Is it cost efficient to Convert a Huffy Mens mountain bike to a road bike like the one I rented at Orange Peel in CO? While on vacation in Steamboat Springs CO I fell in love with a road or trail bike I rented and so did my family.  We road for miles on a trail and it was great.  I want to convert the mountain type bikes that I have in my garage that are falling apart.  They need new brakes, new seats, new tires, and possibly gears.  Is is worth it or scrap and start over?
You can put smooth tires ("slicks") on a mountain bike wheel rim instead of the usual "knobbies". These slicks can be in the typical 1.95" mountain bike tire diameter, or you can go narrower down to 1.75".

Regarding the bikes in the garage which needs brakes, seats, tires and possibly gears; those are not very expensive items. Tires are about twenty dollars; used seats in good condition can be had for a few dollars.

If you'd like higher gear ratios, it is not expensive to swap a casette or freehub. Find out which one your bikes use. Probably, I would guess, these mountain bikes have thread-on freehubs that remove with a FR-1 tool (which should cost under $10, and needs a big wrench to go with it). A new freewheel hub costs about $20 to $30, and gives you an opportunity to increase the gear ratios. The swap is a five minute job.

However, once the repair budget for some cheap old mountain bike surpasses about $50, it quickly stops making sense. Instead of spending $100 on maintenance, you can get something newer and in better condition for $75 and spend $25 on it, ending up with a better bike.

How much is your time worth? Many DIY-ers make the mistake of treating their labor as being free, and only counting the cost of tools and materials! "Look at the bike I put together with a $100 budget!" (And with only 20 hours of my "free" time put in).