My old bike has a 48t large cog at the front. My new mtb has a 44t. I've been running the new one as my exercise bike as the old is worn out enough that I know it can't stand up to the stress of 250km a week.
I noticed though, that while the new bike being newer has better brakes, smoother suspension, etc the top speed down my favorite hill at maximum safe output for the road is only 47.5km/h compared to the 52 km/h I got on the old bike.
I suspect this is because the gear ratio is slightly lower making it better for climbing/slower speeds but not so good if you want to travel faster. I went to the LBS and they say it would be possible, but expensive to put a 48t in place of the 44.
So... is the net gain in speed actually worth spending $200-$300 on? I'm tempted just to do an n+1 and add a roadbike to my stable for the days I want speed...