I'm asking for a little bit of mechanical engineering advice. I only know enough to know that gear ratios are important.
I've built an R/C lawnmower and I've bought another electric wheelchair to harvest parts to upgrade it to add a second set of wheels to make it 4 wheel drive for better handling and load capacity. This would be substantially cheaper than deploying/scavaging tracks (as far as I can tell anyways), which would have been my preferred option. I am using a sabertooth 2x25 motor controller fed by a hobbyking tx/rx pair and would split the signal to go to another sabertooth 2x25 to maintain/create track steering with each "axle" of wheels. The lawnmower is about 6 feet long right now.
I'm pretty sure the second set of motors and wheels has a different gear ratio and that could wind up damaging the grass/lawn I'm trying to cut. I'm not so worried about it in my yard, but eventually I want to show it off on other peoples yards and don't want it to tear things up. My evidence is that the wheel size is different; my theory is that if I put the same wheel size on both sets of motors, each pair will be fine on their own, but will wind up with different gear ratios with the new size wheels.
About how much difference in final gear ratio do I have to work with here? Are there any other obviously fatal flaws in my design? I have the wheelchair and can re-purpose it for another project, though I have not bought the 2nd sabertooth controller.
The problem that I am trying to solve is that the lawnmower is backheavy, slides sometimes, and has poor maneuverability on certain slopes (an angles of attack), which are common where I want to use it.