You said you drove it and it topped out at only 35-40mph...
...was that at full
Engine RPM?
...And were you in the "low" gear or the "high" gear?
The Hondamatic has a low and high gear, that's it. Top speed in low was 60mph and 105mph in high. Also, the CB750A with the Hondamatic boasted only 39hp @ 7500rpm. Hence...I don't really understand the benefit in putting this powerplant in place of the stock Engine (??).
Looks like the gearing from the Engine
through the sprocket-train is unchanged since you've got a sprocket size "A" coming out of the Engine, driving a size "A", steps up to a size "B", but then the final drive sprocket is back to a size "A". Hence, -0- net reduction through those sprockets. Looks like a 14T and 28T sprockets (tough to see)...
> Engine 14T to 14T jackshaft, 14T jackshaft to 28T jackshaft, 28T jackshaft to 14T gearbox, then gearbox 10:1 reduction.
> Math: Engine gearing x (14/14) x (28/14) x (14/28) x (10/1) trans = Engine gearing + gearbox 10:1.
The motorcycle also had a REAR SPROCKET though. Stock rear sprocket on the CB750A was a 42-tooth. So...a 14T drive sprocket to 42T drive sprocket is a 14/42 or a 3:1 reduction.
Tire size differential could be considered null...CB750 rear tire is 130/80 on a 17" wheel = 23-24" and considered the typical tire OD on an Oddy.
Weight isn't horribly different since the hondamatic 750 weighed 560lb.
So the bike's setup would reduce the Engine RPM down 1/3, and the Oddy gearbox is reducing Engine RPM down 1/10. Hence your buggy is 70% slower than the bike (but more torquey, lol).
And this could explain why it only topped out at 30-40mph!!!Slower, heavier, still air-cooled, and required a tank & jack-shafting...besides it being an easier to maintain 4-stroke and having a "granny gear", I really don't know why they did this swap.
In theory you'd have to sacrifice some torque and put a bigger sprocket on the Engine shaft to get more top speed. If you could find a 20T drive sprocket and it'd fit, and use 13T on the jack shaft/gearbox, you could improve the output RPM by 1.5x (~~20/13). This would get your top speed in 'high' out to 50mph, but have no idea what would happen to torque, low end, nor chain life. A 22T / 13T combo would maybe get you to 55mph in high gear.
Or...just rebuild the stock Oddy Engine and use it.
In the end, you've still got your money's worth or better in Oddy parts alone. So, don't sweat it!!