OK guys and gals, I can settle this all.
GVWR is gross vehicle weight rating - meaning the loaded weight of the vehicle, that is where the sandbags come in, and the weight in the trunk to simulate luggage. This usually also includes all vehicle fluids topped off.
Now, GAWR is gross axle weight rating - meaning the MAXIMUM allowable weight for each axle, above and beyond the GVWR.
Obviously now you can see why the difference.
In florida there is a vehicle weight on out titles. It is the dry curb weight of the vehicle, and it tends to be very accurate. It will be close to what Nissan says the vehicle weighs. When a curb weight / vehicle weight is specified it is generally dry weight, no passengers, no weights, no fuel. All other fluids filled.
As for the line thing, Torry is correct in that they weight the vehicle on pads and the weight distribution is calculated that way, if would be interesting if someone also published the L/R distribution.
Another thing, about the RB in S-chassis. Inline 6 engines are far from compact, and thereby heavy. They are also subject to flexing do to the length of the engine so they require extra bracing. The RB in particular is one of the most robust inline 6 engines ever built, so you can bet it is far from being a lightweight, my solution to the problem of adding extra weight to the front is to move the engine rearward. Somebody mentioned the 240 being a master piece that should not be toyed with. HAH dont make me laugh. By adding an RB to the front you HAVE to offset the weight to the rear to compensate. So, absolutely get everything out of the front end that you can, go full carbon on the front if you can afford such niceties. I would cut the firewall and slide the engine under the dash somewhat like the newer F-bodies have, you want all the weight, (or more correctly mass) between the axles if possible.
The vehicle when turning tends to rotate on an verticle axis about 1/3 of the wheelbase ahead of the rear axle, meaning that you want the mass of the vehicle centered as close to that axis as possible to facilitate the car turning easily. The axis about which the vehicle tends to rotate normally is known as the polar moment of inertia, or the center of mass. Ever wonder why a 240 slides so smoothly and controlably ? It is because the polar moment of inertia is located quite far forward in the vehicle due to so much mass in the front from the engine and tranny. It is also why a porsche 911(rear engined) is so hard to maintain a smooth slide in. The mass being centered far toward the rear, once it breaks loose, it immeciately wants to rotate around that axis, and you have to be damned good to drift a RWD 911. Thats also the doubled-edged sword of mid-engined cars; they handle amazingly and turn in like you wouldnt believe, but are accordingly difficult to drive because they rotate so easily. Once it slides you have to catch it quick like.
Now for mass in the front of the vehicle. Well imagine that axis 1/3 the wheel base ahead of the rear axle. The car normally rotates approxiamately around that when turning normally, not sliding. Its about 4 ft from the trunk and about 8ft from the front of the engine bay. Take a 4ft pole and tie a battery to the end and go swing it around and see how easy it is to change its direction. Now try that with an 8ft pole =) thats called momentum guys. So tell me again, where do you want your battery??
And lastly, someone mentioned weight making the car grip. In a straight line this is true. But that same weight, or more accurately mass, wants to keep traveling the direction is it traveling, so when you try to turn it doesnt want to. Lots of weight in the front, the car will understeer because teh front end wont want to turn. Lots of weight in the rear, and that weight will want to swing out; oversteer. Bottom line, reduce weight as much as possible, in all areas, and optimal weight distribution is not 50/50 since that axis I spoke of is not located halfway between the axles. 50/50 would be great if all cars had full four wheel steering, but most dont, and even cars with 4ws, dont turn the rear as much as the fronts. Optimal weight distribution would be something around 45/55, take a look at how any open wheel racecar is setup, they are NOT 50/50, but 50/50 is good to shoot for in a FR street car, and it is acheivable. Always keep the mass as close to that axis as possible. Things that are far away from that axis such as batteries, bumper beams, and heavy body kits will ruin the handling.
Sean D.
P.S. If the DOT come asking, I did NOT tell you to remove your bumper beams,
and on an S13 dont do it in the rear because the gas tank is back there.