[Formulajones]

You have to ask yourself first how much MPG is enough. Is 25 mpg necessary? Is 20 mpg good enough?? That differs vastly for many people.

I played with LS stuff for years. Had a few LS camaros we daily drove for a few years, a Z06 vette. Light weight and aerodynamic with double overdrives 25-28 mpg was the norm on highway trips, even after cam swaps and headers etc....

Then did an LS2 swap in my 72 blazer and daily drove that for a couple years. 20 mpg highway after some tuning with a 4L60E behind it, 3.73 gears and 33" tires. Not bad for a 5300 lbs 4x4 truck shaped like a brick. I was content with that.

As far as cost goes, that swap just in parts was $7000 including $1500 for the price of the engine/trans. So the swaps are not cheap by any means when they are done correctly.

In the end though, I find I can get very acceptable mileage from the classics with the dinosaur engines, especially when overdrive transmissions are swapped in. So I got away from the LS stuff many years ago and just drive classics. I'll never own another new car again, and highly unlikely I'll ever swap another LS in anything. The old iron gets the job done.

For example, wife daily driving our 69Z, when it had a muncie I knocked the rear gear down to 3.55's. It would get 17 mpg highway. I installed a TKO600 and it now gets 21 mpg. I have the engine running a bit on the fat side of things and have made no attempt to lean things out. There might be a pinch more there but I'm perfectly happy with what it's currently doing. In fact it's our first choice for long road trips, it's that good. Cruises 80 all day long. Speed limit here is 75 so it's nice to have something that scoots along out in the open desert without the need to spin beyond 2200 rpm.

My 70 Formula is my driver. Stock RAIII/400 turbo and 3.31 gear. Even without an overdrive I've tickled 17 mpg regularly on highway trips. An overdrive would certainly put it over 20 but at this point it's pretty good as is and economical enough that it's affordable to daily drive. Still better than most SUV's on the road today with EFI and overdrive so it's already ahead of the game from what most are driving out there.

My 454 Chevelle makes 608 hp and I run a 4.10 gear in it. After the 4L80E install, and a Sniper Stealth EFI allowing me to trim fuel in the cruising areas, it's knocked down 15.8 mpg up and down the I-17 cruising at 2600 rpm (70 mph) I'm happy with that considering the nature of that engine.

The real eye opener for me is dad's GTO. A 571 making 724hp you can imagine it has been a thirsty booger. I installed the Sniper Stealth EFI, a 4L80E trans, and knocked the rear gear down to 3.42. After some tuning this one is also getting a consistent 15 mpg cruising, with a best of 15.89 and I still have some tweaking to do. The big reason it's doing as well as my smaller 454 that is making less HP comes down to the rear gear. He's cruising 70 mph at 2100-ish rpm vs my 2600. We follow each other on the same roads, his ability to cruise at less rpm has a big affect on MPG. Both cars are tuned very similar with the same EFI system.

In a nut shell, you can get great mpg with these classics on a well thought out combination, well enough to satisfy most people. 20 mpg with a good engine combination is pretty simple to do on this old stuff if you use an overdrive and keep the rpm in a reasonable area, and know how to tune the engine. I don't mean slap on a carb, set timing at 12 degrees and go. It takes some distributor modifications to get an advance curve that works right, along with a modified vacuum advance curve on top of that. Then a carb that is dialed in really well.

I've done well enough over the last several years with this old stuff on our own personal vehicles that I really don't have the desire or even see the need to do modern engine swaps anymore.