Warning:  The following post contains Math. 

The best thing to do is to leave it as it is to start off.  Once you get the car back together and everything working, use a GPS (your smart phone will work) to measure your actual speed at, say, 60 mph indicated.  This will give you your actual speed vs your indicated speed.  

Just as an example, let's say your indicated speed is 60 mph and your GPS actual speed is 65 mph.  This means your speedometer gear is turning too slowly causing your indicated speed to be lower than actual speed.  Fewer teeth will speed up the speedometer gear so in this case that will be what you need.  For this made up example, let's say you count the teeth on your current gear and it is 19 teeth.  Now for the math...  (60/65) x 19 = 17.54 (teeth).  So you won't be able to have a perfect indicated speed when traveling at 65 mph actual.

So let's see what speed you will indicate when traveling at 65 mph actual using a 17 tooth drive gear.  To calculate this, you get the ratio of 19/17 and multiply that by your old indicated speed of 60mph.  This gives 60 x (19/17) = 67 mph.  So with a 17 tooth gear in this example, your indicated speed at 65mph actual would change from 60 mph to 67 mph indicated.

Now let's see what speed you will indicate when traveling at 65 mph actual using an 18 tooth drive gear in this same example.  You use the same math formula as above giving:  60 x (19/18) = 63.3mph indicated.

This let's you know that in the example where a current drive gear with 19 teeth that indicates 60 mph when you are actually traveling at 65 mph gives you 2 choices of change to your speedometer.  You can change to an 18 tooth drive gear and indicate 63mph when actually going 65 mph.  Or, you can change to a 17 tooth drive gear and indicate 67 mph when actually going 65 mph.  The 17 tooth drive gear would probably be the better choice.