Play with the gear ratio calculator at the beginning of this DIY forum's index.
My approach is to calculate how fast the vehicle would go if the wheel was directly attached to the engine's drive shaft then divide that speed by a reasonably expected speed and you'll have your overall ratio.
The ratio depends on your drive wheel diameter. The same ratio will give you more speed but less torque as you increase the wheel diameter. A reasonable (and easy for calculations) wheel diameter would be 12 inches. The circumference would be pi (3.1416) X the diameter (1 ft.), so the circumference (how far the wheel rolls for each revolution) is 3.1416 ft. X 3,000 rpm. This means the vehicle would go 3,000 X 3.1416 ft. in a minute = 9424.8 ft./min. There are 5,280 ft. in a mile, so we divide the ft./min. figure by 5,280 to get miles per minute. 9424.8/5280 = 1.785 mi./min. We want mi. per hr. so we multiply by 60. 1.785 X 60 = 107.1 mi./hr. Of course that's way too fast. If you want a compromise of speed and torque that results in 40 mph at 3000 RPM, you divide 107.1 by 40 and get 2.6775 which is the ratio you need. If you found a clutch that had 12 teeth, you would multiply 12 X 2.6775 = 32.13. You would need a sprocket on your wheel or live axle that had around 32 teeth. If you couldn't find a sprocket that small for your wheel and used a larger one, you would need a larger one on your clutch or use a jackshaft for a place to put a larger sprocket. If you couldn't find a clutch that fit the 1 inch engine shaft, you could incorporate the clutch into the jackshaft as long as the primary ratio from engine to jackshaft remained in the operating range of your centrifugal clutch. Most clutches for typical 4 cycle engines are designed for the RPM of your diesel as well, so if your clutch is in your jackshaft keep the sprocket size on the shaft the same as the sprocket on your engine (1 to 1 ratio) so it will engage at the right RPM. In this example, if you had a 64 tooth sprocket for your wheel (twice what we calculated), to keep the right ratio you would need to double the sprocket size of the driving sprocket, so 2 X 12 = 24. Typical clutches aren't 24 teeth so you put a 24 tooth sprocket on your jackshaft to chain up to the back wheel. Since we're keeping the ratio from engine to jackshaft at 1 to 1 for the sake of the clutch, we need the calculated 2.6775 ratio to be achieved entirely by the difference in sprocket size betwwen the jackshaft and the wheel.
I think you could have a great deal of fun with a pusher trailer of this size especially if you were hauling heavy loads for long distances. You could carry every conceivable necessity and convenience with you on a cross country trek and still get great fuel economy. I see no reason to avoid road travel. With proper design a single track trailer could be built that would be narrow enough to easily mix with any traffic you would ride with on any bicycle. It wouldn't take much to disguise the fact that you have an engine powered vehicle. Of course you would need brakes on the trailer itself because its loaded weight would be higher than any touring bike is designed to handle even if you used tandem bike components. Personally, I would devise a telescoping tongue that would automatically apply braking when compressed by the bike slowing down. There is a recent example of this on a trailer in the electric section.
I'm surprised you haven't gotten a reply sooner than this. If you continue to ask thoughtful questions and actually begin this project, I'm confident interest and help will appear. There is a great deal I could say about it that would depend on whether you want to design it with one wheel (my preference but design is more critical) or two wheels, if you want max or minimum cargo capacity, whether it will NEVER be used on the road, how rough your off road territory may get, etc. Top speed of 40 mph may be too high if you are in very steep, rough terrain and pulling lots of cargo. 10 hp IS overkill in my opinion if it won't ever be needed due to extreme combinations of cargo load and steep inclines. So think it through and let us know more about how you plan to use this vehicle. The first things for us to determine is what kind of wheel and tire will be necessary and whether there is a clutch available for a 1 inch shaft, and a 10 HP 3600 RPM engine. Start Googling the clutch. If available, its sprocket will determine which chain and additional sprockets to use. Read everything you can find here about pusher trailers, take notes, ask questions, reread, PM me as much as you want. I love this stuff.
