2008 Ford Focus coupe manual shifter. Click image to enlarge
By Jim Kerr
Cars with manual transmissions get better fuel economy – or do they?
It is often thought that manual transmissions get better fuel economy than the same vehicle with an automatic transmission but there is much to look at when choosing a transmission for the optimum fuel economy.
Manual transmissions are more efficient than automatic transmissions. Automatics use some of the engine power to turn a hydraulic pump that provides the oil pressure to apply internal clutches. Manual transmissions don’t need a pump. The only clutch for a manual transmission is between the engine and the transmission and is operated by the driver’s foot.
Automatic transmissions also have a torque converter, which allows the vehicle to be stopped with the transmission in gear without the engine stalling. This torque converter couples the engine to the transmission by forcing fluid from one side to the other in the converter housing. Some slip occurs during this operation so transmission efficiency is reduced, although almost all automatic transmission torque converters now contain a lockup clutch that prevents any slip in the converter once the vehicle is moving and the transmission has shifted to a higher gear. With the converter locked up, the torque converter is as efficient as a manual transmission clutch.
It would appear that the more efficient manual transmission would provide the best fuel economy, but it depends on the driving conditions. Let’s look at what happens when driving around town. Starting out from a stop sign, the driver with a manual transmission has to engage the clutch smoothly (slip the clutch) to get the vehicle moving. To assist the driver with smooth starts, the auto manufacturers often install a lower numerically gear ratio for the final drive so that the engine torque is multiplied more and the clutch needs to be slipped less. The lower gear ratio aids in accelerating the vehicle but the engine rpm is higher so more fuel is used.
In an automatic transmission, the torque converter assists with getting the vehicle moving. By carefully designing the angle of the fins inside the torque converter, the manufacturer can get about a two times torque multiplication effect when the vehicle pulls away from a stop. With this torque multiplication, the vehicle doesn’t need the same low final drive gear ratio, so the engine is turning at slower speeds. Automatic transmissions also shift to the next higher gear automatically at the correct vehicle speed. Drivers with manual transmissions often shift at higher rpms than they need to, so they use extra fuel.
If you look at the fuel economy guide from Natural Resources Canada, you will find that City fuel economy ratings for vehicles with automatic transmissions are often as good as those for manual transmissions and many times are better – and this is done with testing that shifts the manual transmission always at optimum vehicle speeds.
On the highway, the manual transmission has traditionally achieved the best fuel economy, but that is changing as automatic transmissions are built with more gears. When both transmission types have similar numbers of gears, the automatic transmission will often get better fuel economy, because of the lower final drive ratio with the manual gearbox. A higher revving engine will use more fuel. If final drive ratios are the same however, you can expect to get better fuel economy with a manual gearbox.
As you can see, it is not all about the type of transmission. Final drive gear ratios, the number of gears in the transmission, driver shift patterns, where you do most of your driving and even vehicle weight (automatics are heavier) all play an important part in fuel economy. In most cases an automatic transmission will achieve very close or even better fuel economy than manual transmissions.
There are two other factors to consider. One I call the fun factor. I like the fun of shifting manually, but for others it can be a nuisance when driving around town. The other important consideration is repair cost. Shifting a manual gearbox does create wear, and manual transmissions are much more expensive to overhaul than automatic transmissions because the parts cost more. Sometimes economy is looking at the total operating cost rather than just fuel cost.