Still don't understand why we need transmissions?


The explanations I’ve seen for transmissions are:

  1. They stop the engine from breaking when stopping
  2. They move the wheels

Can’t we have a clutch system where the “car wheels” connect directly to the engine crankshaft when the accelerator is pressed. When the accelerator is depressed or you press the brake pedal, the clutch disengages and the wheels coast/stop etc.

The engine spins faster the harder you press the gas pedal etc.

This system is less complex…so why do we need transmissions?


It all has to do with the torque or “twisting power” of the internal combustion (piston) engine. These engines don’t have high torque at low revolutions, hence they need a gear box or transmission. Assuming you have a bike, your leg muscles don’t have the great pushing power (similar to torque) you need to get going fast. So you start in a low gear and when you pick up speed you switch to a higher gear.

Some motors, like electric motors, and steam engines, have maximum torque right off the bat. So they don’t need gear boxes or transmissions.

That’s the best I can do to explain; if you still have difficulty, other posters may be able to explain this principle.


If you had an electric motor powering your car, you would not need a transmission. You could reverse the polarity to the field coil of the motor and have it turn in the opposite direction so that you could back up. An electric motor develops full power as it starts the load at zero rpm. Under Newton’s law of physics, a body remains at rest unless acted on by an outside force. Thus a car has to have an outside force to move. (You can’t get your state legislature to repeal this law and Congress is fighting too much to even consider it.) Now an 4 stroke internal combustion can only rotate in one direction. Therefore you must have some means of reversing the power to the driving wheels to back up. Furthermore, an internal combustion engine has to be turning at a higher rpm to develop enough power to overcome the inertia (back to Newton). Thus, there has to be a way to harness the higher rpm to overcome the inertia. This is done by the transmission. If you have ever ridden a multispeed bicycle, it is easier to get underway when the sprocket attached to the pedals is at the smallest setting and the sprocket for the rear wheel is at the largest setting. Once you are moving, you can have a larger sprocket setting on the pedal sprocket and a smaller setting on sprocket at the rear wheel. The automobile engine must overcome the initial inertia in the same way with something equivalent to the chain and different size sprockets to overcome the initial inertia with the engine turning faster and the rear wheels turning slower. As the inertia is overcome from starting at rest, you want the engine to turn slower while the driving wheels turn more quickly.

On a boat motor, it is different. The propeller has more slippage in the water, but the slippage decreases as the boat gets underway. On the little 3 horsepower Evinrude we owned when I was growing up, you could rotate the engine 180 degrees to back up. On the old two stroke Neptune engines, the engine would turn in the direction that you put the rope around the pulley to start the engine. The engine cold be started in one direction to back away from the dock. You could then stop the engine and wrap the rope around the pulley and have the engine start and rotate in the other direction to go forward. A four stroke engine can only operate in one direction.


I get it now, thanks for the excellent examples and info.


Most car engines produce little power below 1500-2000 RPM. When you are stopped, the driveshaft(s) is turning 0 RPM. The transmission starts off in low or 1st gear, about a 4 to 1 gearing reduction that also multiplies torque 4 times. This allows the vehicle to start off briskly from a dead stop which would be impossible otherwise…Then once in high gear and cruising at high speed, the transmission allows the engine to operate at a lower RPM than would otherwise be possible…


Next time you drive a manual transmission decide beforehand which forward gear would be suitable and shift to that gear and remain there. Please post your results.


xaml, have you ever ridden a single-speed bicycle? Have you ever ridden a “ten speed?” (Showing my age there…) Do you understand the advantage of the 10spd as far as being able to both climb hills AND go fast on the flats?

Well, that’s the basic advantage of “gears” in cars.

P.S. When I was a kid, reaching the point when you “graduated” to the multi-speed bike was a BIG deal. “BMX” bikes were o.k. for tearing around the neighborhood; “ten speed” bikes meant you were no longer dependent on mom’s Country Squire to get you to the pool, the pizza shop, etc. People talk about the sensation of “freedom” of turning 16 and getting a license–for me, it was at least as big a deal turning 12 and getting my Schwinn.


Experiment with your own transmission in this manner ;
Put it in first/low gear, leave it in frst gear.
Now as you drive notice the speed of the ENGINE as you go about as fast as you can in this first gear.
when the engine has reached its maximum ability you can’t go any faster.
– how could you possibly go faster ? –
By changing the ratio of gears ( now change to second gear ) so the engine can begin anew at its lower rpm…advancing to its maximum. allowing you to go that much faster …untill …

now you get the picture.
A transmission changes the ratio of the rolling speed versus the engine speed.
You can’t go faster without more gears . The engine only has it maximium. And then what ?

The 10 speed bicycle is a great pesonal experience of this.

Counting is another.
0,1,2,3,4,5,6,7,8,9 in the engine
10,20,30,40,50,60 etc is the transmission
You can’t count past 9 without ‘‘shifting gears’’ to get to 10,11,12,13,14 and so on.
Then you have to shift again 20,21,22,23
Then faster still…30,31,32,33,34,35


I will add this…I don’t think we need quite as many gears as we have today. I drove many vehicles with “Powerglide” transmissions and they got the job done quite well. I drove a few of them on dragstrips as well in years past. They were 2 speed transmissions and got me from point “A” to point “B” and that’s all that really concerns me.

Can't we have a clutch system where the "car wheels" connect directly to the engine crankshaft when the accelerator is pressed.

Sure. In fact, the Tesla Roadster does exactly that. But it uses an electric drivetrain, and electric motors are perfectly happy spinning at RPMs that would shred a normal gasoline engine.

The reason we have a transmission is because we want to go faster than 30mph, and we don’t want to be screaming along at 6,000 RPM at 30mph either. Changing gears lets us do both of those things while keeping the engine within the RPM range that it has better power output while not destroying itself.

I will add this.....I don't think we need quite as many gears as we have today. I drove many vehicles with "Powerglide" transmissions and they got the job done quite well.

The 2-speed powerglide tranny I had in my 67 Chevelle SS worked GREAT with a powerfull V8. It won’t work so well with a small 4-cylinder or 6-cylinder. Not to mention gas mileage.


The original Chevrolet PowerGlide that became available in 1950 depended completely on the torque converter as did the Buick Dynaflow of that time period. You can picture the way a torque converter works by imagining two electric fans facing each other. One fan is plugged in and turned on and the other fan is not powered. The speed of the second fan will reach approximately the speed of the first fan. To aid in getting the second fan moving, the pitch of the blades of the first fan is changed to increase the air flow until the second fan gets moving and then changed to maintain the speed. However, the torque converter uses oil instead of air as the power transmitting medium. The only way the original PowerGlide or the Dynaflow changed gears is if one manually selected Low range. On the other hand, the Hydramatic transmissions of this time period that was available on the Pontiac, Oldsmobile and Cadillac as well as the Lincoln (yes, GM built and sold the Hydramatic to other manufacturers including Ford), Nash, and Hudson was a 4 speed unit but only had a fluid coupling. The principle of the fluid coupling is like the torque converter except that there is no change in the pitch. In 1953, Chevrolet modified its PowerGlide to start in low range and shift to direct drive about 15 mph or so.
As to the 2 speed Powerglide, my parents owned a 1960 Rambler six cyllinder that had a 3 speed automatic with a torque converter that was built by Borg Warner. In 1963, the Rambler was totaled and while the claim was being settled, the insurance company rented my parents a 1963 Chevrolet Impala V-8 with the 2 speed PowerGlide. My mother, who was usually not sensitive to the way a car drove did not care for the PowerGlide. She said that there were times it was running too fast to be in low, but too slow to be in high. She thought it needed that intermediate gear. Keep in mind that the Rambler was my parents first car with an automatic transmission. My mother learned to drive on a three speed manual transmission. My dad thought the PowerGlide was a variation on the Model T Ford where the pedal was all the way to the floor for low and all the way out for high. Like the Chevrolet, the transmission in the Model T Ford was a 2 speed unit.


About those Powerglides, I drove a 1959 Chevrolet with a 409 and Powerglide that remained in 1st gear well beyond 70 mph at wide open throttle. The torque converter must have been a real slush box to get the car moving against a final drive that high. But who cares about fuel mileage, right.


@MikeInNH…I agree with you about the 4 cylinder but I used to drive a 57’ Chevy Bel Air with an inline 6 and Powerglide 2-speed transmission. It would get up and go. @RodKnox…I do worry about fuel mileage these days because it impacts my wallet. My dad owned a Sunoco station in the 1950’s and 60’s so gas was very cheap or free for me when I pulled up to his pumps.

I agree with you about the 4 cylinder but I used to drive a 57' Chevy Bel Air with an inline 6 and Powerglide 2-speed transmission.

I agree…many V6 or Inline 6’s could handle it. But some couldn’t.


Learning a lot here about the old days.
I rode in a older Chevelle SS back in the late 70s. Don’t know the engine size, just a three-on-the-tree, but Joe put a five dollar bill on the dash…’‘if you can grab it by the time I count to ten , you can keep it.’'
I’m NOT five dollars richer.


“It won’t work so well with a small 4-cylinder or 6-cylinder”.

Back in 1971, I took a 1968 Toyota Corona for a road test. The car had a 4 cylinder engine and a 2 speed automatic transmission. After the test drive, I wasn’t interested. The Toyota was assembled well, but that engine and transmission combination weren’t well matched. Had the Corona been a manual transmission, I might have gone for it. I also tested a 1971 Datsun 510. It had a 3 speed automatic and was a lot peppier. However the Datsun was much noisier than the Toyota.


Too bad Rod Knox doesn’t have that 409 powered '59 Chevy today. The big blocks mean some pretty serious value due to rarity.

I had a '59 2 DR HT Impala back in the day although mine only had the 283 and was Powerglide equipped. Still miss that car although the bat wing rear was a love/hate thing for some people. Love in my case.


I would argue that the transmission is the most important drive train component in most internal combustion power vehicles and in a lot of transportation and work machines as well. The transmission has come a long way, but not in boats with small outboards. You shift gears in a boat by raising the motor and changing out the prop. Transmissions are found everywhere, even walking. You take smaller steps when climbing, carrying heavy loads and accelerating. The transmission in a sailboat is found in the controls that change the sail trim…

In cars with gas motors, the move is toward more gears for increased economy and performance, not fewer as as others point out. The cvt is the end all in transmissions which ever form it takes…
AND THE CVT HAS officially ARRIVED. I stated a couple years ago that CVTs will have arrived when Honda and Toyota used them on primary vehicles. Honda on the Accord and the new model Corolla will have one as an option…it’s now officially here to stay with ICE motors.

A two speed power glide only worked great when compared to the alternatives at the time; back when AC wasn’t standard and zero to sixty times under 10 seconds was looked at as “high performance” and gas was south of a buck a gallon. It worked great if you just hated manual transmissions with many only having three gears often themselves. There wasn’t much competition.


Another type of “transmission” Is the Prius Power split device, There is no torque converter, Its a cvt with no belt or chain. It is actually very simple. There is a planetary gearset, Two motors and an engine. There is no actual reverse gear, reverse is achieved via an electric motor. “Neutral” is achieved by turning both electric motors off.