Propane weighs about 4.2 pounds per US gallon, at 60 degrees Fahrenheit. Propane expands 1.5% per 10 degrees.

40 gals of propane = 168#

Gasoline weighs 6.073 pounds per US Gallon.

40 Gals of Gasoline = 243#

A difference of 75#

The differential would be the weight of a gas tank verses the weight of a propane tank.

Smaller propane tanks such as a 40# tank refer to the amount of propane it will hold. A 20# tank holds 20#'s a 40# tank hold 40#'s etc.. The empty weight of the tank is located on the collar and is marked as T.W (tare weight). You can add 40 to that number and that is the tanks full weight. If you need to know how much propane is in the tank take the total current weight of the tank subtract the empty weight (T.W) from that and divided that number by 4.125 which is the weight per gallon of propane. My guess would be that a filled 40 Gal Propane tank would weigh a little less than a Gasoline tank. Next factor is weight distribution. Where's the gas tank and where's the propane tank going to be located?

EDIT:

**OOOOPS!!!! Forgot you're in Canada! Need to convert to Imperial Gallons????**
<message edited by Foodbme on Fri, 07/20/12 12:06 AM>