Changing the cog will cost a lot less than the chainring and will give you a much bigger change in gearing. I ride a 47 x 17, which is only slightly lower gearing than 48 x 17, so I'd suggest trying a 17T rear cog first and see how it feels. I can still average 20 mph on the flats with this gearing and crank it up to 35 mph on downhills, but still get up hills easily. I don't skid, just backpedal to stop and have a front brake for fast and emergency stops.