DIESEL MATH
headaches, depending on how it’s used. The most common misconception about nitrous is that a certain size jet equals a certain size “shot.” While this is true on gas engines (a 0.062 jet for instance, is a 150hp shot), on diesels it works differently. Since diesels operate over a wide range of air/fuel ratios, more air doesn’t always mean more power. In a lightly fueled truck, an 0.080 jet might equal 50 horsepower; in a heavily fueled truck, the same 0.080 jet might be worth 200 hp. Also, since nitrous flow is based upon the area of a jet (3.14 x radius²), a 0.100 jet is not twice the size of a 0.050 jet—it’s four times larger. Diesels are also capable of huge amounts of nitrous ingestion, even with the turbo. Two or three 0.125 jets on a diesel can be worth 500 hp or more.
Transmissions, Axles, Tires
It’s hard to fit everything about transmissions, axles and tires into one category, but it’s important to remember that they all are interrelated. Change the gear ratio? The overall speed versus rpm of the vehicle changes. It’s the same with transmissions and tire sizes. We’ll start with the easiest calculation, estimated speed versus rpm. Let’s say that our test vehicle is an ’01 GMC with a fivespeed transmission, 3.73 gears and 32-inch-tall tires. In fifth gear, the engine is overdriven via a 0.71 gear ratio, which means the effective final gear ratio is (3.73 x 0.71) 2.65. To figure out speed at, say, 2,000 rpm, our equation is mph = (rpm x tire diameter) / (gear ratio x 336). The number 336 is just a constant that gives you a correct result. So, our speed is (2,000 x 32) / (2.65 x 336) = 71.8 mph. This formula is probably the most useful in determining tire size changes, as we can see switching to 35s would give us a big speed increase to (2,000 x 35) / (2.65 x 336) = 78.6 mph. Just like the other formulas, it’s not 100-percent accurate due to slippage, tire deflection or other unknown variables, but usually it’s darn close.
In closing, breaking out the calculator can help you plan the next move for your truck, calculate power, or just help you daydream. As useful (and accurate) as most of these formulas are, there are still “freak” vehicles that can break the mold and run quicker or make more power than physics should allow. So take all of these formulas with a grain of salt, and remember that math isn’t always scary—it’s helpful!