If you can't figure it out through the formula, look at how your dip switches are set now. Find the corresponding frequency in the chart. Go up or down 5 or so frequency numbers in the chart. I believe last time I set mine I went down in numbers for bigger tires but I don't remember for sure. Test drive it and use a speedometer app on a smart phone. I leave the speedo loose in the dash and can pull over and change dip switch setting on the side of the road in 30 seconds. I've done to fine tune speedometers and get them exact with GPS.