Need help from a speedometer gear size expert!

4 mph at what speed ? The way I do it is to find a mile marker and note your odometer. drive 10 miles and note your odometer. Divide the difference by 10 and multiply that number by the number of teeth. so if your odo says you drove 9 miles and your tooth count is 30 then you will need a 30 x 9/10 = 27 tooth gear. or just use the charts.