Pulley size ?

[Henry Hopper]

I have a generator that only seems to put out 12.5v at higher revs.. during heavy traffic driving it hovers around 11/12v. If a smaller pulley was used would this give me the required voltage at lower speeds ie;30 mph?
Next question, .. if it would work is there a smaller pulley available and what size is it..... part number etc?

Thanks.