RE: RC18T or T2 for a high speed car?
3s is a 3 cell lipo in series, 4s is a 4 cell lipo in series. 3s lipos run 11.1v and 4s is 14.8v. so the advantage is obvious...more power. but with more power there is more heat and wear on you motor. when you are buying a brushless system check what input your motor is rated for against what the esc is rated for. I have seen combos where the esc is rated higher than the motor so if the esc can handle 4s power the motor may only able to handle 3s.
when choosing a motor I look at the kv rating since I havn't figured out exactly how the thurn rating converts to speed. but the kv rating is the just rpm per volt. so if you have a 5500kv motor you will have 40700rpm on 2s, 61050 rpm when running on 3s and 81400 rpm on 4s. motor range any where from 2500kv to 10000kv for these cars.
There are tons of gearing options for these car as well so so 60-70mph isn't only doable I will say it is almost easy even on 3s. My RC18T is geared right now to go about 60-65 on a smooth surface with just a 4120kv motor. but it gets hot quick so I don't run it too long with this gearing setup. I plan to get a 8000kv motor in the next week or two and then shorten the gearing a bit and see where that gets me. so far the tallest gearing I have found that will fit the RC18T is a 54t spur and a 33t pinion. Right now I am running a 60t spur and a 26T pinion and it still wheelies with just the 4120kv motor on 3s. so as you can see there is still plently of room for more speed. and at 60+ the car still feels very stable. Hopefully within the next week or two I will see how 80+feels!