What is the total allowable width for a pinewood derby car?

The full alowable width for a pinewood derby car is 2-3/4 inches.

Actually this would only be the case if the wheels were pushed tight up against the car (causing the car to run very slowly). You see, the block is 1-3/4 inches wide and each wheel is 1/2 inch wide, so if you add that all up (1-3/4 + 1/2 + 1/2) you get 2-3/4 inches. In reality there is a 1/16 to 1/32 inch space between each wheel and the car body. This means the actual allowable width for a pinewood derby car is 2-3/4 + 1/16 + 1/16 or 3 inches total. You'll find that most rules state 2-3/4 inches as the allowable amount, but the race inspectors usually know that there has to be some space between the wheels and the block, so they usually measure using 3 inches. No one has ever corrected the standard rules on this, so it still remains in effect today.

Now, most standard tracks center their lanes on 3.5 inches and use 1-5/8 thick rails. This means that a car can drift up to 3/16 of an inch to either side of the rail. If two cars in side by side lanes both drifted towards each other, there would still be 1/4 inch of space between the cars. Although this is usually not allowed, technically as long as a car's width was shorter than 3-1/4 inch, it would never interfere with a 3 inch wide car in an adjacent lane.