Things that slow down vehicles or speed them up...
Density altitude is a major factor. This is the relative condition of the weather at the time of the run. This is gathered from temp, elevation, barometric psi and humidity. This is how you can correct your times to see if you really are making or losing rwhp.
About hp to weight....great for those that do not race nor understand power under the curve. Take my Lightning, makes 401/487 SAE @ the wheels, weighs 4800 lbs. Truck last time out ran 108.4 mph in the 1/4, as fast as 108.59 mph. This is with a DA of 1500' also. Give me a cool day, mucho baro and little humidity, I should hit 110-111 mph.
General rule of thumb...under 5250, rwtq is moving you, over it, rwhp comes in play. My Lightning shifts at 5000 rpm, makes 475 rwtq @ 3000 rpm, 487 @ 4000 rpm and it slowly drops off. How does your hp to weight ratio deal with such torque that never stops?
Tires...well, tires and rims. More mass = less speed.
There are a number of things that have an effect on mph and et. Those are the big ones.
Biggest one of all.....driver.
