Calculating Fuel Burnoff
At times you may find that you'll need to know how much fuel weight you're going to burn off in a certain distance. For instance, you're in Tennessee which allows up to 20,000 pounds on the steer axle. You just scaled your truck. You're 11,800 on the steer axle and you have steer tires rated for 6,150 pounds so you're legal up to 12,300 in Tennessee on the steer axle. You have 1/4 tank of fuel, you're 210 miles from the Arkansas line, and you're heading to Little Rock, Arkansas. Tennessee allows 20,000 pounds on the steer axle, but Arkansas only allows 12,000. You already know that your truck puts 90% of the fuel weight on the steer axle. You'll need to know how much fuel you can add now and still be legal when you get to the Arkansas line.
When making any sort of fuel mileage calculations, you always want to be conservative. Most companies can tell you what fuel mileage you're getting, and in fact most trucks have a readout right on the dash. Whatever your fuel mileage is, round it off to the most conservative number. In this case, we don't want to overestimate how much fuel we'll burn off because we might wind up too heavy when we get to Arkansas. So if the dash says we're averaging 6.5 mpg, we'll round it up to 7 mpg for the calculation just to be safe.
To figure out how many gallons of fuel you'll burn off, simply divide the number of miles travelled by the miles per gallon:
Number of miles travelled / miles per gallon = gallons of fuel burned off
So we're 210 miles from the Arkansas line and at 7 mpg we're going to burn off approximately 30 gallons of fuel by the time we get to the state line (210 miles divided by 7 miles per gallon = 30 gallons of fuel)
30 gallons * 8 pounds per gallon = 240 pounds of total fuel burned off.
If 90% of the total fuel weight will be coming off the steer axle, we need to figure out how much weight will come off the steer axle if we burn 240 pounds of fuel.
Calculating The Weight Coming Off The Axles
For this example we know that 90% of the 240 pounds of fuel weight we'll burn will come off the steer axle. To calculate the percentage of a value, you simply multiply the total value times the percentage you're looking for, and then divide by 100. In this case we want to know what 90% of 240 pounds is so we plug in the numbers:
(90 x 240) / 100 = 216 pounds.
So if 216 pounds is coming off the steer axle between here and the state line, and we can be 12,000 pounds on the steer axle at the Arkansas line, then we can put in enough fuel right now to put us at 12,216 pounds on the steer axle. We're currently at 11,800 on the steer axle so we have 416 pounds of weight available for the steer axle. From the previous page, we already know the formula to deterimine how much fuel you can take on is:
So we plug in the numbers:
- 416 (amount of weight available for steer axle)
- 90 (percentage of fuel weight to steer axle)
- 416/90 = 4.62
- 4.62 x 100 = 462
- 4.62/8 = 57.7 gallons of fuel you can add right now and be legal at the Arkansas state line.