So why does my old, giant wagon, with a big motor get bad mileage?

So, here's the deal: I picked up my '68 Ford Country Sedan earlier this year. 77k orig miles, 390 FE with a 2bbl carb. Got around, 6mpg around town. I don't really drive it on the freeway any appreciable amount which I would presume would help things out slightly.

I swapped out to a new Edelbrock intake manifold, 4bbl carb, and new dual-out exhaust with flowmasters. It got... 6mpg.

Finally, I swapped out the 4bbl for a FiTech EFI, and it gets... wait for it.... 6mpg.

Am I wrong thinking for even for a 5300lb vehicle with a 390 this is a bit low for mileage? I baby it, barely hit the gas, etc.

Just want some opinions on the matter, thanks.

 

Haha, that's about all, I'm sorry to say. When I was a kid, we had an F350 crew-cab Camper Special with a deluxe Kit camper on it. 390, 3X2, 19 gallon tank (in the days when even the desert stations were only 90 miles apart). The bastard got 4 MPG, no matter what you did: 70 MPH on the freeway, 4 MPG, in-town traffic, 4 MPG, climbing a hill, all six barrels open, 4 MPG, push it off a cliff with the engine not running, you guessed it, 4 MPG. There's just something about how the FE engines were set up, all the way to the mid-70s. I'm sure a re-degreed cam and timing set, along with a re-curved dizzy, could yield some better numbers, but in the late sixties, the majority only cared about power, not MPG, and definitely cared nothing about smog.

 

First, I'd expect your wagon to be closer to 4300 lbs. A loaded Suburban with 4WD is more like 5300 lbs (unless you are talking about the car being fully loaded with cargo/people).

Second, how and where are you driving? If this is all stop-and-go and low speed, you're going to get poor mileage.

Third, what gears do you have in the rear axle? Assuming low 3-series gears, I'd expect closer to 10 MPG, even with the carb.

I have no idea how the FITech is calibrated (other than it probably comes set up for a 350 Chevy), nor do I know how long you have to drive it for "self learning" to kick in. Is the engine in good shape? Is the trans slipping? Is the ignition system up to spec? There are a lot of factors that can affect this, and simply dumping a one-size-fits-none EFI system on the motor is not a magic elixir.

 

5300 was just what I found on several online sites. I've not actually weighed it myself
My driving is all short distance, stop and go. Basically, the worst for any sort of fuel economy. This is my daily driver, and my commute to work is all side streets, about 4 miles.
Gearing is stock. Whatever that is for that make\model.
FiTech starts to calibrate\self-learn once the temp goes past 170. Which this car now does with a thermostat swap (orig was 160). The first week or so I could tell it was 'figuring things out', but now that I've been driving it for several months is sort of 'settled into the norm'. Starts great, runs great, super smooth.
Engine appears to be in good shape. 77k orig miles, my old school mechanic guys said it was in remarkably good shape all things considering. Still a 50 year old engine with some new bits on top now.
Trans does not appear to be slipping, feels good.
Has a new Positronic (edit: Pertronix) distributor, coil, new plugs.

 

'Positronic?' You must be letting your spell checker rule the roost....

 

Sigh... Pertronix. I like making up words. Maybe I'll go with 'flux capacitor' next

 

Thermostat maybe too cold to let computer get out of cold engine program. Not warming up enough

 

FiTech starts 'learning' at 170F: My car gets up to temp pretty quick (which is 195F these days), and definitely is in the 'tunable' realm. I have the lcd handheld hooked up to my dash, so I can monitor the temp and other stuff in realtime.

 

The slushbox will make the engine thirsty. Consider swapping it for at least a 5-speed

 

Yah, that could be a future improvement as well.

 

I have an ez efi setup and I’m not thrilled with it either. Works great on tv and in the magazines everytime though

 

You can get one at O'Really's, under P/N '121G:' https://www.oreillyauto.com/flux-capacitor

 

Your mileage seems about right, perhaps you may get it to 10mpg but would be shocked if you got anymore.
Way back when I was a kid had a 70 GTO convert with a 400 ram air and an 18 gal tank, commute to work was 63 miles on the Pennsy Turnpike
could go to work come home and would have to fill it up didn't have enough to get me to work, so might have gotten 10mpg on the highway but probably closer to 8

 

Thanks for the feedback everyone.
Back in high school my father and I did a frame off restoration on a 55 first-series Chevy pickup. Part of that was installing a 350 from a mid-70's blazer in it. That with a 4bbl got about 12 around town, maybe 15-18 on the freeway (drove it many thousands of miles across country). So obviously this is an bigger motor, based on different design ideas, but I figured I at least get over 10. Figured the EFI would help.
My last vehicle was a 87 VW Westfalia with a hopped up boxer by G0Westy. a whopping 110 hp (compared to the stock... 70?), and I was lucky to get 12-13mpg around town. But that was a little tiny 4 cylinder. in a 2-ton vehicle.

Anyway, I guess this is the new normal. As long as the internet validates me I feel ok