Ask the Mechanic: At what speed is the engine most fuel efficient?
Hello Paul, you have written before that a vehicle’s fuel consumption varies considerably from moment to moment during any trip. When during the trip will it be highest and when will it be lowest? Also, at what speeds is an engine most fuel-efficient?
Hello Bosco, most cars will travel furthest on a litre of fuel if they are driven at a steady speed of about 60kph in top gear. Those are the three fundamentals; steady, 60, top gear, distilled from a balance between the rate fuel flows from the tank (engine revs) and the distance you will travel in a given time at that flow rate (road speed).
Car engines work hardest (and use most fuel) when accelerating through the gears from a standstill to build cruising speed, or when climbing a hill in lower gears. They work least at “steady, 60, top gear”, or when coasting downhill. In judging how hard a car’s engine is working, the rev counter is more useful than the speedometre. At under 1,000 revs, the engine is just idling and under 2,000, it is cruising gently. At 3,000 it is starting to work moderately. At 4,000 and above it is working quite hard, either to accelerate rapidly, achieve or maintain a high speed or in lower gear to keep going up a very steep hill or through deep water or soft sand, and so on.
At 40 in top gear fuel flow will be less than at 60, but you will not go as far in a given time. And at 40, you will have less momentum to help maintain a steady speed in top gear; you might have to change down a gear, so engine revs will rise and fuel flow will increase. At more than 80, the engine revs in top will be higher than at 60, so fuel flow will be higher, but you will go further in a given time and maintaining speed is less likely to require a change-down.
So, why isn’t that the best balance? Because of wind resistance, which is negligible at 60 but geometrically more severe from about 80 upwards. High speeds work the engine hard, even on a flat road; more speed can use more fuel. Do not under-estimate the force of the wind. Storm winds of 150kph can knock people over, flip parked cars onto their roofs, snap tree trunks and demolish buildings. And that force increases geometrically with speed. The drag at 120 is four times higher than at 60.
So, the broad real-life answer is not less than 40 and not more than 80. The principle here is the distinction between how much fuel you consume per minute, and how much fuel you consume per kilometre. For most cars, about 60kph gives you the most economical balance of both.
That equation has a high degree of certainty if driving along a road that is flat and generally straight with a surface that is firm and smooth. But driving conditions are often a lot more variable and complicated than that, with ups and downs, corners and different surfaces and other “traffic”. So some more basic principles also need to be your guide:
High revs mean hard work
Fuel provides the energy that enables your engine to work. The more work it does, the more energy it needs so the more fuel it consumes. If tech-talk gives you brain-ache, look at it this way. Jogging gently for a kilometre is less tiring than sprinting flat out for 100 metres, or walking slowly for 10 metres while carrying two suitcases up a flight of stairs. Your lungs and the lactic acid in your muscles will know the difference. How hard you are working (heart rate equates to revs) matters more than how fast you are going.
At any rev level, the road speed will depend on what gear you are in. At only 20kph, the rev level can be 4,000 if you are in first gear. The speed is low; the work load is high. Conversely, at 100kph the rev level could be as low as 2,000 in top gear. The speed is high but the work load is moderate.
In our current traffic conditions or in mountainous terrain, or if the road has been mutilated by speed bumps, there is little opportunity for maintaining a steady speed. We use extra energy to accelerate to a steady cruising speed, but before we enjoy the fuel-saving benefit of that steady momentum, we have to apply brakes for a bump then go through all the gears as we accelerate again before another bump and/or slow queue.
That pattern is a colossal waste of fuel (and brake linings, and time). It also sabotages another “economy run” principle; that gaining speed is cheapest when going downhill, fast enough that you have enough momentum to coast up the next hill, shedding speed but without having to change down. If bumps or traffic prevent that rhythm, your fuel consumption is significantly increased.
WHY ARE MY BRAKES SOFT?
Hello Paul, my 1992 Toyota Camry blew a leak on the left front brake hose, passed the midway clamp. The pads had been replaced 10 days before. Other than the visual brake fluid near the wheel there was no noticeable brake pressure loss while driving. Challenge is there is a lot of brake pressure while driving but as soon as you start it and depress the brake pedal it drops to the floor with little or no pressure. What can we do?
Hello Moses, your Toyota seems to have brake fluid pressure loss caused by the leaking front brake hose you have mentioned. That is why your brake pedal sinks low when you step on it before you start and run the engine. A car brake system relies on the brake master cylinder and brake fluid to pressurise brake lines.
This delivers brake fluid, which in turn provides hydraulic pressure to stop the car by engaging the brake pads to the discs.
A leak of brake fluid will reduce the hydraulic pressure, make the brake pedal spongy and increase braking distance or delay stopping the car when you need to. It is easy to misdiagnose this symptom as having a bad brake master cylinder.
What you must do is urgently replace the leaking brake fluid pipe with a new one. Avoid the temptation to weld or repair the leaking point if there is the option to replace the leaking pipe with a good one (used or new).
Welding brake pipes should be last resort because it has some failure risk. Renew all the brake fluid as aged brake fluid causes overheating and corrosion damage which damage brake hoses and cylinder seals. Aged brake fluid tends to have moisture which reduces its hydraulic effect and causes corrosion damage.
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