Why You Should Only Use Premium Gas In A Harley-Davidson

Harley-Davidson motorcycles are an iconic American staple, but buying one of them is a big investment. There are a few slightly more affordable Harleys out there for new riders and those who are looking to save a buck, but the vast majority of the manufacturer's bikes generally aren't cheap. So it makes sense that anyone who buys one is going to want to make sure that they're following all the bike's recommended care and maintenance instructions.

One of the most basic maintenance tasks that all riders should be mindful of is making sure that their bike is getting the correct kind of fuel. It might come as a surprise to some that just about every motorcycle Harley-Davidson makes requires premium gasoline. This is typically the most expensive variety of standard (non-diesel) petroleum gas that you find at the pump, which may have some riders wondering why Harleys need it, especially since they've never had to put it in their cars. I've been riding and maintaining motorcycles for over 15 years, and I've found that premium gas is generally best for most bikes, but it's an absolute necessity when it's required by the manufacturer. So, to address the question of why Harley riders should only use premium, we should first look at the difference between standard and premium gas, see which Harley models recommend it in their owner's manual, and then examine why premium is a benefit to performance motorcycles in general.

One of the big myths about gasoline is that premium gas offers higher performance. This is kind of true, but not in the way most people think. There are a few differences between regular and premium gas other than the cost, but none of them affect horsepower or torque. They actually have slightly different chemical compositions, which ultimately boils down to their octane level. The U.S. Energy Information Administration states, "Octane ratings are measures of fuel stability. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. [...] The higher an octane number, the more stable the fuel." See, there are two kinds of combustion: Controlled combustion and spontaneous combustion.

Controlled combustion is what you get when an engine is performing like it's supposed to — with ignition regulated by the spark plug. Spontaneous combustion is when the fuel ignites without a source that is controlled by the engine, which is how you get engine knock (AKA detonation.) If left unchecked, this can cause significant damage to the engine. There is a significant increase in combustion stability when using a higher-octane fuel.

Regular gas has an octane rating of 87, while mid-grade gas is usually 89-90, and premium gas is typically in the 91-94 range. This means that vehicles running on premium gas are much less likely to experience spontaneous combustion, or will at least experience it at a lower rate.

One of the best pieces of advice that I can give to new riders is to always follow the guidelines in their owner's manual to learn the proper way to maintain and care for their motorcycle. There aren't many people in the world who can claim to know more about your bike than the engineers who designed it, and the fluids and materials that they recommend are usually chosen for a reason. This is just as true for fuel as it is for other fluids such as oil.

You can view the owner's manual for just about any modern Harley on the company's Service Information Portal. Once you've found the correct manual for your specific model, click on the tab marked Specifications and then scroll until you reach the section marked Engine and Transmission. This is where you will find the information on recommended fuel.

The manual for the 2024 FLHX and FLTRX models states that they require premium unleaded fuel, as does the manual for the company's Pan America models, Softail models, Sportster models, and Touring models. In fact, I've yet to see a Harley-Davidson manual that doesn't recommend it. Premium unleaded is even listed as the fuel requirement in the manual for Harley's Trike models.

The fact that Harley-Davidson recommends the owners of its motorcycles only use premium gas is hardly surprising. As I mentioned before, it's generally considered good practice to use premium gas in most performance motorcycles unless the owner's manual dictates otherwise. This is because the smaller engines found in performance motorcycles tend to have a much higher compression ratio than those that power many larger cars, trucks, and SUVs.

"Detonation is more likely to occur when the fuel/air mixture is squeezed harder, which is exactly what performance engines, with their high compression ratios, advanced ignition timing, and sky-high rev limits, do," says Ari Henning of Revzilla. "If you're thinking of going cheap and putting regular in a bike designed to burn premium, you're at risk of engine damage. Some modern, high-performance bikes are equipped with knock sensors, but most aren't."

So while there are some bikes out there with lower compression ratios that can get away with regular fuel, it's not a great idea to go filling your tank with low-octane gas unless you've verified that it's okay by checking the owner's manual — and as we've verified — that doesn't seem to be the case for the vast majority of Harley-Davidson motorcycles.