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1st overall gear ratio - from CIMs to 8 inch wheels - recommend you find a link (search CD) for the JVN calculator - nice spreadsheet helps you get reasonable on what ratio will drive you and not overload. I think you will find you want 1.5x or so more ratio than the toughbox gives. This can be done by chain and sprockets between gearbox and wheel, mount wheel with 2 bearings on a dead axle (3/8" bolt) mount a sprocket with a spacer to the hub (sprocket and spacers from AM or VEX), and other sprocket to transmission output shaft. Remaining consideration is spacing of wheel axle to transmission output shaft - use a chain distance calculator (search). If you have a way to adjust chain tension, all the better.