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Razor PR200 Drivetrain (Stock)
This includes the specification on the stock parts and research on alternatives.
1 Watt(w) = 1 N-m/s or 0.738 Ft-lbs/s or 1.341E-03 hp
It looks like the 36V 350W Unite MY1016 motor is the biggest that is a straight bolt-on. It's specs are nearly identical except for the 1.22 N-m of torque (136%). Bigger motors (MY1020) should fit but, you'll have to drill new mount holes and make a spacer (~1") because they're too long to fit between the frame rails. The bigger motors are a better use of your money.
Running the stock 24-volt, 250-watt, 3000 rpm motor at 36-volts got me to about 22 mph at 4,326 motor rpm. This leads me to believe that getting to 40 MPH may be difficult. Even with the tallest gearing available from tncscooters.com (15/55) we're looking at 4,892 rpm but, 35 mph would be a do-able 4,283 rpm based on this single reference point.
Mr. Exon claims to have hit 40 mph but, forum posts show him and others using a 34" (864 mm) wheel circumference based on his math that a 10" tire x pi = 34" (dπ=c) instead of 31.4". Assuming that he never caught that math error then his actual speed was closer to 36.9 mph. Whatever his actual speed was, it does tell me that it is possible to get 150% of the rated rpm for a motor and that the tires probably will not immediately disintegrate at 35-40 mph.
Note: On 4/1/14 zen_racer at modifiedelectricscooters.com reported that my 65T stock rear sprocket size claim was not accurate with his pocketbikes - They were running 55T rear sprockets.
#25 Gearing Links:
The PR200 comes with a 126-link ANSI #25 chain stock. Generally speaking you would want to add/subtract links as you add/subtract teeth on your gearing but, you may be able to compensate for a few teeth with the chain adjusters on the rear axle.
Other small electric vehicles may use a T8F chain which I've also included in the table below for reference.
Tire Circumference: 800 mm
Tire Size*: 10x3x4 or 260x85
Speed Rating: Unknown
My initial searching suggests that your two options are the stock Qing Da tires or Kenda's as an alternative. Neither brand lists a speed rating but internet reports suggest that the Qing Da's can handle at least a few moments at 35-40 mph.
Tire Speed Ratings: A5=16mph, A8=25mph, B=31mph, D=40mph, F=50mph, J=62 mph