Want to save precious fuel? Don't drive too fast or too slow
You have written before that any vehicle’s fuel consumption varies considerably from moment to moment during any trip. On what parts of the trip will it be highest, and on what parts lowest? Also, at what speeds is an engine most fuel-efficient?
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 speedometer. At under 1,000 revs the engine is just idling. 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 or to 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 won’t 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. Nett result: lower speed can use more fuel. 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. Nett result: more speed can use more fuel. Don’t 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 and 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.
In practice, a steady 60 is usually not possible, nor is it recommended except in a fuel-saving emergency. And there is another balance that must be struck. You should drive with the ambient flow of traffic – even if it is not the most economical. While accelerating gently uses less fuel than harsh acceleration, when overtaking you should change down and accelerate hard so those behind also have a chance to use the overtaking opportunity.
When is your engine oil level too low…or too high?
I checked my engine oil level recently, and it was well below the MAX mark so I topped up and checked again. It was still well below MAX so I added another half-litre and checked again. The level was way above MAX. Will this damage my engine? Should I have the extra drained off? How accurate are dipstick readings?
Dipsticks measure the depth of the pool of oil in the sump, physically and accurately. And that is where all the oil will be if the car has been standing still with the engine off for a minute or so. When the engine is running and the car is moving, some of that oil is being constantly pumped and splashed all over the inside of the engine block and the cylinder head, and the level of the reservoir the dipstick measures will be lower…until the engine and vehicle are static again and all the sprayed oil has drained back into the pool. That process is quick, but not instant.
An oil level anywhere between the MIN and the MAX is enough oil for the lubrication system to do its job properly. No immediate damage is likely just below MIN, but there will be less safety margin in the event of further oil loss, and less capacity to absorb and dissipate heat. A fill to slightly above MAX will also do no harm; indeed, the safety margin against loss will be increased. One downside is that the crankshaft webs incur more resistance as they splash more deeply through the pool of viscous liquid than they would if spinning in air and oil spray, fractionally reducing performance and needlessly thrashing the oil’s physical structure.
So, ensure your engine has been idle for a minute before checking the dipstick level. And, after topping up, also wait a minute for the new oil to drain down from the filler cap to the sump before you check again. An overfill of half-a-litre can be ignored until your next service.
The Bugatti Chiron might be the fastest street car ever made, but who’s driving at that speed?
I see that the fastest street-legal car is the Bugatti Chiron, which has recorded a top speed of 490.84kph. Whatever that is worth, surely somebody must be trying to achieve a landmark 500 kph. What is stopping them?
Short answer: The Traffic Act (vehicle design regulations) and fresh air (wind resistance). And also because 500 is unachievable on a “road”, and yet paltry in terms of land-speed records. The fastest “car” (on land, on wheels but not street legal) has more than doubled that landmark… and broken the sound barrier!
Longer answer: The key phrase here is “street-legal” – the Chiron test presumably complied with all the regulations for a car used on public roads. This imposes dozens of restrictions ranging from the fuel type to noise levels, dimensions, weight, control and safety requirements, and, and, and… Almost all of them militate against straight-line maximum speed. And none of them apply to land-speed record attempts.
For all that, the Chiron’s formally tested top speed is nothing more than a curiosity, even for Bugatti. The fastest street-car-in-the-world is a handy marketing gimmick (and kudos to the engineers) and to say it demands independent test proof. But no owner will ever drive one on a “road” at much more than half that speed. Nor, in all probability, was circa 500kph a design goal – numerous other performance characteristics in more useable circumstances will have taken precedence.
The 490 figure is meaningless as a specific, but it reflects a general impression of extraordinary power, performance, design quality, stability and so on. “Cars” that set land-speed records above 1,000kph have rocket engines. If used on roads, when they set off at the traffic lights they would either blow the car behind them onto the pavement or set fire to it.
Not all towing straps are worth your money…
There are many types of towing strap available on the market. Some are quite good, others are flashy but flimsy. Kinetic straps (per last week’s DN2 article) operate on a very different level of strength. Their webbing is at least twice as thick and probably 10 times as strong as conventional towing belts. They are also 10 times the price. Kinetics don’t usually have hooks (the joint between hook and web is a potential weak point), instead, they have loops at both ends, made of their own webbing material in a continuous band. Kinetics are also much longer to give the towing vehicle enough slack to build momentum before the strap jerks taut. The adjacent picture shows what a (much used) kinetic strap looks like. It is nine metres long.
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