If its a regular 225 opti with torquemaster gearcase and 1.75 gears I would say a 26 trophy and maybe a 27. If its a sport XS with sportmaster gearcase and 1.62 gears you will need a 25 or 26. In each case you can also run a TXP OT4 in 1" less pitch. Txp's carry a load well but are usually 1 mph slower than a trophy. They are very durable though.