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Why is your car or truck accelerating more slowly?

There are six main categories of problems that cause your vehicle to accelerate more slowly than you’re used to: mechanical, aerodynamic, load, road, weather and driver. Some of them mean you should get your vehicle checked by a mechanic, while others require a change in driving style or an assessment of what (and how) you are carrying items.

The effects are often noticed more in higher gears where the vehicle doesn’t have the torque to continue accelerating strongly if it has to fight any more than normal to propel the vehicle.

Below are 37 reasons your acceleration could be affected.

How do aerodynamics affect acceleration

Roof rack: Roof racks can cost you as much as 25% more fuel because they increase wind resistance. This extra wind resistance will cause slower acceleration that is more noticeable at higher speeds due to wind resistance increasing exponentially with speed.

Trailer: The airflow of your vehicle is designed to terminate nicely to give good aerodynamic performance, but adding a boxy trailer disrupts this. A cage trailer is more aerodynamic when empty, while a tall, flat-fronted trailer is the worst. Again, you’ll notice the effect more as your speed rises.

Windows: at higher speeds, having your windows down can increase drag by as much as 20 percent which will increase fuel consumption and affect acceleration.

How does the road affect acceleration?

Road surface: the road’s surface influences traction which means that the wheels slip (wheelspin) rather than gripping and providing forward movement. This is most noticeable when there’s ice on the road. Also see traction control under mechanical reasons.

Gradient: even subtle changes in gradient can affect how fast you’ll pick up speed. As you drive uphill, gravity is trying to pull you back down. The effect will be even worse if you’re carrying a heavy load or pulling a trailer

How does the atmosphere affect acceleration?

Temperature: Hot temperatures mean the air is less dense and this affects vehicles with internal combustion engines which need to compress the air to provide power in the piston.

Altitude: Air is also less dense at higher altitudes and this means a reduction in power in the engine. It can be as much as 3% every 300 metres in elevation.

Wind: headwinds can have a massive effect on acceleration, particularly at higher speeds.

How do mechanical factors reduce acceleration?

Mechanical factors tend to reduce power either through limited air or fuel, or some kind of faulty sensor.

Fuel quality: a bad batch of fuel, or adding fuel with the incorrect octane can cause a loss of engine power. You may hear ‘pinking’ under heavy acceleration.

Tyre pressure: if tyre pressures are low, it creates more rolling resistance, meaning the engine has to work harder to maintain the speed, let alone increase the speed

Fuel filter: if the fuel filter is clogged or dirty, fuel flow is affected and engine performance will be reduced. The filter is usually replaced as part of regular maintenance.

Mass air flow sensor: if this malfunctions or is clogged, it won’t accurately measure how much air is flowing into the air intake. The false reading will be sent to the engine’s ECU and it won’t calculate the correct air-to-fuel ratio.

Hoses: if hoses are leaking or blocked, all kinds of issues can arise. In a turbocharged car, air pressure could be leaked, meaning the turbo will under-boost, for example. There are many hoses in the engine bay, all of which will degrade over time and may need to be replaced during routine servicing.

Air filter: if the air filter is clogged, the engine can’t breathe propeerly and you’ll experience a reduction in power.

Throttle position: a sensor monitors the position of the accelerator pedal and that changes the angle of the throttle valve. This information is sent to the ECU to calculate the air-to-fuel ratio.

Spark plugs: the spark plug ignites the atomised fuel in the piston. If the spark is weak or not working, then you may get a misfire.

Timing belt: this keeps the intake and exhaust valves operating in-sync. If they open or close at the wrong time, pressure can’t build up in the cylinder, or it is released too early, meaning the combustion of fuel in the cylinder isn’t efficient.

Traction control sensors: these sensors determine whether a wheel is spinning. If they malfunction, incorrect information is sent to the ECU to reduce the power. Modern vehicles have traction control which will reduce the power to the driven wheels until traction is gained. If this is cutting in to reduce wheelspin, acceleration will be stymied.

Fuel injectors: fuel injectors atomise the fuel in the cylinder. If they don’t provide the fine mist required to achieve optimum ignition of the fuel because they are clogged, the fuel won’t be efficiently burned.

Fuel pump: if the fuel pump is blocked or partially blocked, it can’t get enough fuel to the engine.

Clutch: if the clutch is worn out, it will slip under load. You might hear the engine racing but without you picking up speed. This will be more noticeable under the road and load conditions explained in this article.

Catalytic converter: if the catalytic converter is clogged, air flow out of the engine is reduced, causing too much back pressure.

Limp mode: if the engine has something wrong with it, it might enter ‘limp mode’ which is a drastically reduced power setting that hopefully gives you enough distance to get to a mechanic. It will be accompanied by a ‘check engine’ light on the dashboard.

Engine block: carbon build-ups can cause issues with fuel pre-ignition.

Gaskets, rings and springs: if the head gasket leaks, you won’t build up pressure in the cylinders. Cylinder linings, piston rings, valve springs and worn valves also reduce the engine’s power.

Sensors: there are many other sensors in the engine that monitor temperatures, fluid levels and the operation of the engine. If any of these detect an issue, engine power could be reduced.

Wheels: if you put larger or heavier wheels on your vehicle, they have more rotational mass – this means the engine has to work harder to turn them.

Battery: in electric vehicles, as the battery is running out, it will reduce the amount of power available to the engine.

How does the load affect accceleration?

Weight: the heavier the load, the more mass the engine has to get moving. Don’t carry unnecessary items around in your vehicle as it affects fuel economy.

Weight distribution: if all the weight is on one side of the vehicle, traction is more easily lost on the lighter side. Uneven weight distribution is extremely dangerous. Check how to load a trailer and your vehicle.

Load shape: poor load arrangement will cause aerodynamic problems, as explained above. Cover loads to improve aerodynamic performance.

How does the driver affect acceleration?

Gear: if you’re in a gear that’s too high, you won’t get efficient acceleration. Try changing down a gear.

Engine mode: some vehicles have modes for ‘comfort’, ‘economy’ or ‘snow’, all of which reduce the performance of the vehicle

Speed limiter: if you have activated a speed limiter, you won’t be able to accelerate beyond a certain speed unless you turn it off (usually by pushing the accelerator pedal to the floor or deactivating it on the wand).

Seat position: if your seat is too far back, you might find that you’re not pushing the accelerator all the way to the floor.

Accelerator pedal: is something stuck under the pedal? Is the floor mat flat? This might be preventing the accelerator pedal from being pushed down all the way.

Handbrake: is the handbrake engaged? You’ll soon find out when the smell of burning brake pads fills the cabin.

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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

Posted in Advice
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