The more I learn about cars, the more I don’t know about cars. These engineering stories often start with a simple question. Rather than producing a straightforward answer, I realize the question wasn’t the right one to ask in the first place. This week, a demonstration of the principle: Do today’s cars have too many gears?
We live in a world of eight-, nine-, and 10-speed automatic transmissions. Sure, EVs have simple single-speed gear reductions, and manuals have remained steadfastly at six speeds with a handful of exceptions. But in the world of automatics, you’ll have a hard time finding less than eight gears. How did we get here?
To answer, we need to first understand a bit more about the fundamentals of automatic transmission design. An automatic uses a series of planetary gearsets that are connected in various arrangements by clutches to provide certain forward ratios. A single planetary gearset has a number of possible forward ratios, but not all are useful.
“For a single planetary gearset, there's actually seven possible gear ratios available,” explains Weber University automotive professor John Kelly. “Some of them are ridiculously high or low gear ratios that would be unusable, like an overdrive gear ratio of 0.2:1, five-time overdrive. But when you combine these planetary gearsets in different combinations in a row, then you end up with dozens of possible gear ratios, so you can just pick and choose which ones you want.”
To create a transmission with more usable forward ratios, you don’t necessarily need more planetary gearsets. If you look throughout automotive history, there are examples of four-speed automatics with two, three, or even four gearsets. ZF’s popular eight-speed has four planetary gearsets, and so do GM and Ford’s 10-speed. The former has five clutches, while the latter has six, but the point stands; Having two more usable forward gear ratios doesn’t increase component count by all that much.
“If I was to look at an eight-speed transmission versus a 10, you’d have a hard time telling the difference between them,” Kelly says.
ZF was the first to market with a six-speed automatic. Ford, Aisin, and others, licensed the design, which used two planetary gearsets combined in a single unit called a Ravigneaux, and one traditional gearset. Despite going to four individual gearsets for its eight-speed, the company managed to fit everything into a package roughly the same size and slightly lighter overall.
“We found from our point of view a very smart solution for being able to go for eight speeds and initially everybody said, ‘Well, we don't need eight speeds,’” recalls Dr. Albert Dick, a project manager at ZF who led the development of the eight-speed, with a laugh. “That was very early in the development phase, so we did some demonstration and we found out that it’s just great to have eight speeds.”
ZF debuted its eight-speed automatic, the 8HP, in 2009, as automakers began a trend towards downsizing. With a smaller turbocharged engine in place of a larger naturally aspirated unit, the useful operating window of the engine shrinks. To get the right balance of performance, response, and fuel economy, you basically need more gears.
“The only way you can keep the engine in its power band is to have transmission gears that, when [the transmission] shifts, it doesn't take [the engine] out of its power band,” Kelly says. “So it allows the illusion of a powerful engine to be maintained. When, if you took this same engine and put it with a four-speed transmission, you would hate it.”
More gears allow for a wider ratio spread—calculated by dividing the first gear ratio by the top gear ratio—and as Dr. Dick simply puts it, “The bigger your ratio coverage, the more different running conditions you can achieve.”
In a 2009 SAE paper on the first generation ZF 8HP, engineers from the company noted that the eight-speed contributed to a 6-percent fuel-economy boost over the existing six speed. That was thanks in large part to the increase in forward ratio count (other efficiencies found elsewhere helped, too). The first versions of 8HP had a ratio spread of 7.05 while the latest iterations push up to 8.6 depending on spec.
A wider ratio spread isn’t just for fuel economy either.
“Vehicles with 8-, 9-, and 10-speed transmissions have some really low first, second, third gear ratios….that really give you some hard acceleration. They feel amazing,” Kelly says. This tight gearing equates to high torque at the wheels, which also helps with towing capacity, a big part of why today’s pickup trucks sport ever-increasing tow ratings.
Kelly sees it as a best-of-all-worlds type of deal. In reality, a continuously variable transmission (CVT) is the most efficient of all transmission types because the gear ratio is always the ideal for the current running condition, whatever that may be. But CVTs aren’t the most reliable transmissions, and customers don’t like their droning sensation, so a traditional automatic works better. With more speeds, you can get closer to CVT efficiency without the drawbacks.
Yet, Dr. Dick doesn’t see a need for more than eight speeds now. (ZF does make a nine-speed transaxle, but this is for off-road-type vehicles that don’t have a dedicated low-range transfer case. A super-low first gear simulates low range for off-roading, but in most running conditions, this ‘box behaves like an eight-speed.)
“I think it was 2018 [we had a discussion] with some customers on ‘Does it make sense to add more gears or do we cut off now?” Dick says. “We clearly came to the common conclusion that it doesn't make sense to go to nine or ten speeds, we need to go into hybrids, we need to go into electrification.”
The 8HP was always designed with hybrids in mind, able to support a 48-volt crank-mounted starter-generator, or a larger 400-volt motor either in tandem or in place of a torque converter. ZF helps develop these hybrid technologies as well, so it made sense to focus corporate investment there.
A hybrid system does help flatten an engine’s torque curve, increasing its useful operating range significantly. Doesn’t this mean you could get away with fewer speeds?
“Our customers know that sometimes the batteries are empty and then you are running a combustion engine [alone] and then you are happy you have your eight-speed transmission,” Dick says. “And of course, when you are driving in hybrid mode, you can even be much better in the efficiency using maybe excessive torque for charging the battery.”
The electric motor also lets you run an even smaller motor, further helping fuel efficiency
Plus, automakers love and understand the 8HP, so they don’t want to switch to something new. Transmissions are hugely expensive to develop—hence why Ford and GM teamed up on one—and so new ones only come around when needs must. The eight-speed works, so why replace it?
Reporting this out made me realize something. It’s not really a question of do cars have too many gears, but do cars do a good job managing the number of gears they have?
There’s a ton of good reasons to go with 8-, 9-, and 10-speed gearboxes, but does the car hunt for gears, or does it shift to the right gear at the right time? A huge part of the ubiquitousness of the ZF 8HP is not just the number of forward ratios or the ratio spread, but the fact that it shifts so well. Eight’s not too many when you can downshift from 8 to 2 in an instant and with total smoothness.
Put another way, you don’t bemoan the complexity of modern cars if the complexity is well managed. Do a good job calibrating a gearbox, and there’s every reason to go with eight or more speeds.
