The faster the 1/4 mile times, the more lengths will be between the cars. Think of it this way. If car A can do the 1320 at 15 sec at 80, and car B does it in 16 at 75, then when A crosses the finish line, B will be still be accelerating but will be close to 75 mph already. In other words, it will be traveling very close to 75 mph for that last second. (If you have a car that runs close to these times, you can check the next time you get on the freeway. Notice how little speed you gain in one second from 70 mph.) Simplifying the equation (to get there exactly is a calculus problem), 75 mph = 51.1 ft/sec. Giving a little extra to account for the acceleration in that last second, car B is approximately 55 feet away - about 3 1/2 lengths (using 15 ft lengths).
Now, let's do the same thing with a Top Fuel car vs a Top Alcohol. Top Fuel runs the 1/4 in about 4.5 seconds at around 325 mph. Top Alcohol is running around 5.5 at about 250 mph. Using our same basic equation, 250 mph = 170 ft/sec. Add a little to make up for acceleration over that last second, the TA car is about 185 feet away from the finish line when the TF car crosses - or about 12 lengths back.
Remember that this assumes equal reaction times in a heads up race. If one car gets left at the line or it's a bracket race with a dial-in, the overall margin of victory will obviously change.
