Slow and steady boosts your car's fuel efficiency

BY Jim KerrBackyard Mechanic . Mar 31 04:00 am

Question: I’ve had the theory to do the following to try to improve mileage and could use input from you on the matter. When starting out in my car, I speed up for a couple seconds to put the transmission into the highest/higher gear so when I am cruising down the highway the car will be in the highest gear that is comfortable before it downshifts. This should give me a bit better mileage. Am I right? Thanks.

— Stan

Answer: Yes and possibly no! Generally shifting the transmission up to a higher gear as soon as possible without lugging the engine (the point where it starts to hesitate badly on acceleration or surge when you try to maintain speed) will give you better fuel economy. Many of the manual transmission equipped vehicles in past years have had an “Upshift” indicator light on the dash telling the driver it is possible to upshift. The purpose of this light was to maximize fuel economy by upshifting as soon as possible. Some higher performance cars with manual transmissions even had computer controlled solenoids that would “skip shift” gears and only let you shift from first gear directly to fourth gear if you were driving with a light throttle. Again this was to maximize fuel economy.

Generally I would say upshifting to a higher gear as soon as possible will give you better fuel economy, but do avoid quick acceleration from a stop. You are better to accelerate slowly until you get to your cruise speed and then back off the throttle slightly to let an automatic transmission upshift. Having driven many cars with fuel economy instantaneous readout displays, I can confirm that this will give you the best fuel economy.

As for the no part of my answer, at highway speeds there is often a “sweet spot” where the engine is operating at a certain r.p.m. to provide enough torque to pull the vehicle. This sweet spot will keep the intake manifold vacuum higher because the throttle is not pressed most of the way to the floor, yet the engine still produces enough power to keep the vehicle moving at the desired speed. There are many factors such as transmission ratios, final drive gear ratios, engine camshaft design and vehicle aerodynamics that will determine the “sweet spot” for each vehicle. Even wind direction and force and road texture will affect this. Most vehicles come from the factory optimized to provide fuel economy operation when the vehicle transmission is in high gear at normal highway speeds, but if you load the vehicle, it may actually get better fuel economy in a lower gear rather than high gear. There are many factors to consider when trying to maximize fuel economy, but the best ways are to accelerate slowly, plan your driving so you don’t have to accelerate, slow or stop more than absolutely necessary and combine short trips in to one longer one. With these simple driving techniques, it is possible for many drivers to cut your fuel use by 15 to 50 per cent.


Question:  I just installed a big 1,000-watt stereo and subwoofers in my car and it sounds great but I have a question about power. When I turn up the music, my headlights flicker bright and dim in time with the music. What can I do to fix this?

— John

Answer: The lights are flickering because producing those powerful bass notes take more power than the alternator on the car can put out. The audio system is using all the alternator power and robbing some from the battery, too. Because the voltage is dropping, the lights dim every time a big bass sound is produced.

There are possible solutions. One is to install an alternator with more output capacity. This would provide the power for both the audio and other vehicle components. Others have installed large capacitors (available from your audio store). The capacitor is a storage device that can release power quickly when needed and will smooth out the surges in the electrical system caused by the bass power requirements. Instead of a capacitor, some will install a second battery, which is also a storage device and provides the same benefits as a capacitor.

Before you make any changes however, make sure all electrical connections are clean and tight and that the power cables to the audio system run to the battery and not into a connector box. This will help maintain steady power for the audio system.