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This is a chart of HP Make/size Horsepower Lowest ratio (final:1) The numbers after the HP is the gear ratio Recon 250 15.5 HP 36.8 300 Fourtrax (after 1988) 16.5 HP 55.0 300 Fourtrax (1988) 18 HP 55.0 350 Rancher 21 HP 54.1 350 Foreman 24.5 HP 57.2 400 Foreman 21 HP 56.9 420 Rancher FM/TM 26.5 HP 46.5 420 Rancher AT (IRS) 27 HP 38.8 450 Foreman 24 HP 56.9 500 Rubicon 33.5 HP 58.6 500 Foreman 27 HP 51.0 500 Foreman (FI) 28 HP 53.4 650 Rincon 35.5 HP 17.7 680 Rincon 38.5 HP 17.7   420 Vs 450 HP

thanks Jeep! I thought I had sent this message, but it was still sitting here waiting to be sent lol … Welcome to the forum! let me know your you’re making and model and I can most likely get you taken care of If you’d like to contact me, I’ll send my direct contact information to your message box -AKATV

Welcome aboard.  Post up a pic of your meter on with the headlights on so we can see what you have.  @AKATV will be along shortly.

Try without first.   If it acts like it's running lean get a programmer.   I did one on my wife's 09 DCT Rancher.  I put a Weisco programmer on with the top end, but I honestly think it would have run fine without it.   Didn't make much of a difference at all over the stock 420 cylinder.  Not worth doing unless your original 420 cylinder is gone too far to be bored oversize again.   I was curious as to the 520 top end, but you have to either turn down the outside of the bottom of the cylinder (Reduction Boss sells those ready to bolt on) or grind the cases a little to clear the bigger jug.    I've gotten away from 420's and am sticking with the older machines, so never got around to the 520 swap.    

Thanks guys! Got it fixed up.