MotoMike, I believe you lived n New Mexico, correct?
I'm in Santa Fe and love riding the local ski hill road, as well as the Sandia Crest road. I'd love to see an improvement going up those roads.
Did you notice any negative issues when downshifting going down the mountain roads? How about for downshifting during city commuting?
Also did it effect highway performance in the upper gears? I have a commute that I do which is half hiway, half city.
Yes, I was in Eldorado, just outside of Santa Fe up until two months ago. I can say unequivocally that the 13T was a significant improvement for the CBR250R when riding at altitude. Riding up the S.F. Ski Basin road is a perfect example of this. With the stock 14T front sprocket, 2nd & 3rd gear were just a bit too tall when exiting the tighter uphill switchback corners. Going down the road wasn't an issue, whether in 3rd or 4th gear.
As for highway & freeway riding, you do loose a few MPH on top speed on a flat level road. On the other hand, the bike doesn't scrub off speed on the rolling hills with the 13T, whereas it did with the 14T sprocket. I'm sure you're familiar with the stretch of I-25 between Eldorado and Santa Fe... with the 14T the bike couldn't maintain 80 MPH on those inclines and would scrub off speed, whereas with the 13T it had no problem maintaining 80 MPH.
I would say that in-town riding is also improved with the 13T, again because 2nd & 3rd gear have a bit more grunt.
IMO, changing to a 13T front sprocket is a must for anyone riding this bike at higher altitudes (over 5000 feet above sea level). While this bike has about 24 HP at sea level, at 10,000 feet it's down to about 18 HP.