Ahh, you're catching on fast, whodat. There things are not designed for true high speed running. Bad things can happen fast.

The bigger the tires, the more they rob your power. In theory, the bigger tires should actually give you more top end, but just take you longer to get up there. At least that's my opinion. But everytime I read a post by someone who put on larger than stock tires, they seem to complain about a loss of power. This is more pronounced on the 'smaller' engines (like my 400) than on a 600 or above, I'd assume.

 

Actually the way it works is that horsepower determines top speed, at least horsepower per pounds being moved plus wind resistance. Gear ratio does not change the horsepower as it does torque, so available horsepower will only drive you so fast. The torque multipication by the gear change can change your acceleration rate(how fast you get to top speed) but won't change top speed.

 

All depends. Larger tires will definetly use up any leftover available torque throughout the power band, especially 12" width tires. And they can take away from top speed, but not always.
I installed 27x8 & 27x10 Mudbugs on my 400 manual & picked up 2.8 mph on a radar gun. But they did cut into my lower rpm torque just a little bit. Hardly noticeable.

 

Like I said Horsepower determines your top spped. Then again if the overall ratio isn't high enough the rev limiter determines your max speed. The bigger tires backs you off the limiter by changing your overall ratio(lower number+higher ratio) and makes you faster because the rev limiter doesn't stop you any more. That's why I always ran ignition boxes without limiters.