If I have done my math correctly, 25 KPH is about 15 MPH. I ride hard on my daily rides; and my average right now is 16-17 MPH.
Unless you live in really windy or hilly country, I find it hard to believe you have trouble keeping your speed below 25 KPH/15 MPH. If you truly are, I would shift down to a gear of about 57 gear inches; on my 1970s ten speed, that would be 52-24. At that gear; even a cadence of 90 will keep you at 15 MPH; anything faster, and it is time to apply the brakes.
Which is the other issue. If you have say (just as an example) a time trial bike with the double pivot caliper brakes; you will now be stopping for the two of you, and not just yourself. What may have been adequate when it was just yourself and a light load may now not be enough in an emergency stop situation at 25 KPH+ speeds, with say a little water thrown in.
(I am not trying to lecture to you, just giving you something else to think about.)
-James
Last edited by jhefner; 12-15-09 at 12:32 PM.
Reason: spelling