Wildfire Equipment Research & Development

Roscommon Equipment Center


Calculating the Load on a Slip-On Tank Fire Engine

This calculator was developed to estimate the total load on a slip-on fire truck prior to fabrication. It requires the input of accurate information that you, the designer, must obtain. Your reward for doing your "homework" will be the reduction of costly design errors and the likelihood of a well balanced truck. You will need to enter good numbers into the calculator. This discussion will help you determine the numbers.

If you have not read the REC booklet, Newsnote #3: Guidelines for Designing Wildland Fire Engines, please do so before calculating.  Newsnote #3 provides background information and definitions that will aid you in your fire engine design.

Vehicle Manufacturer’s Weight Ratings

The vehicle manufacturer or dealer can provide you with the pick-up truck's Gross Vehicle Weight Rating (GVWR) and each Gross Axle Weight Rating (GAWR). This information is often found on a data plate on a door or door jam. Military vehicles will have this data on a dash plate.

Bare Chassis Weights and Wheelbase

This section asks for three items that are necessary for the calculations. Determine the bare truck weight by filling the truck full of fuel and taking the vehicle to a set of scales. Without occupants, weigh the vehicle to get a total weight and the weight on the rear axle. Also enter the wheelbase. You may refer to REC’s Weights of Common Components for calculating your load. Estimates include weights of water tanks, pumps, winches, and various other items.

Behind the Front Axle

In this section, you will enter weights of major objects that you place in the vehicle. Estimate the horizontal distance between the object’s CG and the center line of the front axle. Refer to REC’s Center of Gravity Diagram for assistance.

Important: This Detailed Calculator - Pickup Truck Slip On Units is a simplified version of our Detailed Calculator - Truck Chassis > 14,000 GVWR.  This calculator only considers adding items that will be located behind the front axle. If your slip-on unit will have components added in front of the front axle, please use the Detailed Calculator - Truck Chassis > 14,000 GVWR.


After verifying that your entries are correct, click the "Calculate" button. The estimated weight of the completed truck and the weight on each axle will appear.  The results will also display the estimated percentage of the weight on each axle and the percentage of the payload on each axle.  A dialog box will give a general statement of what the results mean.  It might recommend that you make some changes.  Use "Reset" to start over.


Considerable effort has been made to make this calculator accurate and easy to use. The more accurate information you enter, the more accurate your estimate will be.  It is your responsibility to determine the accuracy of the results. Your final check for accuracy should be to weigh the completed vehicle with a full load of occupants, water, fuel, and equipment to verify that it does not exceed the manufacturer’s GVWR or GAWRs.

Tips & FAQs

Please refer to Tips & FAQs for more information about the calculator.

User's Pledge: I have read the instructions and understand that the calculator’s accuracy depends on my understanding of vehicle design principles and accurate input of the data requested. The interpretation of the results and decisions about their accuracy are my responsibility. By clicking the calculator link below, I accept this responsibility:

Detailed Calculator - Pickup Truck Slip On Unit