While the chart on the website that Jerry posted is correct for larger tires and the lower gear ratios (4.10 and lower), it has proven to be slightly incorrect (in my experience anyways) for 3.73 and higher.
I got the recommended 32 tooth gear for my 31s and 3.55 and it was off by 3mph at 75. I was actually going 71/72 by GPS when the speedo read 75. So I actually needed probably a 33 or 34 tooth. So for 3.73, I'd venture to say you'll need a 34/35 tooth gear.
It is however, correct when it says 38 tooth for 4.10 and 31s. I now have 4.10 with my 31" tires and the reading is spot on all the way up to 75-80.
My tires are 31x10.5R15 on a 15x8 rim, aired up to 30 psi. They measure an actual 29.5".
When I do the math for 38 tooth with 4.10, it gives me a proportional value of 34.5 for 3.73. Make your pick, 35 tooth or 34 tooth. You'll be off a little bit in either direction.
My suggestion is measure your tires. If they're slightly larger than 29.5", go with the 34 tooth. If they're smaller than 29.5", go with the 35 tooth. You need a higher number of teeth for smaller tires.
