Gas mileage of older Cadillac -- including 331 OHV V8. Explaining a mystery.

[jdemerson]

My 1952 Cadillac 6219X has just 67,000 miles on the original motor and transmission. It is in good shape and well-maintained, but of course it is 66 years old.

It was the Motor Trend Car of the Year for 1952, with a top speed of 115 mph. In two different assessments, the average gas mileage was reported by Motor Trend as 16.3 MPG and 16.7 MPG. My average mileage has been running between 9.5 and 10.5 MPG over nearly four years. Knowledgable folks have told me that this is about what one should expect from a car this old, but I've always wondered just what accounts for the apparent substantial drop in mileage from when the car was new.

A few weeks ago, the car had a tuneup as well as some other work done on it. Since then, My mileage has improved to 14.2 MPG (over a couple of tankfuls). This large improvement has surprised me!

Can anyone guess what service/services led to such a dramatic improvement in mileage? And what might it take to get the mileage up to 16.3 MPG?

I have a theory about all this, partially supported by some data. I will report on it in this thread, but first I'd like to hear what the experts here have to say about gas mileage in older cars!  What's going on??

See below for a possible explanation

John Emerson
1952 Cadillac sedan 6219X

[Mike Josephic CLC #3877]

What I usually get on the highway with my '55 Eldorado
is around 14 MPG.  So, I'm not surprised at your results.

I think you just found out why "back in the day" we tuned
up our cars (points, plugs, condenser) about every 10k
miles.  They ran noticeably better and the mileage also
improved.

Mike

[jdemerson]

What I usually get on the highway with my '55 Eldorado
is around 14 MPG.  So, I'm not surprised at your results.

I think you just found out why "back in the day" we tuned
up our cars (points, plugs, condenser) about every 10k
miles.  They ran noticeably better and the mileage also
improved.

Mike

Mike --
    Points, plugs, condenser are plausible, but are not the issue in this case (the '52) as they were always kept in good shape and replaced periodically.  Another guess?
John

[Jay Friedman]

My '49 gets no more than 16 or 17 mpg on long highway runs.  It's usually in good tune with modern AC spark plugs and NOS points.  Your dramatic increase in mileage could be due to a number of factors, so I wouldn't hazard a guess.

[Mike Josephic CLC #3877]

You mentioned that you noticed the dramatic
improvement after you had a tune up and some
"other work".

If your convinced the tune up made no difference,
then perhaps you can tell us what the "other work"
was.  Otherwise, I haven't a clue.

Mike

[The Tassie Devil(le)]

It was the Motor Trend Car of the Year for 1952, with a top speed of 115 mph. In two different assessments, the average gas mileage was reported by Motor Trend as 16.3 MPG and 16.7 MPG. John Emerson  1952 Cadillac sedan 6219X   

Back in the day, if the testers wanted to give good numbers, they would use every trick in the book to get what they thought they could attain.

As for the top speeds, this is another figure that is questionable.   If they went by the speedo, then the figures were miles out.

It is a bit like todays' figures.   Nobody can get anywhere close to the stated figures.   Just look at the VW figures of recent years.

Bruce.

[jdemerson]

The more interesting question is why a car that got 15.5 - 17 MPG back in the day would now get only around 10 MPG even though plugs, points, condenser, rotor, distributor cap are all in good shape. I'd not heard a convincing explanation. Now that my mileage is at 14.2 over two full tankfuls, I think I've gained some insight...

Here is what I learned:

• The Motor Trend test figures from 1952 are about right at the time. Walt Woron was the (well-respected) tester and the testing was instrumented. See also Jay Friedman's post above for his 331 mileage.
• In recent years on my '52, the mileage was 10 or even a bit lower.
• Besides the routine tuneup, there were two other changes. One was to replace the choke cover and choke coil. Although that significantly improved the starting and idling when cold, I don't believe it had an impact on the gas mileage.
• It turned out that the vacuum advance on the distributor had a diaphragm that was shot. So there was effectively no vacuum advance. I replaced the unit with a rebuilt unit from Fusick, and the result was the mileage improvement to 14.2, and (I think) a little smoother running engine on the highway. The vacuum advance changes the timing by around 14 degrees -- this was a surprise to me though I'm sure those with much experience in working on older cars would know that.
• If I ever figure out what it takes to get the mileage from the current 14.2 up to 17, I'll report back. 

I hope this information is useful to someone.

John Emerson
1952 Cadillac Sedan 6219X

[Jay Friedman]

Besides the routine tuneup, there were two other changes. One was to replace the choke cover and choke coil. Although that significantly improved the starting and idling when cold, I don't believe it had an impact on the gas mileage. 

I think replacing the choke mechanism can affect mileage, most particularly if the old mechanism was not allowing the choke valve to fully open after the engine was warmed up.  Make sure your current choke mechanism is adjusted correctly so that when the engine is warmed up the valve is vertical and not partially closed. 

[The Tassie Devil(le)]

Don't forget that back in the day, they were using actual petrol, and not any ethanol diluted petrol, or unleaded petrol.

Any petrol with any amount of ethanol will always give poorer consumption figures than straight petrol.

Bruce.

[CadillacRob]

I'll tell you exactly why.  Gas quality. 

Run your numbers on a tank of ethanol, then a tank of non ethanol.  You'll get a couple more mpg with the non ethanol, and even back then they had higher octane gas on average.

I did that test with my 03 f150 and got a couple more mpg, but with the additional cost of the non ethanol it was a wash in terms of cost per mile.

[BJM]

As a former mechanic I'll chime in, and also a guy with a father who was a Chemist. The blended ethanol should cost as much as 6% decrease in fuel mileage.  If you can locate a "pure" gasoline, than try that and see if you get 15 mpg +. 

I highly doubt those testers in Motor Trend.  I think they were fudging to gain favor. I hate Motor Trend even today.  They are NOT unbiased. 

How about things like air pressure in the tires?  I learned along time ago to set my customers air pressure on the high end and throw out manufacturers recommended specs.  1st, the recommended tire settings on the tire are at 80% of maximum.  To prevent liability issues.

Then the manufacturer would recommend even less because they wanted you to have a soft cushy ride. Which means - especially on a heavy car like those - you are running at 55% of maximum air pressure, increasing fuel consumption.  Look at your tires and add 4-5 psi to each, and your gas mileage will go up 2-4%. 

You don't think Walt Woron knew that back then?  In fact, these tips were printed in many gas mileage pamphlets back in the 50's. 

Regarding tune ups, carberators and peak mileage in those days would have been achieved new through maybe 35,000 miles.  Tune ups - adjustment of timing is critical, and electronic performance critical. If such a thing exists, a NOS carberator I would suggest might add .5% to gas mileage.  All of these considerations - fuel, carberator, tune up, and airpressure, should get a 52 Cadillac to 12 mpg to 16 mpg depending on driving habits and highway/city. 

I do appreciate the comment about the vacuum advance issue.  It makes perfect sense to me.  If the timing is not being advanced, then the engine is "dumping" fuel/air into the cylinder chamber far too early, creating increased pollutants and less thermal energy (aka waste).

[walt chomosh #23510]

I've driven my 1955CDV for just under 20yrs now and have never gotten near 20MPG. (maybe 18 once or twice) Generally, out on the road I get just over 16. I've had some "old timers" tell me they got great millage (20plus) "back in the day". Mine is always in perfect tune,all it's ever seen is "real gas", but it also has a 3:36 rear axle ratio whereas the non A/C Caddys has 3:07. I do have a Petronix ignition, and a exhaust "crossover" tube. (misfortunately  I welded a beautiful mandrel bent stainless exhaust system but possibly used too big of a tube vs original) Also, I run radial tires. My motor is fresh and I've always watched millage on my equipment as a method of watching tune, but in recent years gave up on my Cadillac....like...who cares? one things certain, when I show up ANYWHERE,I've got the baddest ride around.....walt...tulsa,ok   

[Maynard Krebs]

I wonder about the effect of differing differential ratios in Cads with A/C versus without A/C.   

I believe that the mid-1960s ratios were 3.21 with A/C versus 2.94 or somewhere close to "an even three" for non-A/C.   This percentage difference seems significant to me.

[V63]

There are so many variables in play.

Namely 60 years of wear and repairs. Including brake adjustments, Dragging brakes could be an issue. Tire inflation. Suspension Alignment and wear could add drag. Driving patterns have changed with more and more street lights. People drive much faster and accelerate much faster today.

Just a few 

[jdemerson]

The more interesting question is why a car that got 15.5 - 17 MPG back in the day would now get only around 10 MPG even though plugs, points, condenser, rotor, distributor cap are all in good shape. I'd not heard a convincing explanation. Now that my mileage is at 14.2 over two full tankfuls, I think I've gained some insight...

Here is what I learned:

• The Motor Trend test figures from 1952 are about right at the time. Walt Woron was the (well-respected) tester and the testing was instrumented. See also Jay Friedman's post above for his 331 mileage.
• In recent years on my '52, the mileage was 10 or even a bit lower.
• Besides the routine tuneup, there were two other changes. One was to replace the choke cover and choke coil. Although that significantly improved the starting and idling when cold, I don't believe it had an impact on the gas mileage.
• It turned out that the vacuum advance on the distributor had a diaphragm that was shot. So there was effectively no vacuum advance. I replaced the unit with a rebuilt unit from Fusick, and the result was the mileage improvement to 14.2, and (I think) a little smoother running engine on the highway. The vacuum advance changes the timing by around 14 degrees -- this was a surprise to me though I'm sure those with much experience in working on older cars would know that.
• If I ever figure out what it takes to get the mileage from the current 14.2 up to 17, I'll report back. 

I hope this information is useful to someone.

John Emerson
1952 Cadillac Sedan 6219X

I'll respond briefly to a couple comments posted after the one above:

• In my case, the choke was simply not working until i got the new choke coil and cover. Choke was not closed when driving on highway. Of course the choke COULD have been the problem, but not here.
• I always use ethanol-free 91 octane gasoline and that has not changed as the gas mileage has gone from under 10 to 14.2.
• There was no other change (in differential ratios, dragging brakes, tire pressure, etc.) when mileage improved to 14.2. It HAS to be the vacuum advance. Both the previous owner (CLC member and good mechanic) and my very experienced old-time mechanic have no doubt that the vacuum advance made the difference.
• Of course several of other factors mentioned could well account for the difference between current 14.2 MPG and the original new-car 16 or 17 MPG. Who knows... perhaps I'll eventually get the mileage back up to over 16 and have a better running engine too! But for now, I'm very happy...

John Emerson
1952 Cadillac 6219X

[Gary McKinney]

I've never calculated fuel mileage on my 1950 Cadillac, but the fellow who drove the car when new told me that on the highway (such that they were in the early 1950's) the car would always get slightly over 20 MPG.  He regularly drove the car from Morgantown, WV to Washington, DC and back and insisted that these numbers were real.  On the other hand, I have calculated fuel mileage on my 1966 Eldorado and, on the interstate, it will get around 15-16 MPG. 

[savemy67]

Hello John,

To get an increase of more than 40 percent in your car's MPG would indicate to me that something significant changed.  I would have to concur that restoration of the timing function via the new vacuum advance is probably the most significant contributing factor.

The engineers called for the timing advance with increasing RPMs because as the speed of the piston increases with higher RPMs it is vital that combustion occurs at the point where the most work can be done and the least amount of energy is wasted.  With a broken vacuum advance, the timing is effectively retarded.  Combustion occurs later than is optimal for a given RPM.  It is quite possible at higher RPMs that the combustion flame front actually follows the piston down the cylinder resulting in very little energy imparted to the piston, and a great deal of energy wasted as heat.

How do you go from 14.2 to 16 MPG?  Downhill with a tailwind!  Do you remember the Shell Oil Company mileage stunts from the '60s?  You could put 80 pounds of air in your tires, but I don't recommend that you do.  You can do a lot of little things, as others have suggested, but to get the next big boost in MPG, you might consider doing both a compression test and a leak down test to ascertain the condition of the cylinders and valves.  If these components are not up to snuff, you may have to resign yourself to your current MPG until you rebuild the engine.

Respectfully submitted,

Christopher Winter

[kgreen]

I'll tell you exactly why.  Gas quality. 

Run your numbers on a tank of ethanol, then a tank of non ethanol.  You'll get a couple more mpg with the non ethanol, and even back then they had higher octane gas on average.

I did that test with my 03 f150 and got a couple more mpg, but with the additional cost of the non ethanol it was a wash in terms of cost per mile.

Pretty much what I found as well - no cost benefit to using non-ethanol gas in my modern car.  My older, carbureted cars benefited greatly in increased performance, easier starting and less vapor lock.  I've not checked the mileage for improvement.

Another factor in gas mileage determination by the authors long ago would be the speed of a car and the weight is carried while performing tests.   If I recall from the old magazine article, the test would be with an empty gas tank, and with gas supplied through a clear glass bottle hanging just outside the car window.  The tester would verify the start of the test at a certain fuel level, then stop the test after a measured amount of fuel was consumed.  They also used a calibrated odometer on a wheel attached to the rear bumper.  As to speed, a car gets much better mileage when it is under low load and at low speed.  Higher loads make the engine consume more fuel and higher speeds are not favored aerodynamically.  For current mileage a running average within 20% of optimally calculated mileage might be acceptable.

As for the OP's car getting substantially better gpm after some work, I'd be guessing.  The old analog and mechanical machinations definitely have a narrower band of tolerance to provide peak performance. 

[Steve Passmore]

In my humble opinion your missing the most obvious John. You said it all in your first post. The car is 66 years old. Fair mileage, not excessive I grant you but a fair amount.   Wear would have started from the get-go in 52. one has no way of knowing if when it was left for long periods any rust occurred inside the motor accelerating wear when it was restarted so you could have wear in the bores, rings, and piston groves, wear in the valve stems and valves. All these things make small differences. You would have to duplicate the factory build technique and clearances to get near to what they tested them at, plus they would have been gently cruising around a track which has no connection to every day driving. Just my 2 pennies worth.

[jdemerson]

In my humble opinion your missing the most obvious John. You said it all in your first post. The car is 66 years old. Fair mileage, not excessive I grant you but a fair amount.   Wear would have started from the get-go in 52. one has no way of knowing if when it was left for long periods any rust occurred inside the motor accelerating wear when it was restarted so you could have wear in the bores, rings, and piston groves, wear in the valve stems and valves. All these things make small differences. You would have to duplicate the factory build technique and clearances to get near to what they tested them at, plus they would have been gently cruising around a track which has no connection to every day driving. Just my 2 pennies worth.

Steve,
     I agree with all you are saying, and that may well mean that I won't find any further improvement!

     But it did change from a consistent 9.5 to 10 over 4 years to a definite 14.2 on each of the last two tank fulls. In effect, this was a controlled experiment, as type of gas, tires, driving habits, etc. etc. did not change. The only explanation for that improvement is the rebuilt vacuum assembly that controls timing. As I've said, I was both surprised and delighted!

     Thanks, Steve.

John Emerson
1952 Cadillac Sedan 6219X