FD's are alot cooler too. A shifter would do wonders for it.A chain drive would be more efficient than a friction drive, owing to tire deflection caused by the loading of the drive roller. As to the 30% figure, I feel this is an inflated number based on personal experiences with both drive systems. The biggest drawback to the friction drive is wet roads, which causes the roller to slip. On the other hand they are less expensive, much lighter and much quieter than chain drives.
I'm currently building another friction drive for my wife, she wants no part of a chain.
the only way to calculate this is too have a chain drive and a friction drive side by side. Since Work = force * distance, hang a 1kg weight on one end of the wheel and see if it can balance 0.9kg weight on the other wheel, assuming the diameters or distance of each wheel is the same. If they werent you would divide or multiply by the gear ratio. So lets say for the chain drive it is 90% efficient because if it was 100% efficient it would balance a 1kg weight. If the gear reduction was 3x, it would have to balance a 3kg weight.... see where i am going. Do the same for the belt drive and lets say the 1kg weight was balanced by a .7kg, now that would mean 70% efficient as the extra work was converted into friction. These are made up numbers, but yeah if anyone wants to do this try it out and post the numbers. The greater the weight, the more accurate.friction sucks spend more on tires and limited gear ratios