BrumJim said:
I'm looking for a high-speed commuting bike, so probably a road machine. Then, if it is light enough and the mechanicals are good enough, I'm hoping to chop 5 minutes off my average time. Realistic?
IMHO 5 mins out of 30 is a big increase, and the determining factor is probably biological, not mechanical. Depends on a few things about your current arrangement though:
-a lighter bike is often quicker to stop and start, but tops out at a very similar speed to a heavy one. Unless your route has a lot of climbs or stop lights, I don't think you'll see much difference here.
-a more reliable and efficient bike will allow you push with greater confidence, and may slightly increase your top end speed.
-a new bike almost always yields some improvement, just because you ride it more and push harder.
I'm assuming your current bike is reasonably fit for purpose, but if its really bad, you could get 5 mins here.
Then there are the special multipliers of 'fit' (is the saddle height correct?) 'cleat' (are you clipless?) and footing (are you on road tyres?).
If any of those is a no, you can expect significant gains, but they won't necessarily require a new bike.
To put it in perspective, I have 2 bikes I use on a regular 17 mile very hilly commute. Both are mechanically sound, both have cleats, both run road tyres. One is super lightweight (for its kind) and is set up for racing. The other is 8kgs heavier and is a very comfortable tourer.
When I only had the heavy one, my average speed was 12 mph. Including time lost to lights and traffic, the journey took around 90 mins. After I bought the new one, I slowly got the average up to 15+ and could do it in an hour and ten. 6 months later and iit takes an hour an five, on *either*.
The main difference has been my fitness. The new bike was a fun way to improve that, but wasn't directly responsible for any of the gain.
Anyway, this is just my experience. There are some lovely machines out there, and I'm always a fan of n+1.