There is a recent post on one of the Thunderbird forums, regarding the gas mileage of 55-57 Thunderbirds with 292-312 Y-block engines. Some owners reported 7-10 mpg, others getting 9-12, another getting between 13-14, and another claimed 20 mpg on the highway, in overdrive. Having inherited my 55 T-bird, I drove it just a couple of times before beginning restoration, so I have no idea what kind of gas mileage I should expect. 7-13 mpg is nothing to brag about. Are these numbers typical? They are awfully low for a small v8 in a 3,000-lb. car. Is there an explanation, other than the given examples are parade cars that are seldom driven on the highway? 

There are lies, damned lies, and fuel mileage claims, er Statistics.

Most any OHV V8 in good tune and suitable gearing in a car though is fully capable of 20 to 25 hwy mpg with careful tuning. There is no need to give up performance either. Most people aren't that interested in the finer points of Tune-Ups.

Keep in mind a well tuned motor will not pass any sort of pollution testing for NOX emissions when running optimally for power and economy. This was one of the insults added to the injury of the oil embargoes, dollar devaluations, and the 1970s federal pollution mandates, the engines Detroit was putting out were notably unreliable low powered absolute gasoline hogs. They broke many engine tuning "rules" to achieve the pollution requirements. There was a whole cottage industry of mechanics restoring engines to their pre-smog configuration.

When I first got my ‘55, mileage was over 20mpg on the highway (in overdrive).  But, after installing a Pony teapot and then rigging my Loadomatic distributor with an aggressive curve (including an advance limiter, which unfortunately did away with the factory ‘crushing vacuum’ economy feature), mileage went to hell (don’t even bother to measure it anymore). But then, I also compound the misery by buying 93 octane (which is likely overkill), and justify it all as a ‘recreational expense’.  

6 VOLTS/POS. GRD. NW INDIANA

What has really surprised me, with any carbureted engine if it isn't tuned properly a truly astonishing amount of fuel can be simply wasted, spewed out the tailpipe, with no discernable smoke (nor any performance benefit).

A mistuned engine will tend to foul spark plugs, when extreme. With modern unleaded oxygenated gasoline formulations especially, if the spark plugs are fouling at all, the fuel curve is extremely rich and crankcase oil dilution and even washing out piston rings is a possibility. The Load-O-Matic was designed with economy in mind wasn't it? But I'm not sure if worked very well in the real world.

I don't think my dad drove my 55 far enough/often enough to calculate an accurate mpg. It was almost 100% original, but I discovered that it had a 57 distributor, even though it still retained the 55 Teapot. Upon learning that the 57 distributor is not compatible with the Teapot, I began the restoration. Before my dad passed, I installed a Casco wp and converted to 12v. It had the Fordomatic/w 3.31 gears, no power steering or power brakes. It will not be stock when I'm finished, but retaining all take-off parts.Even with my engine mods, I expect a light foot, combined with 2.72 gears, will see better highway mpg. 

For what it's worth, mine averaged a little over 16 on the highway the last time I checked.  In the mountains, it was around 12 and on flatter hwy, around 20.  I think it can do better than that because the gearing is not right.  It's mostly in the range of 1800rpm to 2000rpm while keeping up with traffic and while the engine seems to do fine, it sounds happier above 2000rpm and the way it's built, probably should be above 2000rpm.  I've never checked city mileage but for the typical mileage I put on it in a week (Atlanta Suburbs) and the amount of gas it takes to top off, that is around 15mpg, which is about the same as my '88 Firebird, which is fuel injected, roller cammed and should be a little over half the power (stock), for perspective.  The Firebird does get about 28 on the hwy, though.

A T-Bird should be more aerodynamic than my truck, so I think 20s on the highway and teens or better city is possible in good all around tune and setup.  Note that this probably doesn't mean stock.

If at all possible, install an oxygen sensor and tune the carburetor with that.  If you want the interior to look stock,. figure out a temporary means of installing the gauge and remove it when tuning is done.  This takes a lot of the guess work out and provides a lot more info at one time than reading plugs because you can see what's going on at idle, at cruise, under load going up a hill, during acceleration, etc, all in one drive and don't have to deal with pulling hot plugs on the side of a busy road to see how it is doing.  Some of the fancier 02 setups can data log so you can record the data on your laptop and take a look at it later.


Lawrenceville, GA

It would be more correct to say that the LoadOmatic distributor is only usable with the factory Teapot carburetor, as these were designed around two different sources or types of engine vacuum. As you know there is no mechanical or centrifugal advance in these distributors.

On the other hand, a post '56 dual type distributor can be made to work quite well with any common source of engine manifold vacuum. Some people actually prefer this. The LoadOmatic distributor needs the Teapot, but, the reverse is not also true.

As I understand it, interestingly enough T-Birds have somewhat of a reputation for running hot or overheating, and any engine with a manifold vacuum distributor connection and a conventional dual distributor will generally run noticeably cooler at idle, cruise, and stop and go driving. This would probably be worth investigating if other remedies weren't panning out in this situation.

I'll have an O2 bung installed in the Sanderson reducer during rebuild. RPM @ 65 should be about 2200. Cam range is 1800-5000. C4 transmission.

Several years ago I drove my '57 (312 and overdrive) between LA and the Bay area quite often (400+ miles) and routinely got 21/22 mpg.

55Blacktie. A Thunderbird might be different than the full size car but during my transmission and rear end swap I was advised not to use the Ford-o-matic with the new rear I was installing with 2.74 gears so I did the true 3 speed automatic first. I agree that at highway speeds the new rear is great and is not sluggish at all starting out because it’s using 1st gear. Automatics use a lot more fuel at idle which doesn’t help our mileage.

56 Vic, B'Ville 200 MPH Club Member, So Cal.