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Fuel Saving Tips

Fighting R10 fuel – Popular MechanicsFuel is expensive. In fact, it’s close to historic highs, and drivers are trying to find ways to conserve fuel – and protect their wallets. Fortunately, there are ways to save that don’t involve buying a hybrid car, or sitting at home.

Fuel is expensive. In fact, it’s close to historic highs, and drivers are trying to find ways to conserve fuel – and protect their wallets. Fortunately, there are ways to save that don’t involve buying a hybrid car, or sitting at home. Recent POPULAR MECHANICS testing proves what many have argued: that modifying how you drive can boost fuel economy. But not all of the conventional advice is right. To separate efficiency truths from myths, we outfitted an ordinary set of wheels (a 2001 Suzuki Grand Vitara; odometer reading 177 000 kilometres) with a precise fuel-economy sensor, and then started driving. The conclusion: if a formerly lead-footed driver employs the tactics presented here, he could save up to 10 per cent – that’s hundreds of bucks a year. So ease off the pedal, and let’s dive into the data.

Tactic no 1

Coast to a stop

Brakes are necessary (duh!), but they’re inherently wasteful: They take the kinetic energy of a moving car – energy it took pricey petrol or diesel to generate – and turn it into heat that’s lost to the air. Everyone knows that accelerating until the last moment then braking hard to stop is less efficient than slowly coasting to a red light. But PM’s test data (illustrated below) prove what a huge difference coasting makes. The lesson: whenever possible, anticipate that a light will turn red and ease off. Generally, the less you have to brake, the better your fuel economy.

Tactic no 2

Close windows and use a/c at high speeds

It’s a fierce efficiency debate: open the windows in summer to avoid running your energy-intensive air conditioner, or keep the windows closed and the a/c on to preserve your car’s aerodynamic profile. (We’ll leave aside the option of sweating it out.) PM’s testing settled the issue. Driving at 90 km/h with the a/c running, we got 9,84 litres/100 km; turning it off dropped us down to 8,43 litres/100 km. Then we opened all four windows, one at a time, and increased by 0,35 litres/100 km per window until we were back at 9,84 litres/100 km. So at that speed, it’s a wash. But aerodynamic drag rises exponentially with speed – the faster you go, the more the open windows hurt efficiency. The answer? Below 90 km/h, open the windows and leave the a/c off. But at 100 km/h or higher, keeping them closed and the air conditioning running will burn less fuel.


Tactic no 3

Avoid slowly crawling up to speed

Conventional wisdom says that jackrabbit starts consume more fuel. But it turns out that nursing your speed up to the limit too slowly also boosts consumption. How can that be? Cars get poorer fuel economy in lower gears, and accelerating too slowly prevents upshifting at an efficient rate. The best acceleration rate varies with the vehicle, gear ratios and weight. But in our testing we found that taking 15 seconds to accelerate to 80 km/h used less fuel than taking 30 seconds to reach the same speed, because the car entered its top, fuel-saving gear sooner.

Tactic no 4

Cruise at a slower speed

Since the power required to overcome aerodynamic drag is a function of the velocity cubed (in other words, it shoots up quickly), a car’s jump from 65 to 100 km/h requires less fuel than the increase from 100 to 130 km/h. (As the graph above shows, the hit to fuel efficiency is roughly twice as severe in the higher range.) So go slower, right? Well, yeah, but fuel efficiency isn’t the only thing that matters. Some studies suggest that, in the USA, the old 55-mph (88 km/h) limit saved fuel but cost more in terms of lost work hours. Then there’s safety: going 90 km/h when traffic is cruising at 120 can be dangerous to everyone. Just don’t go 130. That will drain your tank quickly – and the costs add up if you also have to pay for a speeding ticket.

Tactic no 5

Climb slowly (when it’s safe)

Imagine driving on a flat highway and approaching an overpass. From a fuel-efficiency standpoint, the best strategy is to turn off cruise control and forget about maintaining a constant speed up and down both sides of the grade. The theory predicts that, and our data prove it. The physics work like this: lifting off the accelerator while travelling up the hill and allowing your speed to decay trades some kinetic energy (related to speed) for potential energy (related to the car’s tendency to roll downhill). You regain the kinetic energy – and get better economy – on the backside. Whereas hypermilers – who are obsessed with getting the best possible economy – claim significant benefits from this technique, our results showed only modest gains. Two things did happen, though: (1) We drew the wrath of a lot of drivers following us, as evidenced by their single-finger salutes; (2) We were nearly sideswiped by an impatient 18-wheeler. Yes, the method does work. But we’ll save it for lightly travelled roads.

Tactic no 6

When coasting downhill, leave the car in gear

There are those who refuse to be shaken from the practice of coasting downhill in neutral to save fuel. This is a bad idea no matter how you look at it. Let’s set aside fuel economy for a moment. Coasting downhill in neutral is dangerous. In neutral, you have no way to accelerate to avoid a hazard, and if the engine stalls, you have no power steering or vacuum boost for the brakes. If the hill is steep enough to call for hitting the brakes to keep you from gaining speed, they’re more likely to overheat – and overheated brakes lose effectiveness until they cool off. They’ll probably do that right around the time the police show up to take the accident report.

Here’s the surprise: there’s no trade-off between safety and fuel economy in this case. Leaving the car in gear while coasting downhill actually is more efficient. Why?

Most fuel-injected engines today use computer-controlled deceleration fuel cut-off: when you lift your foot from the accelerator pedal while leaving the car in gear, injectors shut off automatically, and the car’s rotating tyres – which are connected to the engine via the transmission – keep the engine turning and the accessories running. So, the engine consumes no fuel at all while the vehicle is coasting downhill.

In contrast, the fuel-consumption rate for a petrol engine idling in neutral falls between 0,8 and 1,6 litres per hour (l/h). Splitting the difference and using 1,2 l/h for our example, idling in neutral down a 1-kilometre hill consumes fuel for 30 seconds, for a total of about 10 ml of fuel. Popping the car into neutral actually wastes fuel.

This may seem counter-intuitive, but that’s what data are for – replacing good guesses with solid facts. Watch the data, and over time the savings will take care of itself.

Fuel-sipping basics

Monitor tyre pressure
Keep your tyres properly inflated, because low pressure increases rolling resistance. Few drivers check and adjust their tyre pressure often, but it’s a good idea to do it once a week.

Plan errands carefully
Reduce the distance you drive by running all your errands in one trip. Making a run to the dry cleaners and then picking up the kids after soccer practice? Don’t make separate outings. A little bit of foresight will stretch your fuel economy.

Warm up the engine
Cars get better fuel economy when the engine is warm. So if you have a three-stop run, hit the farthest destination first, then work your way back home. A fully warmedup engine will remain at an efficient temperature even if it’s parked for half an hour.

Make left turns only
The MythBusters proved the principle works: when city driving, make as many left turns as possible, even if it means going a few hundred metres out of the way. Reducing loiter time – or idling while waiting for traffic to clear – saves fuel.

Pop Mech test tools
To develop the best tactics for fuel-efficient driving, we instrumented our car, then hit the road. To settle the long-standing question of whether it’s better to coast along in neutral or in gear, we tapped into the fuel injection harness on the engine, hooking up to an oscilloscope to capture the opening and closing of the injector. For most of the other tests, we used Palmer Performance Engineering’s Dash- Command and ScanXL Pro software running on a generic Windows netbook computer, with Palmer’s cable plugged into the OBD-II port under the dash.

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