Engineering Explained answers the most frequently asked questions.
Since cars have been equipped with sensors to measure instantaneous fuel economy it has been a common pastime for passengers and drivers to try to get that number as high as they can. There are some who delight in the fact that that number can remain low for as long as it takes, but even die-hard horsepower enthusiasts love to see that figure rise, especially when they’re rolling down big hills.
You can do this by simply putting the car in neutral and letting it cruise down the hill while the engine is at idle. This is not best way to get maximum return on your investment, regardless of whether you have an manual or automatic transmission. This may seem obvious to many, but Engineered took a Subaru Crosstrek up some hills and, with our familiar narrator, discussed why it is better to leave the car in gear in the long-term.
This question can be answered in one simple way: it is better to keep your car in gear. The computer will sense acceleration if the transmission is still in gear, whether it’s an automatic or manual. It will then cut fuel to the engine. Yes, the engine is still being run but the drive line is turning the engine over instead of incendiating fuel/air mixtures in the pistons. The computer will recognize that the car has been taken out of gear and will provide enough fuel for the engine to idle if it is.
The video shows this clearly, with Subaru achieving a maximum instant mpg of 99.9 while cruising in first gear. The figure remains high even when the car is out of gear. However, it fluctuates between 60 and 80 mpg.
According to the video, the best way to coast over multiple hills is to shift the car out of gear. This allows for enough speed to climb the hills. Although technically correct, it is not practical nor legal due to the speed required to coast uphill. For all practical purposes, make sure the car is in gear all the time.
This topic has caused more than a few disputes among car enthusiasts. Engineering Explained thanks for clearing this up.