I'm assuming your largest ring is a 42T and your largest cog is a 14T?
I'm also assuming your bike has a Freewheel?
That only gives you a gear ratio of 3 to 1, which is about 75 gear inches.
There are 13 tooth FW's available, which would gear you up about 7%, or 81 gear inches.
Or you could just leave it as it is and coast downhills at 25-30 mph instead of 35-40 (or whatever). I rarely ever need gears higher than 80" or so and suspect that many commuters don't either.....unless you are griding your knees away at slow cadences (poor technique) or just
really enjoy flying down hills (which I no longer do since having a bad accident during a race).
Just a thought -- whatever you like.