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Responded because i'd considered an El Camino myself and fairly intensively researched options for both mileage and towing.

I've seen over 30mpg in one using a 6.2 diesel, and it could tow.

The Oldsmobile 350 V8 and 4.3 V6 diesels are possible options, official ratings of the 350 diesel with a 4spd auto was 26hwy, the 4.3 I dont think came in there but generally got 4mpg better in even full size sedans i'd seen it in. Not much power though. Reliability of unmodified olds diesels is an issue, the modifications are not expensive.

The 4spd TH200-4R is probably the best transmission to use of the era and would have come stock. I dont think the 700-R4 came stock. The early ones are unreliable (the 200-4R is better for early stock ones), rebuilds of the 700-R4 with newer parts tend to be better than the 200-4R stock rebuilds however. The 200-4R bolts more easily to either GM or buick/olds/pontiac type engines.

Gas engines - you could try putting in smaller later model engines. 80's engines are a bit underpowered, swapping in the later 3.4 V6 60-degree (not the DOHC, just the one like in the Trans Sport and such - it had better mileage than the smaller 3.1) would likely improve mileage with no hit on performance. (actually more) Th700-R4's bolt up more easily to the 2.8/3.1/3.4 V6 family as they came in the firebird and such. 4 cyl is even possible, the 2.5L "iron duke" - the 100hp stock for the later ones is actually about what the 350 olds diesel v8 puts out, but with less torque. It's down about 45hp from the 4.3 V6 and such. The 2.5L also came in the 80's Firebirds/Camaros in a RWD configuration so motor mounts and such from there would probably be modifyable to get in the ballpark.

Rear axles - I think it's the 10 bolt that comes in the el camino, the 12 bolt should also be swappable/commonly done by hot rodders but there's reasons outside hot rodding to do that sometimes like ratio choice. In either 10 bolt or for sure 12 bolt ratios as low as 2.29 are available, typically pulled from late 70's sedans as I recall because they had no overdrives in those days and did it with the axle. Combined with overdrive transmissions gives extremely low cruising rpm's. Add in aeromods and i'd expect 30mpg hwy without problem. Stick it on a diesel and i'd expect upper 30's. (a 6.2 diesel in a corvette returned 40mpg averagedue to aerodynamics)

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New info to add, incl concerning your budget and looking at others replies, will be rementioning some of the same data tho:

The 6.2 GM diesel should be within engine budget rebuilt, almost no other diesels will be. (anything else like a 7.3 probably $4k and up) It will be heavier than stock v8's by maybe 100-150lbs but not nearly the weight of most diesels (7.3 alot heavier), not as loud/rough as like a ford diesel, it didn't come stock, but 95% sure the engines bolt up to GM gasoline transmissions and i'm 95% sure the engine mounts are in the same location and I think are even the same as small block chevy. So it will be the easiest engine to swap and the 1st or 2nd least expensive diesel to put in.

The 350 olds diesel is potentially far cheaper because of a bad reputation, but they are reported reliable IF they are rebuilt and some reliability mods made which have only been known more recently by fans of the engine. (nobody knew them at the time so they died left and right, it's simple things like quality main engine bolts and such) They came stock in 82-84.

Smaller gas v6's and inline 4's will be rebuildable within that budget easily if lower tech ones.

One of the few engines i'd consider that's not a GM make would be a VW TDI which they like to swap into everything. Don't know if it matches your budget but it's about as efficient as you can get, only a Cummins 4bt will beat it. Custom mounts and stuff will be a given, and some kind of transmission adaptor likely.

The Cummins 4BT will give the best mileage possible, the problems are it's LOUD, shakes horribly, and usually expensive. That said it will last 500,000 miles and tow a bus. They are often found in panel vans, those giant bigger than 1 ton vans like UPS used to run where they'd get 15-17mpg. Swapped into a Suburban they'll get 25mpg. Put in a minitruck mid 30's probably with no aeromods or careful driving at all. There is a big community somewhere on the web that swaps them into everything, they could potentially help. Did I mention it's loud? Weight is 600-some if I remember. You can find them under 7k but you'll have to find the panel van yourself and you might not be rebuilding it, that said one in good condition might not need any rebuild.


Comments on others:

I'd be surprised if a T56 is a cheap transmission, if they are by now it's news to me but sixspeeds have never been cheap so i'd think it plus the diesel would exceed budget. Or the extra money is better spent on something else. 5 speeds are cheap if you dont mind a manual though. Plus not sure if the diesel will work with a T56 trans - bellhousings will bolt up but there's more parts than just that. Expect hundreds in extra cost minimum. Talk of aftermarket turbos for the 6.2 would utterly kill the budget, a banks kit alone probably exceeds the total engine budget. A lockup automatic will get the mileage, you only lose some performance is all. The 5spd Warner T5's can be used and give better performance due to lower 1st gear ratios and are available for practically nothing.

Ford diesels all out, cross platform swaps are harder and more expensive always. The TDI is more civilized, the 4BT the mileage king if you can stand the teeth rattling. The GM 6.2 reliability has been argued for and against but consider the original AM General Hummers using a 6.2 diesel had a durability test of full wide open throttle for something like 300 hours nonstop without shutting down or oil changes (like driving 20k miles) going between the torque peak and HP peak... a rebuilt Hummer spec engine wont break, and they don't use the older 6.2's anymore so they aren't in demand. The only weak point i've heard on the normal civillian early 6.2's was the crankshaft under high load, thats a critical part, but i'd assume just swapping a military hummer crank and maybe rods during rebuild would solve that full stop.

Small chevy 2.8/3.1/3.4 v6's are not unreliable mechanically for what I heard, every one i've had ran over 200-300k miles without problem, early ones had electrical or fuel injection problems though and the 3.4 DOHC I think was iffy. There are better later years, get those. The larger 3.4 2 valve in 1996 had better mileage than the previous 3.1's, not sure if it was available with RWD boltups though admittedly like earlier V6's. The Ford SHO is unreliable as heck, and most Ford V6's got worse mileage than Chevy. (The 3.0 in my taurus gets 17city/23mpg highway, the 2.8 in my old grand prix got 23city/30hwy driven the same - the cars are both midsize, the grand prix weighs what your camino does but a little more aero and i drove 75mph everywhere) I don't know of any V6's likely to give better ECONOMY than the little chevy V6's, there are ones giving more performance sometime at the same economy, which will cost more and be a PITA to swap in.

Four cylinders i'm less of an expert in, the Iron Dukes had a reputation of lasting forever because they are half a small block v8 basically. They were not very powerful, the ones in the Chevy Astros were 98hp for instance and were RWD bellhousings and such. They were very economical in the cars they were in, I got 35mpg hwy consistently with only a 3spd non over drive transmission in a buick century I had which could seat six. Are newer 4cyl more economical? A 1985 S10 with one and a 4spd auto was 18city/25hwy (this is "newer" aggressive EPA driving mind you), the 1995 S10 2.2 with 4spd auto was 18city/24hwy vs the 2005 Colorado with 2.8L 4cyl and 4spd auto 16city/23hwy hmm... in honesty we figured out all the tricks for good mileage in the 1980's because we had to... that said the newer ones have almost twice the HP. If your sole concerns are mileage and budget newer engines don't offer that much, seriously, but if you need performance then yes you will want late model engines because that has really improved in the last 25 years. The only question is whether an Iron Duke is underpowered.

Probably NOT. The 1978 Camino/Monte Carlo had a 200cid v6 stock with 94hp and 160lb/ft torque, and rear axle ratios then were typically 2.52 or even worse. Do you mind driving a car with late 70's performance or a 4cyl minivan? Actually some of the later versions had 110hp so it's not even all that bad, and if you don't find one of those lightly mod the engine to give that or a bit more - the torque is still a bit less than the 1978 V6 but the acceleration would be comparable. Stronger mods would equal it in torque too. If it's just getting out of the hole consider an axle swap to something lower since the overdrive transmission will still be lower than 1978 cruise RPM. Actually the 2.5x ratio with an OD and a 4cyl may not even hold without aeromods or flat land driving, you'd have to try it and see... the RPM be getting really low by then. What accelerates the car is torque to your tires - if the 0-30mph movement is intolerably underwhelming, and say you have a 2.52 ratio, and you put in the 3.73 from the 1985 Monte Carlo SS you'll have a 48% harder 1st gear pull kicking you back in the seat a bit better (even though once you exceed 3000rpm or so it's less relevant.. 0-60 will be about the same, only horsepower matters after the hole shot) yet the 0.70 overdrive in the TH700 will still have a 2.61 overall ratio. You don't have to touch the rear axle til you see how it drives and cruises though.

The 6.2 diesel will give better mileage because it's a diesel, and have alot of torque, but not be much faster powerwise since it's heavier and cost more.


I've considered several builds of the El Camino, myself I want to go with a 6.2 when I build one because I want the ability to tow around 5000lbs which it would handle without problem and which I prefer to minitrucks of the era. I've long considered the Iron Duke as a perfect mileage swap for any number of project cars that hot rodders would be insulted by - people forget the original inline sixes in them had no more power either. A 4cyl would work for just moving passengers around but you almost have to ask why aren't you driving a Saturn if you just want to move people? :P Yet if all you need to move is the occasional couch or you just like the style, an Iron Duke may very well work just fine. GM had no problem sticking them in Chevy Astro midsize vans fer chrissakes, at 98hp and a heavier curb weight than your camino (3900lbs instead of 3200lbs) albeit typically with the 5spd stick. If you swap the Warner T5 5spd stick you get an even lower 3.50 1st gear and the transmissions cost nothing and are everywhere.