www.dansdiscountprops.com shows a recommendation of 13-1/4 x 17 or 13-1/4 x 19. I also updated your signature so it'd help when you ask these types of questions in the future - you never mentioned the motor year of manufacture on either motor. Also be sure to use a GPS when checking speed (don't rely on a speedo - they are not accurate). Do be aware though you will be sucking 3-4 times as much gas for probably less than double the speed gain. I use to go 3-4 days on a tank of gas with my Force 40hp, that went to just less than 2hrs with the 115.