Diesel is available in almost all stations. You'll be fine with the normal fuel range of a diesel car. I recall some Gobi stretches which were perhaps 300km between towns, but that was heading out into real wilderness. There is no problem with obtaining diesel. The only thing is the variation in price. Currently, T1270 get's you a litre in UB, while the most expensive I found was T1500 in the south of Gov-Altai aimag. Generally, the further from UB or an asphalt road you go, the higher the price. It's normally around T1400.

Daniel

See at the bottom of page for digital map download
Landkarte Mongolei (1:1.600.000)
They are also selling other excellent maps

Driving&Navigation in Mongolia is anyway quite relatively easy. Last summer we had papermap, one Garmin and one PDA-GPS equipped with OziExplorer and Mongolia map downloaded from Reise-Know-How (my GPS)

I never actually felt that i was lost. I knew roughly where we were all the time but sometimes roads disappered and at least once we just headed direction where next city should be and eventually found road. Few times we had to search for the track and we did some extra driving. Also sometimes Garmin showed different route what reise-Know-How map/digital map and there were new roads which werent on my map/GPS nor Garmin but you could always watch where cars were driving and ask from locals (but dont show them map :) )

I think longest distance was something like 250km-280km in so called central route whichout fuelstation but beside that you could find fuel from almost every bigger village. Last option would be also always to find some ger and try to buy fuel from locals (we got petrol sales offer once in one house near lake) In eastern part 80 octane was more common but toward aimags capitals and UlanBataar 92 started to appear. We had our bikes once filled from canisters. After that always from pumps (many times pumped with hands)

That Roadatlas of Mongolia seems nice but quite expensive. GPS information of fuel stops could be usefull even though situations can change and you can see abandoned gas stations in villages. So it doenst mean that if there has been once station that it will be there next year. If I was feeling that i am unsure about petrol availability i tried to ask from village and i think Mongolians and i spoke equally bad russian so we did fine.

Mongolia is great country and now when i am putting my ride report together from last summer and watching videos, photos and tracks from GPS i really want to go back there summer 2011.

Thats the one! highly recommended

(1) Garmin world map is completely useless.

(2) I would get both GPS maps and the atlas mentioned above.

The GPS maps available from Worldwide routable Garmin maps from OpenStreetMap are the best I am aware of for Mongolia