OK. 37 is the tire width in millimeters and 590 is the bead seat diameter (tire inner diameter) in millimeters. 1-3/8 is the approximate tire width in inches, which computes to 34.9 mm, but the exact width per the ETRTO standard is 37mm, so this number should be used when computing gear inches, since the tire width is also assumed to be the tire height. The 26 number is quite useless in determining wheel size and gearing, since the BSD can vary from 559mm on tires with decimal widths such as 26 X 2.125 all the way up to 597mm on tires with fractional widths such as 26 x 1-1/4 EA1 used on rims common to old American Schwinn bicycles. Anyway, in your case using 37-590 and 36 x 16 gearing, I calculate your gear to be 58.9 gear inches. This is not much lower than one of my town bikes that I ride on some dirt and unimproved paths with 61.4 gear inches. Given that your bike is probably heavier and the wider tires have more rolling resistance, if it were me I would not bother to increase your gearing. As to using the front brake to control your speed on downhills, I do this all the time on my fixed gear bikes with no rear brake, and it is never a problem unless I'm riding on a very slippery surface in which case I rely on my legs back pedaling against the rear wheel. Just remember to continue pedaling while going downhill, and let the front brake control your speed.