Does Leaving My Automatic in Overdrive in the City Help or Hurt Gas Mileage?

Your transmission knows what's best for it; just trust it to do its job.

It's long been something of a myth born of intuition that low engine rpm equates to better fuel economy. After all, diesels run at practically idle speed compared to fuel-hungry gas engines, and there's something inherently wasteful-sounding about revving an engine to redline every time you shift. And those things are true, but fuel economy has a lot less to do with rpm than it does keeping the engine in its efficiency sweet spot by always having the car in the best gear.

Overdrive Basics

The simplest way to think of an overdrive gear is as a reverse-sized gearset. In its lower gears, your transmission uses smaller drive gears -- those attached to the input shaft from the engine -- than it does driven gears, those attached to the transmission output. These gears determine the ratio of engine speed to driveshaft speed. For instance, if the drive gear has 10 teeth and the driven has 30, you get a 3-to-1 gear ratio; so, the engine's crankshaft will turn three times for every one turn of the driveshaft. The overdrive top gears are the other way around; the engine-driven gear is bigger than the driven gear, so the driveshaft turns slower than the engine.

Fuel Economy and Engine RPM

There's no direct correlation between engine rpm and fuel economy, since all engines are designed differently, and work differently when subjected to different loads. One thing that's almost universally true is that an engine will return the best fuel economy when run as close to its peak torque as possible. Peak torque rpm is where the engine is operating at its most efficient, when it's doing the best job of converting chemical energy to mechanical energy. Above and below that rpm, the engine is taking in more fuel than it's using to make power. So, keeping the engine at its lowest possible rpm won't necessarily return the best fuel economy if it's running inefficiently at that speed.

Manually Shifting

While all cars use different shift strategies, most employ the same sort of logic when you put your gear selector in first, second, third, drive or overdrive. The first three gears, the numbered gears, exist to assist in one of three situations: pulling a heavy load, going up a steep hill or accelerating with utmost urgency. The car's transmission- and engine-control computer know you're asking a lot from the engine under these situations, so they'll allow higher rpm and will switch to a program optimized for power output over fuel economy. The Drive setting -- or the second-to-highest gear in a manual -- is much the same, but it's high enough to allow you to cruise at highway speed without exploding your motor.

Driving in Overdrive

Assuming you have an automatic transmission and you aren't towing anything or going up a very steep hill, you can count on getting better gas mileage by leaving the transmission in overdrive. When you put your transmission in Drive instead of overdrive, the computer may recalibrate shift points to a higher rpm before engaging the next gear. The overdrive setting is your transmission's default, the one that the car was designed to use under normal conditions. Even on older, less sophisticated transmissions, overdrive won't engage until you get to around 50 mph, so putting the transmission in lower gears can only stand to move your engine further away from the torque peak.

References

• Hypermiling 101: Squeezing Every Penny Out of Every Drop; Kurt A. Clark
• Engine Airflow: A Practical Guide to Airflow Theory, Parts Testing, Flow Bench Testing and Analyzing Data; Harold Bettes