The mapping comes in two different types, a breadcrumb trial you follow and it warns you if of course, or the more expensive satnav type, the cheapest would be the Garmin 200, those above this level will also feature the ability to record other data other than just the ride, like H.R. rate at which you pedal (cadence) and a more accurate speed recording from the wheel (both H.R. and Speed/Cadence require separate sensors, these may be included in the more upmarket packages) they are not necessary to get going and can be added later, in the middle are the like of the Garmin 500/10 and the Bryton Rider 40, these offer the breadcrumb trial, true mapping comes with the Garmin 800/10 or the Rider 50/60.
Garmin do a touring unit that doesn't feature the cad/sp.hr but has true mapping
Prices approx
Rider 40 £130
Garmin 500 £160
Garmin 510 £200
Rider 50 £200-230 but the 60 is about the same price and the 50 can be got for less
Garmin 800 £220
Garmin 810 £300
Garmin £190
These are just quick prices on
ebay for new.
Gears the bigger the gear on the front the higher the gear (harder to pedal) the same on the back but the other way smaller is harder, so big front small back = harder, small front large back=easier, the idea is to use a gear that is easy to keep a constant rate of pedalling (cadence), try not to use the combination of large front and large rear, or small front small rear, this is known as cross chaining and leads to premature wear of the chain and gears.
And finally