If you measure with a full tank and then measure again with an empty tank, you can calculate the spring rate since you will know the deflection and load.
I'm not really sure whether this would be useful to anyone though.
Conversely, if you can find a published spring rate, you should be able to predict the deflection with a full tank or empty tank (or anywhere in between). This might be useful to find out if the spring meets spec. If it deflects a lot more than it is supposed to, I would inspect carefully for cracks.
Of course you have to calculate the weight distribution with respect to all 4 wheels (each wheel carries a different percentage of the weight because the center of the fuel tank does not coincide with the center of the vehicle) Also, you have to take into account the deflection of the tires under the same load (the tires have their own spring constant). The tire companies usually publish data on the loaded radius, so you could figure this out.
Has anyone here weighed each wheel separately on a scale to determine the actual weight distribution? If you knew this, then you could pick springs for each corner with exactly the right spring constant so that your truck is perfectly level. Of course, you would need to decide whether you want it to be level with no driver or if you want it to be level when you are driving. Also, do you want to carry passengers and cargo? How much and where are you going to put it? Level with a full tank or half a tank? Level when driving at 70mph (factor in aerodynamics) or just when sitting at the curb?
Ford has already designed a system to try to do this. It is their Air Ride system. Maybe that is what you need if you MUST have a perfectly level truck.
