I'd go w/ the .15 and if you want more top end you want "taller', or lower numerically (higher) gears. Theoretically, yeah, an engine w/ more torque, but same R.P.M. capability should provide quicker accelleration, if you have adequate traction, w/ the same gears. If you have a wheelspin problem, taller gears will help cut that down, because the engine won't be as close to peak torque R.P.M. until the car gets going faster. If you also want it to handle, the .15 is prob. the best choice, because a .21 makes for a top-heavy 1/10 scale, and you'll find that tire selection becomes more critical. The Fusion just blows the tires off w/ the slightest touch, and does donuts in the blink of an eye Actually, it's pretty cool, but gets to be a P.I.A. buying tires, (you have to get the belted radials, so they don't fly off the rims as easily) and sometimes, like now when it's colder, it's really hard to get it to hook-up. And, the colder air means more power, which exacerbates the problem...Sure does sound nice tho, just like a hi-po dirt bike, when it's shiftin', and on the over-run....Over-all, I'd say it was worth it, 'cuz when the adrenalin's flowin', and she's singin' at top speed, it's wicked cool[8D] Just don't hit your foot! Friend of mine did, w/ his LD3, and went to the emergency room, and when we told 'em "He got hit by a car", none of them would believe us, 'till we said an R/C car