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© 1999 Brian F. Schreurs
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Prince William County Sanitary Landfill, Dale City, Va.

Test session: Saturday, May 1, 1999

Sure, you can take the curb weight from magazines or the gross vehicle weight from your doorjam and try to calculate it. It'll get ya close. But wouldn't it be nice to know what it really weighs?

We thought so. But as racers, we don't like to wonder, calculate, or guess. We like to know. A few well-placed calls provided the answer: the Prince William County Sanitary Landfill has a truck scale, and no, they don't mind at all if we come down to weigh in.

Let's talk specifics. This particular scale has a resolution of 20 lbs. That means it'll round the car's weight to the nearest 20 lbs. It is constructed of five "pads," each of which provide an independent measurement. Each pad is about the size of a car so we tested each pad separately to make sure they were consistent. They were: same weight across the board. Good enough for us.

The cars were outfitted in race trim; that is, no cargo at all except for the driver. We topped off the gas tanks mere minutes before hitting the scales so we'd know exactly how much gas was in the cars.

I brought the Paradise Garage 1998 Pontiac Firebird Formula (manual); Josh brought his 1995 Ford Mustang GT (automatic); Josh II brought his 1992 Eagle Talon TSi (manual and AWD). Josh II had to go a day later than Josh and I, so just for kicks I brought the Paradise Garage 1970 Dodge Charger (automatic) the second time.

The Firebird weighs in...

...and so does the Mustang.

When Josh and I pulled up at the scales, we met Sonya. I explained our mission; she seemed amused but willing to tolerate the eccentric racers. While we were experimenting with the scales, Bob trotted up and helped us check the scales, position the cars, and zero in. He apparently used to campaign a first-generation Camaro that ran 9s in the quarter so he knew exactly what we wanted. Cool!

The Firebird checked in at 3640 lbs. 15.5 gallons of gas, at 6 pounds per gallon (specific gravity 0.72), is 95 lbs; the driver is about 200 lbs. That leaves the Firebird with a dry weight of about 3345 lbs.

The Mustang checked in at 3560 lbs. It carried 15.4 gallons of gas, but the driver is a comparative lightweight at 140 lbs. This leaves the Mustang with a dry weight of about 3325 lbs.

The Talon, despite its heavy all-wheel-drive, checked in at 3480 lbs. It carried 15.7 gallons of gas, and the driver is a sliverweight at 120 lbs. This leaves the Talon with a dry weight of about 3265 lbs.

By way of comparison, the Charger checked in at a portly 3800 lbs. Even allowing for the driver and the 19 gallons of gas, the Charger still sits at an impressive 3485 lbs dry.

The obvious question is, what good is this information? Well you can use it to figure out how much your car weighs on race day. All it takes is a little planning ahead.

On your last fill-up before you race, be sure to completely fill the tank. That way you know what you started with: a full tank. It doesn't matter whether you fill up mere moments before racing or fill up two days ahead to race on fumes, although this procedure is more accurate if your tank is closer to full than empty. Be sure to note your odometer reading.

At the track, note the odometer reading when you enter the gates. Note it again when you leave. This tells you how many miles you drove at the track.

EXAMPLE OF CALCULATING RACE WEIGHT mileage at fillup = 21,000 (mf1) mileage entering = 21,120 (me) mileage leaving = 21,125 (ml) mileage at fillup = 21,200 (mf2) gallons at fillup = 11.764 (gf) mf2 - mf1 = total mileage (mT) 21,200 - 21,000 = 200 mT/gf = fuel economy (fe) in miles per gallon 200/11.764 = 17.0 (me - mf1) / fe = gallons consumed entering (ge) (21,120 - 21,000) / 17 = 7.0 (ml - mf2) / fe = gallons consumed leaving (gl) (21,125 - 21,000) / 17 = 7.3 (ge + gl) / 2 = average gallons consumed (gc) (7.0 + 7.3) / 2 = 7.15 IF YOU KNOW THE VEHICLE DRY WEIGHT: car weight = 3400 (cw) driver weight = 200 (dw) tank capacity = 15 gal (tc) gas weight = 6.0 lb/gal (tc - gc) * 7.5 = tank weight (tw) (15 - 7.15) * 6 = 47.1 cw + dw + tw = race weight 3400 + 200 + 47.1 = 3647 lbs IF YOU KNOW THE VEHICLE+DRIVER+TANKFUL WEIGHT: full weight = 3712 (fw) tank capacity = 15 gal (tc) gas weight = 6.0 lb/gal fw - (gc * 6) = race weight 3712 - (7.15 * 6) = 3669

Next time you fill up, again completely fill the tank. Now you know how many gallons of gas you've used (check your receipt silly). Record your odometer reading. Now you know exactly how many miles you've driven since your last tankful (subtract your first reading from your last reading to get miles driven). And somewhere in there was a race.

To find your overall fuel economy, divide the number of miles driven by the number of gallons consumed. That's your fuel economy. Hang on to that number.

Now we get to find out how much gas was in the car at the time of the race. Take the odometer reading from when you entered the track and subtract your first fill-up reading. That tells you how many miles you drove from the gas station to the track. Multiply that number by your fuel economy (this is why this procedure works better with the tank closer to full) to see how much gas you consumed. Do the above procedure a second time, using the odometer reading from when you left the track. Odds are it will be close, but if it's not, you may want to average the two numbers for any further computations.

If you know your car's dry weight: Take the number of gallons consumed and subtract it from the number of gallons in a full tank. This gives you the number of gallons remaining in the car. Multiply that by 6 to get the weight of the gas in pounds. Add that and the driver's weight to the car's dry weight to get the approximate race weight!

If you, like us, know your car's weight with driver and full tank: Take the number of gallons consumed and multiply it by 6 to get the weight of the gas consumed in pounds. Subtract this number from the car's weight, and you get the approximate race weight!

Yep, that's a lot of math. But it's not so hard really. We've included an example of how the math works so you can see that it takes a bit of patience, but it's not rocket science. Now that we have this cool information, what should we do with it? Well, you could try plugging it into those silly caluclators on the internet that purport to give you horsepower and quarter-mile estimates, but we've tried them all with our own numbers and have gone away impressed only by how far off they are.

No, there are two main uses for this number. The first is to plug it into a G-Tech and let that guesstimate your horsepower. It's more accurate than those calculators, but still not as good as forking over the money for a dyno run.

The second and better use for this data is to track how weight affects your runs. If you're consistent about this, you can see, over time, whether you're actually picking up any time or speed by losing weight in the gas tank. This information can help you make a better race strategy.

Good luck with the calculations! And if you find another use for race weight, please let us know!