Technical 55 Pontiac speed question

Hi everyone,

I have a 55 Pontiac 2 door sedan. It has the original 287 with 2 bbl, original 4 speed automatic and, I'm ***uming, the original rear end. It's had 215/70R15s on it since I bought it.

The speedometer reads about 12 mph faster than actual speed when cruising. Here is my main issue: she cruises in 4th gear at about 38-40 mph. If I even push it to 45, she's wound up a little much.

Is this normal for this car? Should I be looking at a rear end swap to get up to highway speeds?

Thanks for any help.

 

When you say "wound up a little much" what is your actual RPM?

 

In my teens I worked for my Uncle at a mom-pop grocery store. Our delivery vehicle was his 56 Pontiac wagon. I drove that a lot and it would cruise easily at 60 - 70 without sounding "wound up". Went with him on a couple of family trips and he ran between 70 and 80 all the time.

 

That 287 has pretty small intake & exhaust ports, difficult to make decent power but it is a torquey ****er. With a factory rear end, she out to be good for the century mark, no prob.

 

I have a 56 Pontiac wagon with the 4 speed hydro currently, drove it tonight actually. If you are comparing it to your modern daily driver then yes they sound a bit wound up at 70mph because they were designed to be driven all day at 55mph. These are not all that peppy cars being a bit heavy and not all that powerful at only 200-220hp I would not mess with the gearing as they need it to get out of their own way and just enjoy the experience of driving an old car for what it is, an old car.

 

It's probably got either 3.08 or 3.23 gears. The removable carrier had a single digit number code stamped in it around the 7 or 8 o'clock position to identify it.

 

Looks like the original pontiac tires were over 28" diameter, yours are under 27" so that accounts half the error.

 

As stated above, you don't have the post-OPEC 0.7:1 overdrive, so do not expect to be burbling along at 70.

40-45 in top was always in the realm of 2000 rpm. You want to do 70? 3500.
Also, if you have a fairly quiet muffler and intake, the b*** notes will be gone and you'll be hearing the second and third order harmonics, which make the engine sound quite different at speed.

My speedometer over-reads too. My tires are a little lower profile than stock. Must be a Pontiac thing.

 

I’m betting on the “just bought this car and it sure winds up” vs the daily driver the OP has been used to.
Heck, my neighbor brought home a fairly new JD, super turbocharged 120 HP backhoe. When he’s working it at high RPMs, damned thing is quieter than my MF65 when it just above idling.

 

Very much so. If you close your eyes and just listen, comparing my OT truck (5.7 Dodge) at the same engine RPM the truck has a deep wubba-wubba-wubba, which is the odd/even pulses combining in the exhaust. You don't hear the actual engine note.
The Chieftain has no such beat to the exhaust and as such sounds like you're wringing its' guts out at 1200rpm- until you realise you aren't.

Likely just a case of perception.

 

Start by "correcting" the speedo with the proper size (taller) tires (at least in the back) and drive it for a while that way. That will settle it down for cruising somewhat and perhaps be enough to get you where you want to be. Don't be tempted to compare it to modern cars with overdriven gearing in the transmissions.
In 1955 there were barely any sections of interstate highways yet built. Your car was engineered to drive on state and local highways and to pull over for every other tourist trap. Think of it as a time machine that is still looking for the type of roads it likes.

 

Load the speedometer app on your phone to verify your speed and get a tachometer to verify the actual rpm. Unless your transmission isn’t shifting into 4th gear I think you’ll find the rpm to be normal. It’s probably just a noise issue with the fan or something touching metal to metal.

 

Thanks for all the replies. I'll install a tach and put taller tires on the back.

 

An old Chilton manual list a 3.23 or a 3.41 axle ratio Even with the 3.41 gear 27" tires at 40 mph the RPM would be less than 1700 Rpm are you sure the trans is going into high gear/

 

It shifts 3 times when accelerating.

 

Throwing this out there. Your linkage to the transmission may not be adjusted right. To make a short story long. I put new seals in my 56 transmission. Linkage adjustment got played with while out. I had no idea what proper adjustment was. Ran like ****. Much like you note. After I learned how it works, all was good. Shifted and cruised fine.

 

It does shift into 2nd pretty quickly. Should I adjust the linkage to be longer or shorter?

 

Might be the right time to suggest a factory manual for the car, them books of paper have a lot of good info in them

 

On mine the perfect adjustment was when there was no pull on either end.

These cars cruised all day even then at 55-60. There are some here that are far more an expert than me on adjustments.

 

Low gear was 3.82:1, so I think that a quick shift to second was normal.

 

I think in general that HydraMatics tended to have a pretty early 1 - 2 upshift, especially at light throttle. Low gear seemed to be there mainly as a "break away" gear to get you moving away from a dead stop with at least a little more pep. These were big heavy cars with comparatively lower powered engines.

 

Yup, 1-2 will change at a fast walking pace under light throttle, about 12-14 mph with your foot to the floor.

Normal and by design.

 

They needed a steep low gear, seeing that they had a fluid coupling rather than a torque converter. The fluid coupling is more efficient at road speeds.

 

You can view a factory shop manual on the PontiacSafari.com website: http://www.pontiacsafari.com/L1Garage/Garage.htm#Pontiac_Do***ents It should have instructions on how to adjust the linkage to the transmission.

 

Stock 55 Pontiacs had 7.10 15 tires that are right at 28.88 Inches tall (Coker Firestone blackwall 7.10x15)
Your 205-70 15 is 26.9 inches tall giving up 1.98 inches in diameter.
The top four of these images show what rpm the engine turning at said speed at gear ratios the guys memtioned above. 26.9 tall tire, 3.41 gears at 60 mph = 2556 rpm at 40 it equals 1704 rpm
With a 28.88 tall tire with 3.41 gears at 60 you would be turning 2380 rpm.

 

The rod from the throttle linkage to the transmission is very touchy, it is adjusted in 1/64" increments as it has a fine thread adjustment and very easy to not set right. Proper idle setup is part of it too. If it is going down to first at stop signs (might take a second or two to down shift all the way thus the old advice of coming to a complete stop and counting to two) and getting you all 3 up shifts I would leave it alone.

 

Dad's 55 in 1959. My cousin still has it!

 

That's slick!

 

Agreed. It should only be reluctant to find first if the engine is very cold and the throttle idle-up stop is on its maximum position.

Light throttle you'll be in 4th by 15-20mph.
Accelerating away in traffic, by about 35.
Hard acceleration about 55.

If it's changing like that then yes, it's set right and don't mess with it. It's awkward to get right.

 

This is very key, thanks for the reminder. If the car is still on the choke it will not down shift to first as RPM induced pressure will be too high and throttle position linkage to far open. This means basically you cannot drive until it is warmed up without shifting first to Neutral and then into Drive at each stop (otherwise you try to pull away in second which is no good). This is a common occurrence for me if I am in a hurry and do not let it warm up in the driveway for 5 before heading out. Nothing is like trying to turn with oncoming traffic and finding you are lugging forward in second and having to jam on the brakes, throw it quickly into neutral, wait a second or two and back down to drive before you can complete your turn.