# How to Calculate Gas Mileage When Towing a Trailer

Diana Gosi/Demand Media

When you tow a trailer, the added weight lowers your miles per gallon. How much the gas mileage drops will depend on the combined weight of the trailer and cargo. Other factors such as the design and condition of the trailer, as well as road conditions and driving speed, also play a role in determining your gas mileage when towing a trailer. However, you can estimate your gas mileage in advance. If you keep mileage and fuel consumption records, you will be able to calculate the exact gas mileage afterward, providing you with an accurate estimate for future trips.

## Step 1

Determine your vehicle’s gas mileage without the trailer -- if you don’t know it already -- by recording the odometer reading the next time you buy gas and fill the tank. Drive until the gas tank is near empty -- around a quarter tank -- then fill the tank up again and record the ending odometer reading. Subtract the starting from the ending odometer reading to find the miles driven. Divide by the number of gallons of gas purchased to fill the tank the second time to calculate your vehicle’s normal gas mileage.

## Step 2

Estimate the weight of your trailer including cargo. The trailer weight will be listed in the trailer's owner manual. If you are renting the trailer, the dealer can tell you the weight of the trailer when empty. You’ll need to estimate the amount of cargo you are loading onto the trailer. Alternatively, you can stop by a weigh station and have the loaded trailer weighed. In most states, the highway patrol operates such stations. Be aware that there may be a small fee.

## Step 3

Estimate the drop in gas mileage. For light loads -- under 2,500 pounds -- subtract 10 to 15 percent from your normal gas mileage. For medium loads -- between 2,500 and 5,000 pounds -- subtract 15 to 25 percent; and for heavy trailer loads of 5,000 pounds or more, subtract 25 to 35 percent.

## Step 4

Use mileage and fuel purchase figures to calculate the actual gas mileage when towing a trailer so you will have an accurate measure of fuel consumption in the future.