Two ranges, and only one of them is the plan
A tank has a main portion and a reserve portion, and they answer different questions. The main portion is what you ride on, and the distance it covers is the number that decides where you stop. The reserve is the margin, and the distance it covers is the number that decides what happens when the plan was wrong — the station was shut, the detour was longer than the map, the wind turned.
Both are the same arithmetic:
Range = gallons × miles per gallon
A 4.5 gallon tank with 0.9 gallons of reserve, at 42 mpg adjusted down 10 percent to 37.8, gives 3.6 × 37.8 = 136 miles to reserve and another 0.9 × 37.8 = 34 miles after it, for 170 miles total. The useful thing about splitting it is that the second number is small. Thirty-four miles of reserve is a comfortable margin on a highway and no margin at all somewhere the next fuel is eighty miles away.
Where the mpg figure should come from
Fill the tank until the pump cuts off, zero the trip meter, ride, and fill it again at the same pump on the same setting. Miles divided by gallons is your machine, your riding, your roads. Published figures are measured on a test cycle that does not resemble anything you do, and they are usually optimistic about steady high speed, which is exactly the condition where a motorcycle burns the most.
Consistency at the pump matters more than it sounds. Filling to the pump cut-off one time and to the brim of the filler neck the next introduces an error the size of half a gallon, which on a 4 gallon tank is an enormous fraction. Same pump behaviour every time is the whole method.
One tank is a sample of one. Three or four tanks over different roads gives a range rather than a point, and the low end of that range is the honest planning figure.
What actually moves fuel consumption on a bike
Aerodynamic drag rises with the square of speed and the power to push through it with the cube, and a motorcycle is a bad shape moving fast. The consequence is that speed dominates everything else on a road machine. The same machine that returns 50 mpg at a relaxed pace can return low thirties held at a high sustained speed, and no amount of careful throttle use recovers that.
| Condition | Typical direction | Why |
|---|---|---|
| Sustained high speed | Large loss | Drag with the square of speed, power with the cube |
| Headwind | Large loss | Adds directly to airspeed, and drag follows it |
| Luggage and a pillion | Moderate loss | Frontal area and weight both up |
| Soft ground, sand, mud | Large loss | Rolling resistance and slip, often at low speed in low gears |
| Altitude | Small, either way | Less drag in thin air, less power to work with |
| Low tyre pressure | Small loss | Rolling resistance |
The adjustment field on the calculator is a blunt instrument for all of this, and it is deliberately blunt. What it is really for is forcing the question: is the figure I measured from the kind of riding I am about to do? If it is not, the honest move is to knock a fifth off it and see whether the plan still works.
Reserve is the least reliable number on the page
On a carburetted machine with a petcock, reserve is a real geometric thing: a standpipe that draws from higher up the tank, and a lower port that draws from the bottom. Switching to reserve opens the lower one, and the difference in level between them is what you have left. That volume is a manual figure and it is reasonably honest.
On a fuel injected machine there is usually no petcock and no reserve position — there is a level sensor and a warning light, and what remains when it lights varies with how the machine is sitting. Attitude matters more than people expect: a slope, a lean, hard braking, or a rough trail can leave the pickup drawing air with fuel still sloshing in the tank. Some machines add a countdown that estimates remaining range from recent consumption, which is a better guide than the light but is still working from an estimate of what is left.
Treat the reserve distance as the softest figure in the calculation, and treat the main-portion range as the one to plan on.
Planning a leg rather than a range
The useful inversion is not "how far can I go" but "what economy does this leg demand". A 150 mile leg on a 3.6 gallon main portion needs 41.7 mpg with nothing in reserve at the end, and 33.3 mpg if you are willing to arrive on the reserve. Comparing those against what you have actually measured is a far sharper question than comparing distances, because it tells you how much your economy would have to disappoint you before the plan fails.
Add the detours. Map distance between two towns is not the distance you will ride, and the difference is rarely in your favour. A closed station on a remote road turns a 150 mile leg into a 210 mile leg with no warning, and the only defence is having asked the question beforehand.
Finally, running to the last of the tank is not free. Fuel pumps in injected machines sit in the tank and are cooled by the fuel around them, and whatever sediment a tank has collected lives at the bottom. Neither is a reason to fear a low tank, but both are reasons why planning to arrive dry is a worse plan than it looks.
Questions people ask
How many miles can I ride after switching to reserve?
Reserve volume times your fuel economy, and both parts of that are less certain than they look. The volume is a manual figure for carburetted machines with a real petcock, where reserve is the fuel below the level of the main standpipe, and it is usually somewhere around a fifth of the tank. Multiply it by the mpg you are actually getting — not the published figure, and not the figure from easier riding than you are doing — and that is the distance. On a typical machine it lands somewhere between 25 and 50 miles. On a fuel injected machine there is often no reserve position at all, only a warning light, and how much is left when it comes on depends on the tank shape and how the machine is sitting. The number is worth calculating and worth treating as approximate.
How do I work out my real fuel economy?
Fill the tank until the pump cuts off, zero the trip meter, ride a normal tank, then fill again at the same pump letting it cut off the same way. Trip miles divided by gallons taken is the answer. The single biggest source of error is inconsistent filling — topping up to the brim of the filler neck one time and stopping at the first cut-off the next can be half a gallon of difference, which on a small tank swings the result by ten percent or more. One tank is a sample of one, so three or four over different roads gives you a range instead of a point, and the low end of that range is the number to plan with. The onboard economy display, where a machine has one, is derived from injector time and is a good relative guide but not a substitute for measuring it at the pump.
Why is my range so much worse on the highway?
Aerodynamic drag, and it is not a small effect on a motorcycle. Drag force rises with the square of speed and the power needed to overcome it with the cube, so speed dominates fuel consumption once you are past about 50 mph on a machine with little or no fairing. A rider sitting upright is a poor shape moving through air, and adding luggage or a pillion makes it worse. The practical result is that the same machine can return 50 mpg on a relaxed back road and low thirties held at sustained highway speed, and the difference is bigger than anything careful throttle use can recover. A headwind adds directly to the airspeed the machine is fighting, so a strong one is equivalent to riding a good deal faster than the speedometer says.
Is it bad to run a motorcycle tank almost empty?
Occasionally, no. Habitually, there are two arguments against it. On a fuel injected machine the pump sits in the tank and is cooled and lubricated by the fuel surrounding it, so running consistently at the very bottom gives it a harder life than it needs. On any machine, whatever sediment or water the tank has collected sits at the lowest point, which is exactly where the pickup is drawing from when the tank is nearly dry. There is also the practical problem that a low tank plus a lean, a slope or a rough surface can uncover the pickup while there is still fuel visible in the tank, which is how machines cut out with fuel still in them. None of that makes a low tank an emergency; it makes planning to arrive empty a worse plan than planning to arrive with something in hand.
Should I plan on the whole tank or only to reserve?
Plan the route on the main portion and hold the reserve for the things you did not plan. That is what the split is for. If a leg only works by counting the reserve, you have not planned a leg with a margin, you have planned one where the margin is already spent before you set off — and the reserve figure is the least reliable number in the calculation, because it depends on tank shape, machine attitude and, on injected machines, on a sensor rather than a standpipe. A useful habit is to check what economy the leg demands on the main portion alone and compare it against the worst figure you have ever recorded. If the leg still works on your worst tank, it is a plan. If it only works on your best, it is a hope.