The stock ratios are usually alright, not perfect, but alright ... You have to understand that most companies don't tune their gearboxes for performance, even in their performance vehicles ... You'll often see very tall top gears for several reasons, not one of which is performance (comfort, long-term reliability, fuel economy, etc. at highway speeds), so often times the stock gearboxes on road cars leave a lot of room for improvement ...
Here are the general rules for acceleration in a vehicle ...
1) A car will ALWAYS have the greatest acceleration in a given gear at peak torque ...
2) A car will ALWAYS have the greatest acceleration at a given speed at peak power ...
3) If two cars have the same mass, gearing, aerodynamics, etc., the one with the greatest average power over the RPM ranges that the gearing dictates will have the greatest average acceleration ...
Now the reason why 1) isn't the most important is because it's in a given gear ... Since we don't drive around in 1-speed cars, peak torque isn't ideal place to be ... However, this isn't to say that a high peak torque isn't important ...
Allow me to demonstrate ... In this demonstration, we will have two cars that both redline at 7000 RPM, both have the same wheels, both have the same gearing, and both have the same aerodynamic properties ...
Here we have a theoretical car (we'll call it an S2000) that has a completely flat power curve ... It makes 250 hp @ 7000 RPM and makes 187.6 lb-ft of torque from 0-7000 RPM ...
Next we have another car (we'll call it an Evo 8 since I took it from an Evo 8 dyno graph) that has a much wider power band, as you can see ... it makes 236 hp @ 6750 RPM and 240 lb-ft @ 4250 RPM ...
Here we have the gearing and aerodynamic properties of the vehicles (same for both) ...
Both cars are set up to hit 175 MPH @ 7000 RPM in 6th gear and both have gears set to shift at 7000 RPM and end up at 4900 RPM in the next gear ...
So let's see the results of the Force vs. Speed graph ... Remember, Acceleration = Force/Mass and, since Mass is the same for both vehicles, the difference in acceleration will be directly proportional to the difference in force ...
As you can see, the S2000, despite having a 14 hp advantage, gets absolutely decimated ... Why ? Rule number 3) ... If two cars have the same mass, gearing, aerodynamics, etc., the one with the greatest average power over the RPM ranges that the gearing dictates will have the greatest average acceleration ...
Average Force (20-175 MPH)
Evo 8 - 1526 lbs
S2000 - 1427 lbs
Average Power (4900-7000 RPM)
Evo 8 - 224.9 hp
S2000 - 214.3 hp
Unfortunately, you'll never make this completely even over the entire speed range ... However, what you can do is make it a little more even on the track by concentrating the available power at the speeds you'll most commonly be in ... Say you're on a fairly wide open track with only several lower speed corners and your threshold speed through all of these corners is 80 MPH ... At the end of the longest straight, you should be just barely approaching 170 MPH, so you don't need to tweak the top gear or final drive ...
Average Force (80-170 MPH) with equal gearing
Evo 8 - 428 lbs
S2000 - 407 lbs
Average Power (80-170 MPH) with equal gearing
Evo 8 - 225.9 hp
S2000 - 218.1 hp
In this scenario, you decide to run 2nd up to 85 MPH and leave the remaining 4 gears to concentrate on the straights and high-speed corners ...
Meanwhile, the Evo's gearing has already been fully optimized so it needs no changes ... It will take the corners at exactly the same speeds as the S2000 ...
Let's see how much of a difference we made ...
There we go, now it's actually competitive from about 80-170 MPH and, since we don't plan on being below 80 MPH except at the start, we actually stand a chance at winning ...
Now let's compare average force from 80 MPH to 170 MPH ...
Evo 8 - 428.0 lbs
S2000 - 443.4 lbs
And average power from 80 MPH to 170 MPH ...
Evo 8 - 225.9 hp
S2000 - 229.8 hp
Awesome, so not only did tweaking the gears make it competitive, it actually gave us a slight advantage because we raised the average power above the Evo's at the speeds we'll both be using in the race ...