This is what I wrote on the Miata forums...BTW, NA = 89-97 Miatas, not Normally Aspirated.

First off, I am a very science/physics oriented person, so while I was bored today at work I thought of something.

How much gas should you run in your tanks if you're going to be racing autox?

I know that has been asked a million times, but read on, maybe this hasn't been brought up before.

I'm trying to see what would be best for your car, running a full or near full tank, or a half full tank. I don't think you can run less than half or else you might run the risk of sucking air into your lines right?

I don't know how big an NA's tank is, but for sake of arguement I will use 12 gallons. Let's say you fill up your tank and go racing. If gas weighs 6.2lbs/gallon, you would have 74.4 lbs of static weight in the rear. You would never have an odd load shift due to a dynamically changing load center, so your left, right transitions would be more consistant correct?

Now if you had a half tank of gas, you'd only have 37.2lbs in the rear, but it would all be dynamic, moving all over the place. Now if you could pull 1G in a turn or in transitions through the sloalams, that gas in the tank would move from side to side at the rate of 9.8m/s (the speed of gravity which is also the lateral rate of acceleration while pulling 1G). Bare with me as I have never taken a hydrodynamics course yet So given that I'll estimate the gas will crash into the opposit wall and settle in 1 second, that gives a force of 365ft.lbs pushing against the side of the tank as opposed to the 74.4ft.lbs of having a full gas tank.

After that 1 second when the gas has settled to either side of the tank, you will only have 37.2lbs of gas to contend with as opposed to the 74.4 for a full tank.

So what's better, a car that wants to get tail happy both upon entering a turn and during transitions but is more planted in the middle of the turn, or one that is more stable at both the beginning of a turn and in transitions but slightly looser everywhere else?

Wow.. Well now you know why road racers put foam in the fuel cell...

You can redo some of those numbers down to 2 gallons, almost every production car since the errrr mid 70's has some form of a baffled trap/well around the fuel pickup and shouldn't suck air until the tank has less than a gallon...

2 things to play with in your numbers are,the weight that does transfer to the loaded side is less than you may think as you need to take in rebound of the fuel that hits the side first and bounces back into the remaining oncoming fuel.

and secondly, the fact that the fuel tank is centered between the suspension pickup points left to right, for the most part and is not cantalevered outward. being centered between the pickup points nulifies the effect that fluid transfer has on handling, (this is done on purpose) as is mounting the lowest point of the tank as low as possible to be on the centerline of the axle height...

don't drive yourself to crazy, but it is fun trying to sort it all out