Tuesday, August 12, 2008


I have found a new interest, "Hypermiling".

In a nutshell, "Hypermiling" is the art of driving with as little fuel as possible - or attaining the highest possible mpg while driving. You can hypermile with just about any car - you don't need a Prius however it does help if you have a manual transmission aka. stick.

My 08 Subaru Impreza is rated at 20/27mpg, however using simple hypermiling techniques I have been able to get anywhere from 27.5 to 31.5mpg so far on short 10 mile runs in a very hilly area - and the car hasn't even been broken in yet. This car is not an optimal choice for hypermiling. I chose it primarily for its all wheel drive feature. The question remains however, how much mileage can it get given optimal driving techniques ?

Some common techniques which work with this vehicle are:

1. Gentle Acceleration

Accelerating rapidly forces more gas into the engine and as a result it gets burned less efficiently. By accelerating at a slower rate you get better mpg.

2. Driving at low RPM

For each (engine) rotation, the engine sucks in gas (in one cylinder), compresses it(in another cylinder), ignites it(in another cylinder), and dumps the exhaust out ( from another cylinder). So the more rotations per minute, the more gas you are igniting. Lower rpm means less gas burnt. There is of course a limitation. if the rpm gets too low the car will lose power and then stall. The trick here is to find a comfortable place where the engine runs optimally. In short this means driving at the "bottom" of each gear. In my case, doing 40-55mph in 5th, 30 in 4th, and staying in 1 and 2 for as short a duration as feasibly possible.

** Correction - there are cases where the ECU stops injecting fuel into the engine. In cases like this the fuel efficiency would be far better than idling. As I understand this will happen when you coast down hill and engine-brake. not sure what triggers the ECU to do this **

3. Short Shifting

This hinges off driving at low rpm. If the car has enough torque, you can shift up to the next gear early - say at 2,250 to 3000 rpm instead of 4,000 to 5500 rpm. The car has less power in the lower rpm range, and more power in the higher range. Each car has its own unique "curve".

4. Coasting in neutral
Again, this is about reducing rpm. The idea here is to switch to neutral when you see an opportunity to coast down hill. I've read articles that discuss turning off the car while going down hill - while this is more optimal its not safe. If you value your safety and the safety of others over a few mpg then turning the car off while moving is not an option. Coasting lowers the rpm to idle ( 750-1000 rpm in my case ) which is a significant savings in fuel efficiency.

5. Pulse and Glide
As explained in other articles this involves accelerating past the speed you want to travel at, and then cutting off the engine while you drift down slightly below your target speed. The result is roughly double your mpg however this cause wear and tear on your starter and means having to turn off your car. I have applied the same principal to my driving but without having to turn off the car. Here is what I do.

A. When a hill is approaching, gently/gradually accelerate to about 10mph more than the desired speed.
B. If the decline after the crest is going to be large enough to accelerate the car, then lift off the pedal and shift to neutral before hitting the crest allowing the car to slow slightly as it climbs using its existing momentum. Once past the crest gravity will take over and pull it down - ride it out from there.
C. If the decline after the hill is shallow to flat, then continue to gently accelerate until the crest is reached, then switch to neutral and ride it out as far as possible.

The results of doing this will vary given the road's inclines and declines so its hard to estimate how much this actually saves.

Below is a small listing of the results I achieved so far:

Commute to train via Highway = 11.4 miles
Commute to train via back-roads = 10.4 miles

Day 1

To Train: 29.7mpg = 0.3838 gal
Round trip: 27.6mpg = 0.8260 gal

To Train 28.2 mpg. = 0.3638g
Round trip: 27.9mpg = 0.7526g Traffic stalled at one point

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