As Evan mentions below, the change in effective wheel/tire diameter gives
less 'thrust' at the road surface since the torque is acting over a greater
moment arm (distance from the center of the wheel to the ground).  There is
also the inverse problem, the braking force/torque is also reduced by the
same amount, so your braking, already a bit iffy sometimes gets worse.
Pensionerd. (Al Knol)

>
> There are downsides, also, that need to be watched out for. Increasing tire
> diameter lowers your effective torque at the road surface. Since we are SO
> torque limited to begin with, you may not want to go to the biggest tires
> you can fit in the wheel wells. There are wheel bearing life issues, not to
> mention speedometer accuracy problems.
>
> Ask careful questions, decide on your trade-offs, and make your change. Any
> increase will help. And even if you keep the same effective rolling
> diameter, but on 15" or even 16" wheels, it will be noticable.
>
> Evan Mac Donald
>