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I know this has been addressed here, but I am having a hard time finding the postings. As seen in my avaitar, is my car. 1949 Deluxe Touring Sedan, 288 engine, 3 speed with overdrive. Here are my questions. I want to determine what my engine rpm's should be, especially when on the highway. At looking at the spec sheet for the car,
(https://www.packardinfo.com/xoops/html/downloads/SC/SC-VOL23NO5.pdf)
I'm not quite sure if I understand what I am looking at. About the middle of page 26 under the transmission heading, it shows the standard rear end rear end ratio is 3.9 - 1. Yes? My car has the overdrive. So the rear end in my car is 4.1 and the output of the overdrive is 2.96. Yes? I'm having a hard time wrapping my head around how to use this info to calculate my speed and engine rpm. At the bottom of page 22 it shows that with the standard rear end, engine rpm is 2824 per mile. I am assuming that means at 60 mph(a mile per minute). According to my GPS, (speedo is out - I'll get to that), at 55 mph, in overdrive, the mill is turning about 2250 rpm. (225-75-15 radials about 28 inches diameter)I have installed a tach(12 volt, # of cylinder switch set at 6, fairly accurate compared to my dwell/tach meter). Does all this seem to make sense? I have read here that some have no problems running 70 - 75 mph. What rpms is the engine at? If these engines are comfortable at a 2500 rpm as a safe maximum, from what I am seeing in my situation, running at 70 mph, the engine is turning faster than that.
Now for the speedometer. Around town it seems to be about 5 mph faster that what the GPS shows. When I got out on the highway it's about 10 mph faster. I was up to 60 on the GPS and the speedometer was reading 70. And at 60 (GPS) the engine was almost at 2500 rpm.
Thanks again to all who have helped this Packard newbie(purchased Semptember 2012) figure out his car, and thanks to all who help in the future. It was a topic I think in the "General" forum, thanks to BigKev for your dedication to this site.

Others can give the info on the rpms. Math and I don't get along.

As to the speedo, I don't believe any of the typical mass market old speedometers were known for great accuracy. If the tire diameters are within the 1/2" difference Diamondback charts say theirs would be -- radial vs bias -- and your radials are not turning way faster than a bias tire would at a given speed, there is not much to be changed.

Other than tires, there are only two variables. One is the drive gear at trans. The other is an adjustment in the speedo via a spring. Assuming the drive gear is the correct one, then it drives the cable to spin a magnet at a given rpm. That spinning magnet sets up an eddy current and tries to pull an aluminum cup attached to the needle around with the magnet. Tension delivered by a hair spring attached to the cup via the needle shaft opposes the pull and is the calibration. Since the mismatch increases with speed instead of being a constant value, doubt the tires or drive gear is the problem.

If radials are smaller and numbers overall were a consistent amount off, possibly finding a drive gear for a higher axle with one more tooth would bring it down closer to actual. If that wasn't satisfactory, another (harder and more expensive) option would be to see if one of the many modern and popular brand speedo adapterstransmissioncenter.net/speedometer_calibration_______va.htm could be fitted inline into your existing cable.

Even with a proper drive, I'd almost bet the spring has lost a bit of tension over the years. Sending the unit to a speedo shop that has the necessary calibration equipment could possibly get it closer but I still wouldn't want to bet much on the absolute accuracy. There's only the one spring so any adjustment will be done (and good) at the most common midrange speed. Depending on how progressive the spring and how good the tension still is, results will probably be compromised somewhere above and below that calibrated speed.

At least the current mismatch is in the right direction. If the speedo said 60 and you were really doing 70, you might be meeting your friendly local mountie and it probably wouldn't be because he wanted you to stop so he could admire your car.

From post #1:
"1949 Deluxe Touring Sedan, 288 engine, 3 speed with overdrive. ... I have installed a tach(12 volt, # of cylinder switch set at 6,..."

If that is a 288 engine then that means it is an EIGHT cylinder engine??? No??? If it is an 8 cylinder then why is the tach sw. set at 6 ???? IT should be set at 8.

From post #1:
" My car has the overdrive. So the rear end in my car is 4.1 and the output of the overdrive is 2.96.... (225-75-15 radials about 28 inches diameter)

Assuming the final overall ratio of 2.96 and the 28 d
diameter of tire is correct then:

At 60 mph the engine rpm is 2133 rpm.

FOR EACH 10 mph INCREMENT the rpm is 355.5 rpm.

So at 10 mph engine turns 355.5 rpm.
At 20 mph,711 rpm
at 30 mph 1066.5 rpm
at 60 mph 2133 rpm
at 70 mph 2485 rpm.
at 100 mph 3555 rpm.

I definately would NOT try that 100 mph for very far.

I'd say your good to 70 mph at 2485 rpm SUSATAINED but that about tops off any reasonably safe sustainable engine rpm for long distance cruising for about any pre-1955 engine.

PackardV8's numbers are quite right given your tire diameter. From the numbers and from my own experience there is not the slightest trouble running 70 -80 mph, especially with the 288 which has the sweet short stroke. The bottom end won't notice a thing but the gas gauge will have a tendency to plummet.

Dear Deluxe:

According to my calculations for the standard 7.60x15 tires your car should be turning 27.93 mph per 1000 rpm in overdrive top or 2148 at 60 mph.

Out of overdrive your car will turn 2976 rpm at 60 mph

I frequently run my 1950, which is equipped with a 288 and overdrive, at 70 mph. In my experience the car cruises just fine for hours at that speed. I just make sure the tires are all at 35 psi, the fluids are topped off and, most importantly, I have plenty of money for gas!

PackardV8 - You're right...eight cylinders. I had my "old school" dwell tach meter from Sears Roebuck(picture below) hooked up to verify because it was a 12 volt tach. It was not correct on the 8 cylinder setting. Go figure. So I changed it to where it did read correctly. When I get a chance I am going to hook the old school meter back up inside the car and compare the two when driving.

Howard - So full of insight. Made me laugh when you said about meeting my local Mountie. But can you imagine the look on his face when he clocked a car that old going that fast. That would be a hoot! Might be worth the ticket to see that face.

Tim and PackardV8 - Your numbers are close to what I am seeing. What I see though is 55 mph (GPS) at about 2250 and almost 2500 for 60. Maybe my tire size is off just enough to screw this up. But as stated above, I need to verify the tach setting.

Ross and CaptainBristol - We were out for a drive today. Round trip was about 110 to 120 miles. Tried to keep it around 50 to 55 MPH. Depending on how accurate my gas gauge is I think I used at least 3/4 of a tank. Would you consider that normal? I'm new to this car and still figuring it out, but having a blast doing it. Lots of "thumbs up" today.

Gentlemen - Thanks for all your input. Want more if you have it in you. I have to verify the rpm's on the highway and maybe tweek my speedo a bit, like Howard mentioned, to try and get it a bit closer to the speed that I am travelling.

I raise a glass of a barley based beverage to you for all the assitance.

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The speedo on my 53 Pat w/Ultramatic is at least 5 mph faster than actual speed by my estimation. Assuming that it was correct what would be a safe max speed/engine rpm for this car. The engine always seems to be racing with the indicated speed at 60 to 65.

Wow, if what you say is true, you got 10 mpg. I sure hope not! My 51 200 (exact same drivetrain as your car) turns in a regular 17 mpg on a trip, 18 at the speeds you mention, and 16 if I flog it. I don't know what it does in the city because I can't hardly think of a reason to visit a city.