I am using the stock 602 pulley arrangement. In the low/low configuration, the pulleys provide an 11.5:1 reduction in speed. In the med/low configuration, it is 5.75:1. The OEM motor is a 1725 rpm while most treadmill motors are over 3,000 rpm so there is an additional 2:1 speed increase that needs to be addressed as well. Pulleys will do the job but in order to keep the driven pulley a manageable size, the drive pulley must be fairly small in diameter which leads to the possibility of belt slippage. A solution would be to use gear belts rather than v belts.
I am finding that an overall speed reduction of 5.75:1 from my 4090 rpm motor plus the controller with feedback meets 99% of my requirements, giving me an operating range from 10 rpm to 800 rpm ( I limited the maximum motor rpm). If I require a higher speed, a belt change can get me to over 6,000 rpm if I were crazy enough to do so. From a practical standpoint, I could work nicely with two pulley settings, the 5.75 and 1.44 speed reductions giving me an operating range from 10 to 3200rpm.