NASCAR engines like to run at about 8000-9500 rpm (revolutions per minute); however, the tires on the car rotate around 2400 rpm at 200 mph. The gearing in the transmission and the rear end gear reduce the rotational engine speed, with different gears providing different reductions. When you talk about the size of a gear, you’re actually talking about the relative sizes of a pair of gears. The gear on the left in the diagram has 20 teeth, while the gear on the right has 10 teeth, so this gear would be a 2:1, meaning that the smaller gear rotates twice every time the larger gear rotates once.
If the engine is running at 9000 rpm and goes through a 3.0 gear, the result is a 3000 rpm revolution. Pretty straightforward calculation. A car is just a little more complicated because there are two sets of gears. All that means is that you multiply the two gears together to find the effective reduction in speed. In the diagram above, if you were in first gear, 9000 rpm coming from the engine would be 9000/(2.66*3.80) = 890 rpm at the wheels. Note that the fourth gear is the smallest it’s allowed to be in NASCAR: there’s no change in rotational speed. NASCAR doesn’t allow overdrive, which would be a number less than one.
There is actually no rule against shifting – but there are rules about which gears you can use in the transmission and rear end gear. The third hear in the drawing that third gear is 1.14, which is the gear you use at Pocono. At a place like Michigan, you’d use something more like a 1.30 gear.
It’s a seemingly small difference in numbers, but it makes a really big difference in how useful third gear is on the track. Let’s say you’re running 8500 rpm in fourth gear. That translates to 2237 rpm (186.4 mph) at the wheels. If you have a 1.30 gear, running the same speed of 186.4 mph, the engine would have to turn at 11,050 rpm, which is well beyond the range in which engine builders start to get ulcers. If you want to use third gear and keep the engine at 9000 rpm, then the maximum speed would be about 150 mph.
On the other hand, if you’re only running a 1.14 gear, running at 186.4 mph requires you to run at 9,690 rpm. All of a sudden, third gear gets useful on the track.
Remember that horsepower and torque depend on the engine speed, as the graph at left shows. There’s a sweet spot (a peak) where you get maximum horsepower or maximum torque. (The position of that peak changes depending on how the engine is built and tuned.) One way of changing the engine rpm is changing speed. Another is changing gear.
Pocono features two very long (3000+ feet) straightaways and two very flat (6 and 8 degrees) corners. This means that drivers pick up a huge amount of speed coming into turns 1 and 2. Turn 1 (where drivers have been entering at 211+ mph) has 14 degrees of banking – but teams are going into the 8 degree banked turn 2 at 200 mph. You can’t take the turn that fast, so there’s a lot of slowing down going on. Instead of staying in fourth gear and letting the engine run slower, you can shift into third gear and keep the engine in its most favorable operating range.