Why Your 2018 KTM RC 390 Is Using More Fuel Than It Used To
Shop parts for a 2018 KTM RC 390 Motorcycle.The 2018 KTM RC 390 is a liquid-cooled, fuel-injected, four-stroke single with chain final drive. It is a small-displacement sportbike built to be ridden hard, and that matters when you start asking questions about fuel consumption. A single-cylinder engine working at high rpm, with a rider who enjoys the top half of the tachometer, will always use more fuel than the same bike cruising gently. So the real question is not whether the RC 390 is thirsty in absolute terms — it is whether your particular bike has started using noticeably more fuel than it used to. When that happens, the cause is usually something ordinary and findable rather than something catastrophic.
First, Make Sure the Change Is Genuine
Before chasing a mechanical fault, confirm that your fuel economy has actually changed. The reliable method is tank-to-tank measurement: fill the tank to a consistent level, reset your trip meter or write down the odometer reading, ride normally through the tank, then refill to the same level and divide the distance ridden by the amount of fuel you added. Do this over several tanks and average the results. A single tank tells you very little, because one long highway run or one tank of city commuting can swing the number considerably.
It is also worth checking whether your odometer is telling the truth. If you have changed tire sizes, or altered the sprockets on this chain-driven bike, the speedometer and odometer readings can drift away from reality. A shorter final drive ratio also makes the engine spin faster at any given road speed, which genuinely increases fuel consumption rather than merely appearing to. If you have made either change recently, factor that into your comparison before assuming something has failed.
Conditions and Riding Habits That Quietly Cost Fuel
Many riders notice reduced range before they ever calculate a number, and the explanation is often environmental rather than mechanical. Cold weather is a common culprit: the engine takes longer to reach operating temperature, cold air is denser, and fuel blends change seasonally in many regions. Ethanol-blended fuel also contains less energy per volume than straight gasoline, so the same riding produces slightly worse economy.
Beyond weather, consider how the bike is being used. Sustained highway speeds into a headwind, carrying a passenger or loaded luggage, and repeated hard acceleration all increase consumption. Short trips are especially costly, because the engine spends most of the ride warming up and running rich. Long idling warm-ups in the driveway burn fuel while covering zero distance, which drags your calculated average down without any fault existing at all.
Rolling Resistance, Brakes, and the Drive Chain
Underinflated tires are one of the most common and most easily corrected causes of poor fuel economy on any motorcycle. Low pressure increases the contact patch and the energy required to roll, and it also wears the tire faster. Check pressures cold, with a quality gauge, and check them regularly rather than once a season.
A brake caliper that is dragging will hold the bike back continuously and force you to use more throttle for the same speed. After a ride, carefully feel the temperature of the brake discs — an unusually hot disc on one side points toward a sticking caliper or a piston that is not retracting cleanly. Finally, because the RC 390 uses a chain, inspect it. A dry, rusty, or overtightened chain robs power and increases fuel use, and it is one of the cheapest problems to fix.
Air Intake and Ignition Quality
A clogged or heavily soiled air filter restricts airflow and disrupts the air-fuel mixture, which can push consumption upward. If you ride in dusty conditions or have not replaced the filter in a long time, this is a sensible early check. Spark plugs matter just as much: worn, fouled, or incorrectly gapped plugs produce incomplete combustion, wasting fuel that never contributes to forward motion. Pulling the plugs and reading their color tells you a great deal about how the engine is running.
This bike uses a fully electronic ignition, so there are no contact points to wear or timing to drift. That removes an entire category of causes that older machines deal with. If you suspect an ignition problem, the more likely candidates are a failing coil, a degraded plug cap, or a damaged lead rather than anything adjustable.
Fuel Injection: Sensors, Injectors, and Leaks
Because the RC 390 is fuel injected, the causes here look different from a carbureted bike. The oxygen sensor in the exhaust is central to how the system manages mixture; a lazy or contaminated sensor can leave the engine running richer than intended. A faulty engine or coolant temperature sensor can cause a similar problem by telling the ECU the engine is colder than it really is, prompting extra fuel delivery during warm-up.
A dirty or leaking injector is another possibility, since it distorts the amount of fuel actually delivered versus what the ECU commands. Stored fault codes are worth checking, because the system may already know something is wrong even if the bike still runs acceptably. With a single throttle body there is no synchronization to worry about, which simplifies diagnosis. Finally, look for fuel leaks — a weeping line, a loose fitting, or a seeping tank connection wastes fuel and is a genuine safety concern.
Engine Condition and Modifications
Low compression from general engine wear reduces efficiency, meaning more fuel is burned for the same output. On a high-revving single, tight valve clearances or worn rings can both contribute, and both typically come with other symptoms such as hard starting or a drop in performance. Because this engine is liquid cooled, a thermostat stuck open is also worth considering: it prevents the engine from reaching full operating temperature, keeping the fuel system in a richer warm-up mode longer than it should.
Modifications deserve a mention as well. Fitting an aftermarket exhaust or a high-flow air filter without matching fuel injection tuning can leave the engine running rich across the range. If the mileage dropped right after a performance part went on, that timing is your strongest clue.
When to Hand It to a Mechanic
Some symptoms point toward a deeper issue than a dirty filter or a soft tire. A strong, persistent fuel smell, black smoke from the exhaust, sooty spark plugs, rough running alongside the mileage drop, or a dashboard warning light if your bike is equipped with one all justify professional attention. The same is true if you have worked through the basic checks and the consumption stays high. Describing what you have measured, what you have already inspected, and when the change began gives a technician a strong starting point and usually shortens the diagnostic process considerably.
Related Shopping Categories
Shop Fuel System Parts for a 2018 KTM RC 390 Motorcycle.
Shop Air Filters for a 2018 KTM RC 390 Motorcycle.
Shop Tires for a 2018 KTM RC 390 Motorcycle.
Shop Brake Parts for a 2018 KTM RC 390 Motorcycle.
Shop Spark Plugs for a 2018 KTM RC 390 Motorcycle.
Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.
