Wait, you found 25 inch All Terrain or Mud Terrains? Tell us more! I'm running snows offroad. Or are you running four wheeler/ATV tires?
Edit: Found your other thread. Four wheeler tires and wheels.
Offroad tires cause a lot of drag. The more knobby, the more drag. ATV tires? All the smart builds I've seen using ATV tires say the car can't even go more than 45 on a road. Unless those are DOT approved four wheeler tires, you shouldn't be using them on the road, anyway.
Going with bigger diameter tyres effectively changes your gearing for the worse. My smart performs just fine offroad and on road. It's only slightly slower but that's expected when you shove tires the factory never expected you to have on them.
As for more speed, you can do a cold air intake, exhaust, and a GOPedal. The first two do help performance a little while the last only makes the car react faster.
The only true way to get a lot of speed out of a smart is turbo or engine swap, but I think either is overkill for an offroad build.