The Search for Improved Gas Mileage in a ’77 Jeep Wagoneer 401 V8

When I bought my ’77 Jeep Wagoneer, I intended to use it as a tow rig and (eventually) as a daily driver for my wife and ever-growing family. I rebuilt the AMC 401 myself not long after I bought the Wag, and rebuilt the Holley 4-bbl carb (a 600 cfm, 4160-style 0-1850-5) at the same time. I was told by numerous people on the FSJ-List that even a 401 should be able to get 12 mpg, with 360’s routinely getting 12-15 mpg. To my dismay, my fresh 401 was only getting 8 mpg around town or cruising at 75 mph, and maybe as much as 10 when cruising at 55-60 mph. Since poor gas mileage wasn’t a good match for my Wag’s intended use, I began a witch hunt to find the source of my poor mileage.

First off was to eliminate any vacuum leaks. I replaced all the vacuum hoses in the engine bay, even though none of them looked bad. Manifold vacuum was around 17″ Hg, which is acceptable.

Then I got new CTO switches, cleaned up the EGR valve to break free the stuck piston & make sure it worked properly, and got all the vacuum lines routed properly for the EGR and the distributor vacuum advance. I’ll post diagrams for those here eventually. No noticeable change in mileage or power, but at least it should run a bit cleaner now.

Then I made sure the distributor’s base ignition timing was set properly. Factory spec for the 401 is 8 degrees BTDC. Since my aftermarket Crane 260 cam may have modified this somewhat, I advanced it a little to improve manifold vacuum slightly. I’ve currently got it at 10 degrees, with only a tiny difference in manifold vacuum.

Even though the spark plugs looked great, I next decided to change the main jet size on the Holley carb from 66 (the factory size) down to 64. This improved the mileage at 80 mph from 8 up to 10 mpg when driving 80 mph, but the leaner setting also appears to have sacrificed some power (seat of the pants feel while accelerating uphill). It still accelerates just fine from 70 to 80 mph on the interstate. We’ll see how it does when towing. On another highway trip with speeds from 55 to 70 mph, mileage was up over 12 mpg. The leaner setting should also work better at higher altitudes when we use the Wag to tow my Wrangler through the mountains of Colorado next month. If I run it on the highway for a while to warm it up, then sit & idle (like at a drive-through just off the interstate), it tends to get kinda warm. I don’t do this often enough to know if it’s a result of the leaner jetting or if it’s always done this.

I should still be able to add a couple mpg, though. Next up is to upgrade the stock Prestolite ignition system. I bought a gently-used MSD 6T ignition box from a friend. I plan to install it along with a Motorcraft distributor from a ’78-91 Wag (the MSD won’t work with the Prestolite) and a TFI coil. That should work wonders for the ignition, which hopefully will give me the remaining gas mileage that I’m looking for. If it does, I may put the 66 jets back in. Read about it here.

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