I've noticed my BA consistently posts better results down the quarter mile when there's at least two-thirds of a tank in there. Anything less, and it'll loose a few tenths of a second. Is it suffering from fuel starvation? It doesn't feel like it's stuttering or anything but it's definately quicker with a fuller tank. It's a BA XT with the standard fuel system, dynoed at 185rwkw.. is that pushing the stock injectors/pump etc?

doubt it very much

what it could be is the extra weight of the fuel is helping with traction over the rear wheels

I did briefly think of that but it's an auto with the stock diff ratio and 17" BA XR's - it doesn't have any problems whatsoever with traction off the line unless you drive on gravel or something.

How much has the car been driven before you do the 1/4 mile?

Because just about all injected cars a recirculating system, the fuel gradually heats up in the tank as warm fuel from the engine bay is pumped into the tank.

The more fuel in the tank, the longer it takes to heat it up and also the more heat is dissipated to the surrounding air, slowing down the tempertaure rise further.

Differences in fuel temperature theoretically will make marginal differences in performance.