A833 Speedo Gear Drive Question

Since my speedo was out on my '70 Swinger, I put a new speedo drive in her A833. Since I dont know my diff ration for sure, I did the tire/driveshaft rotation trick and figure I have a 3.55 installed; I measured the circumference of the shaft and laid a piece of masking tape on it measured out in inches, it came around 3 full turns, and stopped somewhere between 3.55 and 3.73, probably around the 6-6.5 mark, so I just put it off to rough measurements and called it 3.55. With a tire height of 25 1/4"(again, rounded down to 25" for the chart lookup), Brewers chart tells me I need a 37 tooth drive, so thats what I put in her. The speedometer is slow, so I double checked the numbers tonight and I got the same as the first time.
So rather than play an expensive game of guessing the gear, should I go with a higher or lower tooth drive to increase the speed approx 5-10 mph?