How do they know?
 
• The standard fuel dump can weighs approximately 95 pounds when filled with 12 gallons of racing fuel;
 
• A gallon of racing fuel weighs about 6 pounds (the actual weight will vary depending on temperature);
 
• The dump cans are weighed before and after refueling the car during pit stops to get a precise measurement of fuel consumption.
 
If a can weighs 30 pounds after a pit stop, then 10.8 gallons of fuel were dispensed into the car (95 lbs.– 30 lbs. = 65 lbs., and 65/6= 10.8)
 
To determine fuel mileage at that point, a team would divide the amount of fuel replaced into the number of laps completed since the last fuel stop – for instance, if it had been 45 laps, then 45/10.8 = 4.15 mpg.
 
Because of a number of variables that impact mileage — is the driver out front in clean air or battling traffic?, for instance) — it’s impossible to know exactly how much fuel is in the tank or how far a driver can go before his car begins to sputter.
 
It’s an educated guess, but a guess just the same.
 
• • •
 
Among the various track sizes found on the 36-race Sprint Cup Series schedule, eight are 1.5-mile venues. So fuel mileage should be approximately the same at each one, right?
 
Approximately, yes. But approximately can sometimes be the difference in speeding across the finish line first, or coasting across in 10th or lower.
 
"It changes. At every track you go to, it changes," said Rodney Childers, Harvick’s crew chief.
 
The reasons are numerous. Some 1.5-mile tracks are faster than others — at Texas Motor Speedway and Kentucky Speedway earlier this season, the pole-winning speeds varied by nearly 7 mph. And faster speeds mean an engine is burning fuel at a faster rate.
 
Running a race during the day versus running one at night also impacts fuel mileage; cooler conditions under the lights often mean higher speeds with, again, engines burning fuel at a faster rate.
 
Driving styles differ, too, and charging deeper into a turn before easing off the gas pedal can also use more fuel.
 
Even the racing surface (abrasive versus smooth) plays a role, impacting tire wear, which in turn affects fuel mileage.
 
"(At) some places that don’t have much falloff (in speed), your race fuel mileage would be the same, or close to the same, that it was in practice," Childers said. "At other places where the falloff is a lot, your fuel mileage is often way better than what it was in practice."
 
What does practice have to do with calculating fuel mileage?
 
Everything.
 
The process begins before the cars are on the track. Prior to fueling the cars, the fuel dump cans are weighed. That weight, as well as the time and temperature, are noted on the outside of each fuel can.
 
Temperature affects the weight of the fuel — "the warmer the fuel is, the lighter it weighs," Len Wood, co-owner of Wood Brothers Racing, said earlier this year during a break at one NASCAR Sprint Cup Series stop.
 
"If it’s a 60-degree day at Richmond, and you come to Talladega where it’s 85 degrees, your fuel is going to be 60 degrees at one place and 85 at the other … so you have to adjust for that.
 
"If (temperature) stayed the same, say you ran all 1.5-mile tracks — 20 laps, that’s 30 miles. That doesn’t matter if it’s Kansas or Charlotte or Las Vegas. But that has nothing to do with it. It’s how much you’re on throttle at those tracks and things like that, yeah, that’s what makes it different."
 
Teams begin charting fuel mileage during opening practice each weekend. When a driver heads back to the garage for adjustments, the car is often refueled. And the amount of fuel that’s been used is carefully noted.
 
"I’ve already done three (fuel) checks here in the last 50 minutes," Wood said. "When you come in and change tires, that’s when you typically add fuel during practice.
 
"If you were going to run one set of tires at Texas for 35 laps during practice, then you would only get one (fuel) check at the end. But if you stopped with 15 minutes to go and said, ‘Let’s throw a set of tires on and see what we’ve got,’ say you’ve run 24 laps and go out and run 12 more, then you figure each one of those separately. In that case, I typically would add them together and average them."
 
In addition to figuring mileage based on how much gas a team is putting in the tank, electronic control units also provide information that can help a team determine fuel mileage.
 
"So after practice," Childers said, "you have two different figures you can look at.
 
"Then you’ve got to look at past history — ‘Every other time we’ve been here, our mileage has been two-tenths better than it was in practice.’ Kind of bank on that a little bit.
 
"But you really don’t know for sure until after your first pit stop or first two pit stops. By the time you get toward the end of the race you’re pretty confident on what you’ve got."
 
Of course, just as the competition on the track changes during the course of a race, fuel mileage can change as well.
 
"The thing than can kind of mess you up a little bit," said Childers, "is if you ride around 20th most of the day and the next thing you know you get up there in the top four in clean air. The lap times are almost a second a lap faster than what you were running (in the pack), and your mileage goes way down. You’ve got to be a little careful.
 
"It’s kind of up to the engineers and the crew chief to almost remind each other, ‘Hey, we’re way faster right now and our mileage isn’t going to be as good. We probably need to knock a couple of laps off just to be safe.
 
"It’s not easy, but the tools just keep getting better and better."

MORE: