6th Gear for Increased Fuel Economy?

I’ve noticed a lot of cars in Europe have a 6th gear, and consequently the RPMs run (relatively) low, even at higher speeds. What couldn’t more and more gears be added, with every gear providing increased fuel economy?

I think that the added speeds are there because internal combustion engines operate most efficiently over a relatively slim rpm range. The added gears keep the engine in this range. When I began driving in the 1950’s, most manual transmission cars had only 3 speeds. A popular option wsa the Borg-Warner overdrive which did provide a higher gear ratio above 30 miles per hour or so in overdrive high, but also gave an added ratio, overdrive second, that was between the second and high gear. Adding more gears does not necessarily cut back the RPM. My 1950 Chevrolet pick-up truck had a 4 speed transmission. High, or 4th was direct drive. Most of the time I started in second gear. First gear was for heavy pulling–I actually used this gear in the truck to stretch fence.

Many larger trucks use a 2 speed axle to have more gear ratios so that the engine can run in its efficient RPM range. As I remember, an accesory for Ford cars through the 1948 model was a Columbia 2 speed axle. Highway mileage was improved using the higher gearing of this axle.

There is a limit as to how high a gear will increase mileage. As an example, try riding a 10 speed bicycle in its 10th (or highest speed). You will put forth a great deal of effort. In my case, I rarely use anything above the 6th ratio, because it takes too much energy for me. The same is true with an engine–after a certain point, it takes too much energy in the form of gasoline to power the vehicle if the gearing is too high.

I agree with Triedaq. On the cars with which I am familiar, sixth gear is no higher on the 6 speed than 5th gear on a 5 speed.

On a gasoline engine, there is such a thing as ‘too high a gear’ for a couple of reasons.

  1. In general, a gasoline engine has a ‘sweet spot’ in its RPM where it runs smoothest and the most efficiently. The cam, intake manifold, exhaust manifold, etc. are designed with that RPM range in mind. Running at higher or lower RPMs is less than optimum.
  2. Running in too high a gear for your speed ‘lugs’ your engine. You have felt a shudder go through your car when an engine lugged down as it tried to pull at too low an RPM. That shudder is very hard on engine components and should be avoided.
  3. If your car has a relatively large engine that can run smoothly and has enough power to pull the car at low RPM, there is one other down side to very high gearing. At very low throttle opening, your engine pulls a very high intake manifold vacuum. Pulling that vacuum takes energy, and you don’t get that energy back later in the cycle. In that situation, you are better off to have a smaller engine that runs at a higher throttle setting to pull the car, so you are not wasting energy pulling a vacuum. This applies to gasoline engines only. Diesel engines have no throttle valve and therefore don’t pull vacuum on their intake manifolds.

Many new cars come with 6-speed transmissions, and there are even 7- and 8-speed transmissions out there. More gears does not necessarily mean higher mileage, however, and the overall gear ratio of these multi-speed transmissions is not significantly higher than a 4- or 5-speed. The larger number of gears means the engine can run at an efficient speed regardless of vehicle speed.

Your response about lugging the engine in a gear that is too high for the conditions made me think about the Borg-Warner automatic overdrive tht was popular from the 1930’s through the mid 1960’s. I wondered why the manufacturers used this rather than having an overdrive gear in the transmission. My guess is that too many people would lug the engine in the top gear. The overdrive was designed so that the car wouldn’t go into overdrive until the speed was about 30 miles per hour or higher.

Gearing plays a relativity minor roll in fuel economy. What kills mileage is aerodynamic drag caused by high speeds…

European consistently drive cars with smaller engines than we do. They do not embrace automatic transmissions like Japan and North America. If they use 5 speeds with tall final drive ratio, it would hurt low speed acceleration and their clutches. But if they drive around in 5 speed with short final drive, that would hurt their mileage and they pay double of what we pay.

They need their 6 speeds to preserve low speed response and cruise speed mileage.

I had a 49 Chevrolet Pickup,it had the 3 spd on the column and the side of the trans blew off when I was backing up a steep gravel hill,replaced it with the 4 spd,very easy swap.

It had the 235 in it, I had it bored .090 and put a hydraulic cam in it,I could out pull 292’s up hills. I drove that truck Tucson to Milwaukee and Tucson to Seattle,Just loved that old truck (didn’t mean to hi-jack the thread)

Saw a nice one yesterday for sale $7000.00

My 1950 Chevrolet pick-up was the 3800 one ton. It was geared for pulling power, but not for speed. The speedometer never worked the whole time I had the truck, but I doubt that it would go over 55 mph. I don’t know if I had the 216 or the 235 cubic inch engine. I bought the truck in 1972 for $120 and sold it in 1975 for $110.

All cars should have a gear that is too tall. An 87 Plymouth Horizon could get 42 MPG on a long trip. Fifth was too tall and fourth was perfect for climbing hills.