48-18 is the perfect ratio to start with. thats what I ride and I have to go up cat-4 and 5 hills everyday from and to school,work,and commuting. I commute about 25-30 miles a day. right now im doing 48-17 because im in training mode. but riding SS/fixed is all about spinning. so try a lower gear ratio, you'll be spinning and it will seem like your not getting nowhere but keep at it and in a couple months you'll get to cruise at 20mph. not only that since you a little heavy-dont take this personal- spinning will get you to do more aerobic workouts which will-if your trying to lose weight- help you burn more calories and lose more weight.
Keep at it.