1928 Plymouth 1937 Plymouth Plymouth: The First Decade

How Fast Should I Drive It?

“Cars whisking by on 66. License plates. Mass., Tenn., R.I., N.Y., Vt., Ohio. Going west. Fine cars, cruising at sixty-five.” -- The Grapes of Wrath, John Steinbeck, 1939

Elsewhere I have presented some thoughts on how fast a 1933 Plymouth DeLuxe Six could go. Another question then comes up: How fast should you go?

Original Speeds and Road Conditions

Speed Limits

[Ohio 2002] gives a history of maximum speed laws in Ohio. The maximum speed was 35 MPH until 1940 and was 50 MPH from 1941 until 1958 (excepting the national 35 MPH limit of World War 2). In California [Chartrain 2005], the maximum speed was 45 MPH from 1931 until 1941. In 1941 it was raised to 55 MPH.

Jim Benjaminson [Benj 2011] Gives 1941 speed limits as follows:

StateSpeed Limit  StateSpeed Limit  StateSpeed Limit
Alabamareasonable and proper Maryland50, dual lane highways 55 Oklahomareasonable and proper
Arizonareasonable and proper Massachusetts50, 45 night and rain Oregon45
Arkansas60 or posted Michiganreasonable and proper Pennsylvania50
California45 Minnesota60 day, 50 night Rhode Island35
Colorado60, 20 to 40 in mountains Mississippi55 South Carolina55
Connecticut50 day, 40 night Missourireasonable and proper South Dakota40
Delaware45 Montanareasonable and proper, 55 night Tennesseereasonable and proper
Florida50 day, 45 night Nebraska60 day, 50 night Texas45
Georgia55 Nevadareasonable and proper Utah50
Idaho35 to 55 posted New HampshireAs posted (45 maximum) Vermont50
Illinoisreasonable and proper New Jersey40 (certain zones, 45) Virginia55
Indianareasonable and proper New Mexicoreasonable and proper and zoned Washington DC25
Iowa45New York 40 or posted Washinton50
Kansasreasonable and proper and as zoned North Carolina60 West Virginia45
Kentucky40 North Dakota50 Wisconsinreasonable and proper
Louisianareasonable and proper Ohio45 Wyoming60

Actual Traffic Speed

Then, as now, there were a fairly large number of speeders and it is easy to believe that the actual average speed on rural highways was a little higher than the legal maximum. Many people drive at a speed that they consider “safe and reasonable”. The safe and reasonable speed is largely determined by the car’s capabilities and by the design and condition of the highway.

I don't have references that directly relate to highway design in 1933 but I have some from a little later era. [Hewes 1942 58] indicates that primary highways were designed for 50 to 60 miles per hour while [Whitney 1941 143] indicates primary highway speed in the early 1940s was 50 MPH.

It should also be noted that continuous operation at speeds of 50 MPH or higher was unlikely even in 1940 because highways did not bypass small towns and most roads were not up to “primary highway” standards. [Whitney 1941 144] notes that in 1941 only half the roads in the United States were “improved highway”. In the lingo of highway engineers “improved” can mean graded, it does not necessarily mean paved. [Reck 1938 200] notes that “today there are more than three million miles of road of which nearly four hundred thousand are surfaced and may be driven over in any weather.” Thus in 1938 87% of all road mileage in the United States was not paved.

Historical Speed Summary

Thus highway speeds were likely to be in the range of 35 to 60 MPH. Sustained high speed driving was probably the exception rather than the rule.

Wear and Tear

Plymouth engine design was quite advanced for the era. With full pressure lubrication to all bearings, a four bearing fully balanced crank, aluminum pistons the engine was designed to with stand full power operation for long periods of time. This is contrasted with Chevrolet which had splash feed lubrication for the rods, a three bearing crank and cast iron pistons. For this reason the following pertains more for a Plymouth and other Chrysler products than for some other makes of cars.

Road speed calculator
Engine RPM
Axle Ratio :1
Mounted Tire Diameter Inches
Mounted Tire Diameter

Tire and Wheel
X Inches
Approximate Road Speed MPH
Engine Displacement  
Manifold Vacuum (in. Hg)
Estimated MPG

Assumptions, Notes and Comments

Mounted Tire Diameter Estimator Tire aspect ratio of 1.01:1 That is the height of the tread is about 1.01 times the stated tire width. This empirical number was arrived at by looking at the advertised mounted tire diameters for a number of vintage tire sizes. If you know the mounted tire diameter, you should use this actual number rather than the estimator.

MPG estimator - Based on treating the engine as an air pump and calculating the input volume. This is basically the engine displacement times the RPM with adjustments for the four stroke nature of an automobile engine and for the pressure in the intake manifold. The intake manifold pressure is based on a user provided estimate of “manifold vacuum” (i.e. the amount the pressure in manifold is below atmospheric pressure). An ideal mixture of 14.7:1 by weight is assumed.

Physical constants used
25.4 mm = 1 inch
Density of mercury at 0°C = 13.5955 g/ml.
Density of dry air at 0°C, 760mm = 1.2929 g/liter
1 Lb = 453.59237 grams
Density of gasoline = 6.1 Lb/Gal

High revs on the engine can lead to premature wear. Truck engines are expected to last for long periods of times under heavy duty operating conditions. So for continuous operations governors are used to limit the engine to speeds that can be maintained indefinitely. [Koldjeski 2000] notes that many Dodge trucks fitted with essentially the same motor had governors that limited the engine to 3200 RPM. [Cannon 2002 14] Notes that over speeding an engine is defined as exceeding the RPM that maximum BHP is generated. In addition [Cannon 2002 14] notes that “for long engine life, maximum engine RPM should be held a couple of hundred RPM's below the rated RPM at maximum rated brake HP”. Thus is is reasonable to assume 3200 to 3400 RPM, as a maximum sustained RPM for the engine. Since [Breer 1995 86] indicates that all Chrysler engine designs were tested at maximum brake HP in 50 hour tests, production engines should give good service when run below that RPM. (All Plymouth 6 cylinder engines of this era had maximum BHP at 3600 RPM.)

For the 1933 Plymouth DeLuxe Six equipped with the standard 4.375:1 rear end and 5.50x17 tires 3300 RPM works out to approximately 63 MPH. Interestingly, the Standard Six had a 4.11 rear end ratio and 3300 RPM works out to 67 MPH. Apparently the top cruise speed was not as much an issue as the acceleration off the line. Both cars had the same engine and transmission but the DeLuxe was slightly heavier. So the DeLuxe needed lower gearing to have the same or better low speed acceleration.

Interestingly, I have noticed that people are no longer used to the sound of a high revving engine when cruising at highway speeds. My guess is that this is due to the change in final drive ratios that occurred during the 1970s gas crisis and due to the better level of sound insulation in modern cars. So even though the engine and drive train is capable of sustained operation at above 3000 RPM, drivers younger than around 45 years old fear that they are beating the machine to death. There are lots of Internet posting and queries about changing the final drive ratio or installing overdrive transmissions to solve the non-existent issue of the engine turning over too fast. Many of those have not considered that the limit on high speed driving might not be the engine but rather safety issues with the brakes and suspension.


Equipment related issues

The narrow tires with a small contact area and small brake drums limit braking effectiveness. The stopping distance is much longer in older cars than in newer models. As noted on the headlight repair page, [Whitney 1941 32] gives the average stopping distance of 193 feet for a car going 50 MPH with brakes in “first class condition” on dry level concrete. Contrast to that to a test of a 2007 BMW which is able stop in 89 feet on dry pavement or just 195 feet when stopping from 70 MPH on wet pavement with new tires.

The long wheel base, slow steering, lack of reserve power, and high center of gravity limit evasive maneuvers.

Modern cars have energy absorbing crumple zones, collapsible steering column, padded interior, air bags and properly engineered lap and shoulder belts. While the 1933 Plymouth boasted a “safety steel body” it lacks any other significant protective mechanisms for the occupants. Installation of seat belts will help, but even these are not likely to have been fully engineered and may fail to provide the maximum possible protection.

In any case, we are talking about driving equipment that is three quarters of a century in age. Despite the best inspection and maintenance equipment failure will be more likely than in a modern car.

Traffic Flow

Large differences in vehicle speed, especially on crowded highways, can contribute to accidents. Impatient drivers attempting to get around slow moving vehicles can cause chain reaction evasive maneuvers by other drivers. Studies have shown that there are generally two groups of drivers: One group drives at the posted speed, the other at what they feel is safe and reasonable. If there is a large difference between the safe and reasonable speed (generally considered to be the speed that 85% of the traffic is actually traveling at or under) and the posted speed limit then accident rates can actually be reduced by raising the posted speed limit.

Posted speed limits on freeways in California are generally 65 MPH but traffic often moves faster, sometimes much faster. A antique car traveling 50 MPH when all the other cars around it are moving at 70 MPH constitutes a safety hazard. One traveling 60 MPH is less likely to induce accidents.

Consequences of an Accident

With the poor evasive response and high center of gravity of the car a roll over is much more likely than in a modern sedan.

The forces acting on the suspension and especially the wheels and tires increase dramatically with speed. The energy that needs to be safely dissipated in a collision increases as the square of the speed. So speed will both increase the likelihood of failure and make the consequences worse. It is quite likely that you can walk away from a collision that occurred when you were traveling 60 MPH in a late model car. It is very likely that serious injuries or death will result from the same accident in a 1933 car.


Based on the original highway design speeds and original governor settings on same era trucks, and with some thought but no specific numbers from an examination of some safety factors, I feel that a maximum cruise speed of between 50 and 60 MPH is reasonable. At 60 MPH there is 44% more energy to be dissipated in an accident than there is at 50 MPH. That could be the difference between life and death.

Based on the above, I generally cruise at a 50 MPH pace where traffic is light but move up to 60 MPH when required by the demands of traffic flow. In any case, extreme defensive driving must be used.