One more comment related to the post above... to make this possibly work, I would need a narrower sprocket retainer nut. If none are available, perhaps this stock one could be made half width to allow enough threads after the double front sprocket is installed. Even with that mod it might not work. If someone wants to attempt this, I can get them exact measurements and some good close up pics. I think it is possible, just not easy.
Currently I have 9 and 89 tooth sprockets on my Razor E300S which makes the top speed about 8 MPH (stock top speed is about 15 MPH). This really helps going up hills such as pedestrian ramps, and across grass, but the stock motor still struggles. At this point, since there are no more cost effective bolt on sprockets to lower the "gearing", I have a few choices.
1) Find someone (perhaps a machinist) that can somehow bore my 8 tooth 6mm sprocket to the same bore as the E300S motor shaft (8mm I think), and somehow weld it to a 9 tooth D bore sprocket.
2) Pay $60 for a larger rear sprocket but then I would have to purchase a longer chain (another $10) and pay someone to deal with clearance issues (perhaps another $10 or $20). That seems too expensive to me for a moderate improvement.
So I am leaning towards candidate solution 1 but need to find someone willing to try this. Any retired machinists out there or students machinists that have access to the proper machines to do this? Please see the pics of both my current setup (9 and 89 tooth sprockets) and the pic of my 8 tooth sprocket. Note that the 9 tooth sprocket would hold the D motor shaft but the chain would be connected to the 8 tooth sprocket. I am not even sure if the motor shaft is long enough to accommodate both sprockets and the retainer nut. Maybe this candidate solution is not even possible.