Are 10-Speed Automatic Transmissions Necessary?
When are too many gears too much for a transmission? Jason Fenske investigates.
In this recent episode of Engineering Explained, Jason Fenske asks a very interesting question. Having recently driven the Lexus LC 500, he questions if a 10-Speed automatic transmission is necessary or overkill. Doing what he does best, he goes straight to the whiteboard to carefully explain why a 10-speed transmission is perfect for the LC 500. Jason breaks this all down with 3 reasons why going with 10 gears makes the most sense.
1. Acceleration
In the first graph, Jason shows the how the LC 500’s 10-speed transmission performs in regards to its acceleration. The optimal curve can be determined by looking at the wheel torque on the left of the graph compared to the vehicle speed on the bottom. To remain on the ideal curve, the engine needs to be at peak horsepower at 7,100 RPM. Knowing this exact RPM goes to show how a CVT (Continuously Variable Transmission) would be perfect because it would allow the engine to stay at peak horsepower without having the change gears.
“There’s one problem. Nobody would by a $100,000 luxury sports coupe with a CVT. If you’re going to pay that much for a car, wouldn’t you want something exciting that gives you goosebumps each time you pull a paddle?”
Compared to a CVT, a conventional gearbox will always suffer from acceleration loss. To help reduce this loss of acceleration, more gears can be added. With minimal loss, acceleration performance increases yielding faster speeds and more fun. It’s a win-win.
2. Efficiency
The second graph to Jason shows us applies for BSFC (Brake Specific Fuel Consumption). BSFC is how much fuel is required to make a specific amount of power. For the greatest efficiency, you want your plot points to be in the area of high load and low RPM. The reason why CVTs are more common with cars now is because of how efficient they are. When it comes to automatic transmissions, the more gears you have will allow for maximum efficiency over ones with less gears.
3. Gear Range
Last but not least, Jason dives into the effects of gear range with the LC’s 10-speed gearbox. Using the RC F (with the same V8 engine) with an 8-speed transmission, you can clearly see that the LC 500’s gear range is wider just by adding an extra two gears alone. Not only that, but the LC’s first gear is shorter than the RC F and gears 9 & 10 are taller than the RC’s 8th as well. With so many gears, the spacing between the gear ratios are closer to provide better acceleration while maintaining maximum efficiency.
All in all, Jason believes that a 10-speed transmission suits the LC 500 quite well. With its big and thirsty naturally aspirated V8, he was able to achieve 29 MPG on the highway. That’s not too shabby at all considering how big the engine is along with the heft weight of the LC. Shifts are crisp and quick while driving hard, but feels some work could be done at lower speeds. Lexus have outdone themselves with this gearbox. It’s only a matter of time until other models get the bump to 10-speeds. Are 10 gears too much or is this the new standard for transmissions? Let us know on the forum!
