First is the gear ratio. This is very important. The 1:15 gear ratio is the start of the deficiency of your dragster. So you'll have to find a smaller ratio. I find that using the 1:8.333 ratio works the best with stock* parts. The bigger the gear for the drive and the smaller for the output is usually what you think but if you compare a perfect gear combination to a high gear combination the perfect gear would be crusing quickly while the high gear will be stalling every few seconds.
Next is wheel circumference. The phrase bigger is better fits here perfectly. You can use stock wheels and get 6 MPH out of it. (If you have enough track, 7 MPH is able to achieve.) Now find the largest wheel you have. The Green Exo-Force wheel is probably the largest. Why does this work? Because the larger the wheel, the bigger the circumference. The bigger the circumference, the more it travels in one rotation.
Now is the important part, you battery options. The Energizer E2 is probably you pick of choice. It is super light weight and can give a ton of boost. But the cost is not. If you are sticking with rechargeables but have a big fat wallet you might want to try the LEGO rechargeables, sure not exactly called fast charging or lots of power but it has a plug in charge that makes it easy for multiple runs. It is light to counter act the downsides. Now if you don't have a such big wallet and don't mind 120 grams (30 grams heavier than LEGO's rechargeable battery pack) then try the nice GP batteries. They are only sold in 3 stores in the US but they give 0.1 V more than the rechargeable (I pretty big difference). They also are not pricey but can withstand a huge load for a long time considering they are just normal batteries. My cheep 1300Mah batteries can withstand 3 amps for 20 min! That is more than 3 motors under complete stress. Though it is not recommended to stress all 3!
Now the weight comes to mind. The phrase bigger is better really doesn't fit here. You want to make your dragster as light but as strong as possible. Try only using essential elements and ignore the decorations. The decorations can produce drag and unstability of the NXT so just leave them behind. When your robot is traveling near 5 MPH and is only a few hundred grams, you want to try to keep it as aerodynamic as possible as the wind can effect it at this speed. But, don't ever keep strength out of your mind. Toque is lost due to the friction of the gears. The more friction the less energy is transferred to the wheels making it lose its speed dramatically. So strengthen the gear connections by placing beams on both sides and use 2 gears instead of one to increase torque to give it more power.
Also this last tip is to make the vehicle have only back and forth and have rubber front wheels. The swivel in the front makes it slippery and can make it slip and slide. The front swivel can also make it lose control when attempting to change directions. The swivel has to turn 180 and your robot is feeling that friction created by that 180 spin by that swivel and will make the dragstor do a uncontrolible spin. And if your robot is able to accelerate long enough it might crash!
That's about it for this time. I hope these tips helped you as I have been experimenting with my dragster traveling at a top speed of 6.7 MPH with only stock* wheels and mostly stock* elements (except for the front wheels and a extra 40 tooth to improve torque and allow steering).
* Stock means the stuff that comes with the set.
** See blog post a bit below on my dragstor
*** You probably seen this somewhere else but I thought I might as well post it here (yes it is my own work)
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