When making these performance comparisons , you have to be sure that you're comparing apples to apples as they say. You don't mention what type of airspeeds the given figures are. What you are after are TAS (true airspeeds) not IAS for conditions on a given day. Also other conditions like loading of each airplane, how they are faired (ex. are the lift struts airfoiled) tire size (that you did mention) and prop type and pitch, type of gear etc . If the IAS airspeeds in each plane aren't correct than the the airspeed numbers you see will not be correct. I have a Series 5 with a Continental IO-240 engine (125HP) . empty weight is a porky 960lbs (heavy engine at around 250lbs, autopilot and stuff) well faired with airfoiled lift and stab struts , grove gear with Desser 8.50 (22") tires , Sensenich wood prop (70 3/4"dia. x58 pitch) . Having said that my TAS is 120 mph at 2500 RPM @ mid density altitudes of 3000-8000' . average day climb rates on a standard day at a solo weight of 1200-1300lbs would be around 800fpm at Vy. So before doing an engine change , I would verify that all your numbers are good and to do this you have to do a side by side test to each airplane you are comparing to. Bruce N199CL