A gift to all full version users - unlimited number of offline and custom maps for FREE until the end of June! Be sure to grab it in "Advanced Settings" > "In-App Purchases".
You can always have one offline and one custom url map with no need for in-app purchase!
In case you need more maps, here is information on what types of maps are provided and what are they good for.
You can download maps when you have a good unlimited connection (e.g. via wifi at home) and then use maps with no need for internet. To download a map, you point speedometer to a map server/source that provides map tiles (images). Those images are then saved to your iPhone/iPad to be used later with no data connection. By default, in speedometer you have download sources for MapQuest street, MapQuest satellite and author's lab/beta map servers. Speedometer allows you to add more sources, here is a list of a few available.
What's the download size?
Total number of tiles/images depends on area and zoom levels you include into download. To give you an idea:
Monaco and Monte Carlo
Size of download from the lowest to the highest available zoom levels will be around 10MB.
Las Vegas
Zoom levels between 6 and 16 should be totally sufficient for driving, making whole Las Vegas map a 71MB download.
That's a lot of data?
Take it the other way. That's how much data you'd need to drive with an online map and that's what you can save by downloading it and driving with not even a bit spent from your data plan.
There are many map servers providing you with awesome and specialized maps (topo, cycle, marine, etc). Most of them are based on OpenStreetMap - a map of the world, created by people like you and free to use under an open license. Examples of such maps are OpenCycleMap or Stamen maps.
Some map servers provide data on a subscription basis, as MapBox, and you'd need to add your license key into the url. All those servers can be added as map sources.
Once you have url for a custom map server entered, you can either use it online or for downloading offline maps.
In the future we plan to add support for an ellipsoidal mercator (Yandex maps is the only known provider) and quad url schema (used by Microsoft Bing maps).