Packard Forum Index » V-8 (1955-56)

G'day Guys, I've got a technical carby question about the mileage I've just recorded in my '56. I actually think the mileage is pretty good, although I stand to be corrected on that, but it is the variation that I'm asking about. Last week we travelled a total of over 450 miles (I won't talk metrics) on relatively flat country roads, and I did it in two distinct driving styles. The first and last totalled over 310 miles at speeds probably averaging around 60mph and achieved aussie 17.3 mpg (US 14.4 mpg). The middle bit was 140 miles at about 55 mph and that produced 21.6 mpg (US 18 mpg). Like I said, that seems pretty good to me but for a variation of only 5 mph in speed, mileage dropped by about 20%, which seems a fair bit.

I was wondering whether this might be due to the secondaries coming in too soon and if so, what is the best method of adjustment. I'm not overly concerned about power as I seem to have plenty but was thinking that for steady throttle cruising, shouldn't we only be running on two barrels. Mind you, this is only guesswork on my part as I really don't know whether I'm cruising on two or four barrels. Regards, John

Slight incline on elevation and a little head wind eats your mileage fast. I'm not sure what the formula is but for every 10 mph increase the resistance of "pushing" the car through the air goes up significant as well.
Besides that its still a large difference. Do you know what your rpm's are at both speeds?

This website reports that gas mileage drops once you get past the cars "optimal speed," that is, 55 to 60 MPH.
mpgforspeed.com/

According to studies backed by the department of energy, the average car will be at its advertised MPG at 55 mph. But as the speed increases:

- 3% less efficient at 60 mph
- 8% less efficient at 65 mph
- 17% less efficient at 70 mph
- 23% less efficient at 75 mph
- 28% less efficient at 80 mph

In today's modern cars all it takes is an increase from 55 to 70 to get a 17% drop in fuel mileage. So your results are a 20% drop. That begs the question of how different is your 57 year old car from today's average car?

I would guess that your Patrician is brick when compared to the wind drag coefficient of 0.31 for a Cadillac CTS or 0.19 2014 Volkswagen XL1. So add something there.

Next would be your drivetrain. You have to wonder what your axle ratio is compared to modern cars designed for high mileage. Add a few factors such as these (ethanol?) and it might be all attributable to the 57 year old differences in design as compared anything amiss with your carbs.

By the way, compared to your Patrician, my 1941 is a cement block.

The book "Packard Gold Portfolio 1946-1958" ISBN 1 870642 198 is a compendium of contemporary published road tests. Many are of the 1955-56 Packards and Clippers and some have specific fuel mileage.

One (pg 104) tested the 1955 Clipper Custom with 352CID & 3.23:1 rear gear: 18mpgAt30mph, 20mpgAt45mph & 16mpgAt60mph.

Another (pg 116) tested the 1955 400 with 352CID & 3.54:1 rear gear: 18.4mpgAt30mph, 16.3mpgAt45mph, 14.6At60mph & 12.5mpgAt75mph.

Another (pg 141) tested the 1956 Executive with 352CID & 3.07:1 rear gear: 26.0mpgAt30mph, 25mpgAt40mph, 22.5mpgAt50mph, 20.5mpgAt60mph, 18.5mpgAt70mph, 16.5mpgAt80mph, 14.0mpgAt90mph. Note that the "economy" rear gear really made a difference in this one.

Another (pg 146) tested a 1956 Clipper Custom with 352CID & 3.07:1 rear gear: 19.1mpgAt30mph, 17.6mpgAt45mph, 14.9mpgAt60mph & 12.4mpgAt75mph. This seems more realistic than the Exec above.

There are no detailed gas mileage numbers in this book for 1956 Senior (374CID & 3.54:1 rear axle). One report says to expect around 12mpg average gas mileage.

Craig

Craig - thanks for digging out that info. We should remember to subtract about 7% from those numbers if you are using 10% ethanol gasoline. I know I've posted this before, probably more than once, but here are the figures from Motor Trend for the 1955 Packard 400, Clipper Custom, and competitive makes.

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Thanks Men, much appreciated and very interesting. My car has the 3.54 rear end and if I compare it to the '55 400 mentioned in a couple of posts, I'm almost spot on with those 60 mph figures. BDC mentioned the adverse effects of a head wind on mileage and interestingly, the day I recorded the best mileage it was windy but I'm now thinking it might have been more a tail wind than head wind - does this reverse the head wind impact on mileage? Probably! So, maybe the good mileage figure is inflated and the real difference between the 55 mph the 60 mph figures is not as great as I thought.

I'd still be interested in any advice about checking the setting of the secondaries. I've read the service manual but either I didn't understand it or it doesn't go into much detail about it. Somebody recently suggested why not disconnect the linkage to the secondaries and monitor what happens (???). I have to admit I can't tell when the secondaries come in, although I appreciate that under acceleration that will be happening. But when cruising, I just don't know. Regards, John

Head wind or tailwind with my 2011 1ton pickup @80mph makes about 5-7mpg difference!

You should be able to feel an extra resistance to your gas pedal foot when you get to where the secondaries open. This will be when the gas pedal is half way down or thereabouts. If you want to check this have someone watch the carburetor (with the air filter and engine off of course) and tell you when they start to open.

In normal driving you should be no where near the secondaries. They kick in only under hard acceleration.

There are no secrets to better mileage. Inflate your tires to 32 PSI. A change to synthetic lubricants can help a bit, although I am not sure if they are suitable for a 1956 car. Slow careful driving.

I know it is painful filling up a Packard at today's fuel prices. But size, weight, power, comfort and convenience cost money. We are spoiled today when the cheapest cars have features that used to be exclusive to the luxury models.

BDC comments "at 8o mph" ...phew! In my State in Aus, our max speed limit is 110 kph, which is about 68 mph. The highway patrols do enforce it and you can expect a hefty penalty if caught breaking it. I know Peter Packard has a propensity for moving things along with a heavy foot, but I wonder if he gets into atmosphere that high.

Thanks Rusty - I've tried "feeling" when the secondaries have come in but haven't managed it yet. I'll definitely try your other idea of having somebody keep watch on the carby while we give it a serve. Also, my tyres are 235 x 75 radials inflated to 35 psi and I reckon this, combined with gas shockers, means the car feels pretty sure footed on the road.

The other issue of course is fuel. I don't know whether our fuel is any different to yours, but the various brands available here certainly are different when it comes to the Patrician. I always us mid-range premium (95 RON) and it's not advertised as containing ethanol, but it might have some as from what I understand it only has to be stated if it's at least 10%. In any event, Shell and to a lesser extent Caltex, will make the old girl ping, but if I run her on BP, that's not an issue. Why so? Buggered if I know, except that both Shell and Caltex are able to be bought using supermarket fuel discount vouchers. Hmmm! Cheers, John