Weather in May for the route you describe should be no problem , I´ve done the routes in May and June .Mex 85 is in the coastal plain and the piedmont to south of Cd.Valles and can get nice and warm in the low 30sCelsius or to high 40C if you hit a heatwave but very bearable on a bike.Jalpan is higher up in the mountains, cooler and once you get over the top you will be in semidesert, dry and not hot at all.The rainy season is starting then but that means it has a greater frequency of afternoon thunderstorms, not all day unending rain.You can almost set your clock by them, the day starts sunny and clear , the around 10 the clouds start to build and by around 2 they will be forming thick billowing thunderheads that can then dump hard rain , stopping by late afternoon or early evening. In the arid interior the thunderstorms are more spotty, you may not hit any at all. The stretch of Mex 85 south from Tamazunchale is really worth going through if you have the yen for some real highland Mexican tropical canopy rainforest and lots of curves. This part can be rainy and foggy in spots any time of year, as any of the highways which climb into the mountains from the Gulf or Pacific sides. If you do go that route you could turn west to Queretaro and you will be back in the drier interior and fast highways for a rapid return north up Mex 57 .All the heavy international hauling truckers use those interior routes because they are so direct and thinly populated
ADD. Just reread your post, again . Guadalajara Mazatlan and Durango are feasable in the timeframe you describe if you go west thru Cd.del maiz which is already in the arid zone. Note that from Mazatlan to Durango will take you a very very long days ride, best to stop at a motel in El Salto' the one at the north end Pemex station on Mex 40 by the IMSS hospital is okay and cheap with a place to park the bike on the verandah. Also at the lower end of Mex 40 there is a vado which can have some pretty fast deep water if there has been rain up in the mountains.Or it may be dry as it was the first time I went there in July1979, but two years ago in Sept it was too deep for my taste and I mooched a ride across on the back of a pickuptruck.
[This message has been edited by Sjoerd Bakker (edited 05 February 2006).]