As you are aware, some toll road agencies charge tolls based on weight and vehicle height. Tollguru™ toll calculator uses a relationship table between truck weight and number of axles. For example, you can see the table showing relationship between weight and axle for Pennsylvania turnpike here. Please refer to individual state pages for information for all facilities where tolls for trucks are not charged based on axles.
If we do not use a relationship between weight and axles, we will not be able to show total tolls for routes where one toll agency in the route charges tolls based on axles and another agency charges tolls based on truck weight.
On many toll roads you have to pay different tolls if you drive a 2-axle pick-up with 4 tires, 2-axle pick with 6-tires, or if you drive a 2-axle commercial truck. Often the tolls are same but on some roads they are not. You can pick appropriate vehicle class (personal pick-up truck or 2-axle truck) to get the correct toll.