of those 4th gen Econolines in my line of work. They are junk. One thing I've noticed is that the door latches gradually tear away from the body. Just about all of them I've seen that have substantial miles on them, have door latch problems. I don't see those getting new engines or transmissions because they get limped along until they have a major failure, then they get scrapped.
By contrast, there are plenty of 2nd gens still out there working even though the newest one is 30 years old. I have one. The doors still work fine. It has a 7.3 IDI International engine, an E4OD transmission (its weak point btw) and a full-floater D70HD with limited slip. It's a former ambulance.
Back in the early '90s I drove an '86 E150 on a delivery route that covered about 300 miles per day of everything from 4 lane highway, all the way down to dirt 2-track. I had to drive across creeks and through mudholes, keeping my momentum up so as to not get stuck. That van held up great. It already had well over 200K miles on it when I got it. It had a 300 inline six and a ZF manual 5 speed. It never had a major failure in the 2-3 years I drove it on that route. It got about 18 mpg. I wish I had bought it when I quit that job.
The 4th gen Fords I have driven, besides falling apart, only got about 15 mpg on a far less demanding route.
The van I drive currently is a very late model Chevy with a 5.3 and 8 speed automatic. I like it a lot better than the 4th gen Fords, but I'm under no illusion that it will be worth fixing once the engine and/or transmission bites the dust. It gets 16.9 mpg, which includes about 75 percent highway miles. And I normally don't speed. I'm trying to make money, not lose it by paying tickets. Not to mention that too many tickets would result in the loss of my job.
I'll touch on one more thing real quick: wheel bearings. Most light vehicles now (including my F350 stupid duty) use non-serviceable "lifetime lubricated" unit bearings. Most people really like the sound of that; "It's lubricated for life and I don't have to do any maintenance on it!" Actually that means the life of the bearing, not yours or the vehicle's life. And "I don't have to maintain it" is better stated as you
cannot maintain it. You just have to go with it until it inevitably fails, leaving you stranded until the rollback truck comes to haul it to the nearest repair shop. And all vehicles with that type of wheel bearing (which is most of them now) have a date with the rollback truck... if the rest of the vehicle lasts long enough.
My F350 with 130K miles is on its third one on the right front. The part costs $300 and it's another $300 in labor to replace it. Plus tow charges. And I used factory Ford parts; no cheap Chinese stuff. Those things have discernible slop in them too, right out of the box. No wonder they fail. And since they're non-serviceable, you can't adjust that slop out.
Contrast that with the old school inner and outer tapered roller bearings. Like for example my mid-'90s C3500HD, which btw is way stouter than the new truck GM calls a "3500HD." A bearing set for one wheel costs $30 for that truck. And it's so simple, you can change it on the side of the road if you need to. I've done it. Not on that truck, but on others with similar bearings. When you install one of those, you tighten it enough (by hand) to just eliminate the slop, then lock it down. From that point, if you don't maintain it, it will likely still last about as long as a non-serviceable unit bearing. But if you clean, regrease and readjust it when you do the brakes, it will usually outlast a non-serviceable bearing. And even if it doesn't, it's 1/20th the cost to replace.
Now, about those engines. That 400 hp Ecotec3 engine gets 20 mpg because its computer shuts down half of its cylinders under light-load conditions. You can't slap a carb and a points distributor on that and have it still work. Even the Magnum 5.2 (318) in my Dodge van would have to have a different transmission to play nice (or at all) with carb and distributor. I have zero doubt though, that I could double its gas mileage and greatly increase its power by doing so.
And btw, my '72 318 not only got double the gas mileage of my 5.2 Magnum; it also had more usable power. That old two barrel 318 was about out of steam by 3500 rpm, which is about where the modern engines start working. It doesn't make the big horsepower numbers that the new ones make because the higher RPM the newer engines need to make those numbers are completely outside its operating range. But between 1200 and 3500 rpm, it more than holds its own. That's why the old school engines were fine with a 3 speed. The modern engines really
need all those gears. The old school, low performance V8s and L6s don't.