Fuel Mileage Calculations: The People Who Know Aren’t Telling

Fuel mileage races have always been part of NASCAR, but increased access to data is giving fans new insights into the competition to reach the checkered flag. More than once this year, fans have perched on the edges of their seats during the last few laps of a race, listening to a crew chief tersely instruct his driver to ‘save fuel’.

And yet, even with all that data, fans have had some surprises this year.

At Pocono, Chase Briscoe should have been long out of fuel by the time he took the checkered flag. On his last pit stop, Briscoe left pit road before the team was done filling the tank. Crew chief James Small estimated they were more than a lap short. Briscoe saved enough, though, to hold off Denny Hamlin and get the win.

William Byron ran the last 144 laps of the Iowa race with crew chief Rudy Fugle in his ear reminding him to go easy on the throttle. In the end, though, Byron and Fugle had enough fuel left to not only win the race, but to do a pretty spectacular burnout.

So why do crew chiefs push it so close on fuel? How come the television graphics tell us a driver should be out of fuel before he actually is?

Let’s answer your fuel mileage questions.

How Big is a NASCAR Gas Tank?

Not to be pedantic, but… OK, never mind. It’s actually my job to be pedantic. NASCAR racecars don’t have gas tanks. They have fuel cells.

A fuel tank is pretty much a receptacle you fill with fuel. A fuel cell, on the other hand, is a much more complicated piece of equipment.

The outer container of a fuel cell looks like a fuel tank on steroids. In NASCAR, that means a protective steel case, as shown below.

The steel plates at the bottom of the diagram connect to the rear bumper assembly. That was one challenge with the chassis: The bumper must be strong enough to protect the fuel, but with enough give to protect the driver.

So a Fuel Cell is Just a Beefed Up Tank?

No. The fuel goes into what’s called a ‘bladder’. NASCAR fuel cells are made of ballistic Kevlar — a material used in bulletproof vests. The upper cell in the image below (which I modified from the ATL catalog) shows a Cup-series-type fuel cell.

The ballistic Kevlar prevents the bladder from being punctured in a crash. That keeps fuel contained — and away from the driver and rescue personnel. Although the bladder looks like box, it’s flexible, so that it can be easily installed into its protective container.

So It’s Just a Flexible Gas Tank?

Putting fuel inside a bare bladder would create a problem for a racecar driver. NASCAR’s Sunoco fuel weighs about 6.2 pounds per gallon. Sixteen gallons of fuel means about a hundred pounds of liquid in the rear of the car. When a driver turns at high speed, all that liquid would go sloshing around.

Race cars operate on the very edge of traction. A hundred pounds of weight shifting each time they turn is more than enough to unbalance the car and cause a crash.

That’s why foam — a type that is impervious to fuel — fills the bladder. The foam has small enough pores to keep fuel from moving rapidly within the tank, but large enough pores to allow quick refueling.

Fuel cells also have check valves on the top that ensure fuel cannot leave the fuel cell if a car goes upside down.

Here’s one other neat feature of the fuel cell. NASCAR’s rules allow it to replace some foam with solid materials, which would decrease the overall fuel cell capacity. So if NASCAR wanted to vary the fuel window, they could without too much trouble.

OK, OK. So How Much Fuel Does the Fuel Cell Hold?

Twenty gallons. ATL’s Ron Ross regularly visits NASCAR shops to check in with crew chiefs. He told me that, at one shop, a crew chief told him that they measure every single fuel cell they get and couldn’t find any meaningful difference in volume.

When someone stretches their fuel mileage, it’s not because they have a larger fuel cell.

Where’s the Fuel Gauge?

NASCAR does not allow fuel gauges. The cars do have fuel pressure gauges, which plummet just as you start running out of fuel. That’s not much advance notice.

One reason there is no fuel gauge is that there are no sensors in the gas tank. Anyone who purports to tell you how much fuel a driver has left, or what mileage they’re getting, is giving you either an estimate or a guess.

The Fuel Window is an Estimate?

The fuel window is the number of laps a car should be able to run under green flag conditions. At its most basic, it’s a simple formula: the number of gallons of fuel times the miles per gallon divided by the track length.

At Iowa, NASCAR specified a fuel window of 110 laps for the 0.875-mile track. If we work backward, we arrive at an average fuel mileage of 4.8 miles per gallon.

But television broadcasts often provide a broader estimate. Their estimate sometimes brackets NASCAR’s numbers — and sometimes doesn’t.

For example: NBC Sports estimated the fuel window at Iowa to be between 115 and 120 laps, which translates to 5.0 — 5.2 miles per gallon.

What About Cautions?

The rule of thumb has been that race cars get about twice the fuel mileage under caution as they get under green-flag conditions. But that’s a pretty general estimate, especially if the driver if using extreme measures like turning off the engine and coasting during green-flag laps.

Is Fuel Mileage the Same at All Tracks?

Just as your car has estimated mpgs for city driving and highway driving, NASCAR race cars get different mileage at different tracks.

The cars get the best fuel mileage (according to NASCAR’s calculations) at the two big superspeedways:

• Daytona: 6.00 mpg
• Talladega: 5.98 mpg

At the regular-season-ending Daytona race this year, Steve Letarte showed iRacing simulations of how fuel mileage changes in the draft. According to their calculations, a lead car might get 5.2 mpg because it incurs the most drag, while a car five or six back in line might get 6.1 mpg.

So, especially at drafting tracks, a single fuel mileage estimate can be pretty misleading. But even at regular ovals, a driver may have to negotiate lapped traffic, or fight hard with another car for position.

What’s the Fuel Mileage Window at New Hampshire?

At this week’s race at New Hampshire, NASCAR estimates an average of 5.3 mpg. That gives us a nice, round 100-lap fuel window for the first race in the second round of the playoffs.

Where is Fuel Mileage the Worst?

Road courses have the worst fuel mileage. That makes sense because drivers are speeding up and then braking hard a lot. It’s the same reason city fuel mileage is worse than highway fuel mileage.

• COTA was estimated at 3.9 mpg in 2024 and 4.3 mpg in 2025.
• NASCAR rates the Charlotte Roval at 4.3 mpg
• The Chicago Street race was about 4.4 mpg
• Watkins Glen comes out to about 4.41 mpg

So Nobody Really Knows How Far the Car Can Go?

Nobody knows exactly how far a car can go. But the people with the best estimates are exactly the people who are not going to announce that data to the rest of the world.

So much data is shared in NASCAR today that teams must seize every possible advantage, no matter how small. That’s why you’ll hear crew chiefs call for ‘Quarter Pounder’ or ‘Cleveland’ as pit stops approach. They don’t want to announce what they’re doing in advance.

Personally, I would agree with my driver ahead of time that I will tell them two less than the actual number of laps they can go so that anyone listening will think we’re in more dire straights than we are.

How Do the Teams Estimate Fuel Mileage

First of all, they know how much fuel went into the car. They weight the gas can before and after fueling. Unless there was a lot of gas spilled, that number is pretty accurate. It’s also a number they keep private.

Second, teams have access to real-time data like the engine rpm, the brake and the throttle trace. Other teams have access to that info, also. But each team knows details about the specific engine in the car that no one else knows. Engines are one of the few secrets left.

Fuel mileage depends on a number of parameters:

• How long and how hard the driver applies the gas pedal
• How much fuel is injected into each cylinder each time
• How fast the engine runs
• How much fuel is combusted and how much is released as flames in the exhaust.

The ‘war rooms’ at the team’s headquarters (and their manufacturer’s headquarters) are getting enough data that they can make much more precise fuel mileage estimates than anyone else. If anyone ever asks you what good calculus is, this is the answer.

And again, they’re not sharing this info. I couldn’t even get crew chiefs to tell me how much better their estimates were than NASCAR’s and the broadcasters. One summed it up by saying “You’ve got no idea how much we know.”

So They Might Have Plenty of Fuel Left for a Burnout?

While all the noise and smoke suggests that a burnout uses plenty of fuel, Ron Ross told me that you don’t need much fuel to do a burnout. In retrospect, this makes perfect sense, especially if you park the nose of the car against a wall and just let the rear tires spin. The engine isn’t moving the car, just the rear wheels.

The fuel mileage summary: The folks who know aren’t telling. The rest of us are just guessing.