I used a chart like that on Vikki's site and the drive gear I needed was discontinued/out of stock/etc. I ended up with a set of gears my local transmission shop set me up with that is close. With the compromised gears, 30 mph shows 32 mph on the speedo, and 70 mph shows 75 on the gauge. You can check it by using a GPS. My Magellan tells me how fast I'm going. A Garman does too.