Would you use a chain or pulley type clutch and what ratio?

I'm starting to get everything together to build something like a 'Gator. I have a Peerless gearbox, numbers are 820 037 then 1064 1248. My plan is to use a 15.5HP Briggs to power it. I'm not looking super fast, maybe 20-40MPH tops. My choices for a clutch with a pulley are 1200RPM with a 3 inch pulley then it goes to 1800 or 3000RPM with a 3 inch or 1800 or 2300RPM with a 3.7 inch pulley. I do have the option of using a chain drive, but considering that the biggest set of teeth for a clutch is 17T, that would create some problems I think. In real life this is going to be driven by my significant other just riding around the farm as basic transportation. She's not real thrill seaker if you know what I mean. Do you think the 1200 RPM would work OK, that way the engine could be locked to the rear end at a low engine speed without having to really wind the motor up? I think if I go with the 1200 RPM, I can get it to work, but I'm torn between using the 1200RPM and the 1800 RPM clutch. If I use the 1800 RPM clutch, that would give me more belt contact with the 3.7 pulley and more pulley options down the road on the rear end. I'm open to suggestions. I'm looking at the clutches from the Surplus Center.

so youre trying to use go-kart type clutches on your mower?

Yes, that's what I'm planning to do. When I lift off the throttle, it will be able to "coast", I will also be able to do a few other things that a direct coupled belt drive will not allow me to do.

Idle speed on OPE engines tends to be 12-1400RPM spec. If you were NOT using a tranny with a neutral position, I would strongly advise against a 1200RPM clutch. Between creeping at idle, and engaging before the engine's power curve, it just wouldn't be a great idea. That said, you're not building a go kart, you're building a utility machine. Having the benefit of a neutral position and low gearing negates much of that.

The only thing I'd be concerned with is starting in a high gear. If you're geared for 40MPH in high gear, that's a lot to ask of an off-the-shelf centrifugal clutch. Now, some people insist you can safely shift an 800-series Peerless on ly fly (start in a lower gear and work up), but others insist you will chew up shift keys on a regular basis doing this. Food for thought.

I have no idea what you have in mind for a chassis, but be aware that factory stock lawn tractor running gear and steering components are simply not up to the task of moving you around safely at 40MPH.

It's going to be a WIDE body, non-stock steering, independant front suspension, soild rear mount. I plan to use wide ,short automotive tires no taller than 26 inches at the most. High spped wheel bearings on all corners. I'm working out right now how to extend the rear axles out more. I'm leaning towards pulling the rear end apart and drilling and tapping the axle ends, installing a 1x4 shaft coupler and about 6-12 inches of keyed 1 inch shaft with a bearing behind the wheel end and have a bolt through the "new" axle end into the OEm axles end to keep it from coming off. All the weight will be on the outer wheel/pillow block bearing. Who knows, most of the time it will spend just above an idle on the farm pulling a small manure spreader. The Cub Cadet I modified with the stock Hydro drive only does about 10-12 MPH flat out. I lifted it and installed 26 inch tall lug "tractor" tires on it. That's what she's using now. It needs rings, she's complaining about it smoking some now. The Bolens she was using, broke a belt. I'd rather put spark plugs in a VW through the tail pipe than put another belt on it.

For the tires you could get some 13" honda rims and they are about 24-26" tall with the tire mounted and at correct air pressure.