ORIGINAL: SAVAGEJIM


Smaller tires will sacrifice your top end speed. But smaller tires will help acceleration due to a couple of factors: less weight to accelerate, and the reduced diameter combined with less weight means less moment of inertia. And reduced moment of inertia = faster acceleration.

Larger tires, ones with larger diameters, will give a higher top speed, that is if you have the torque to turn those wheels for the gears you have. But, as a direct converse of smaller tires, larger tires will have more weight and more moment of inertia, thus, you will sacrifice your acceleration.

Now, tires with a higher moment of inertia will help you control the RC's flight attitude while in mid-flight after a jump. Gas it and you can pitch the nose up. Hit teh brakes, and you can pitch the nose down. Using the moment of inertia of teh wheels in mid flight can allow you to control your RC such that you can even land on all four wheels. I know I can do this very easily for nitro RCs, since they are 4wd and the proportion of teh RC chassis to teh body is such that teh wheels are much larger, but in gassers, the 2WD will only give you half teh amount of the moment of inertia control. Moreover, the mass of teh wheels in relation to teh rest of the RC is not like that of nitros, thus it is much harder to control the flight attitude of a gas RC.

As for skinny versus wide tires, skinny tires are great for top speed for a couple of reasons.
First and foremost is you reduce the road contact area and thus the amount of rolling friction incurred. (This is assuming the tire is relatively stiff enough and not running flat. A flat tire is the worst offender of rolling friction.)
Second, the thinner tires at speed present less of a crossectional area for teh sake of aerodynamic drag. Wide tires only add to the cross sectional area.
Now, skinny tires , since they have less of a road contact area, will not be useful in hard acceleration. The tires will break free and spin out more easily than what wider tires would. So if you are in loose dirt or other conditions where you want grip, you want wide tires. Or if you want great grip under hard acceleration, such as drag racing, you want wide tires to the powered wheels. That is why top fuel drag racing rails have massive rear wheels (the powered wheels) but tiny skinny frunt wheels (they are only there to steer, since they do not power teh rail, it only makes sense that they do not contribute to rolling friction and aerodynamic drag)