I did som research on the LRDG myself some time ago, but I have no references other than internet.
Wikipedia might not always be correct but in the section about Operation Caravan you can read "the jeeps were "self supporting" for about 1,448 km (900 miles) and the Chevrolets for about 2,414 km (1,500 miles)". It does not really answer your question but mpg and average speed was probably very dependent on the terrain.
Hope that helps a little.
A few minutes with google came up with this:
Engine: 216 CI 6 Cylinder inline.
Gears: 4 forward x 1 reverse
Steering: Left Hand
Fuel Capacity: a single 20 gallon tank
MPG: about 12
Wheelbase 134 inches
Payload: 3,000 pounds
Max range on a single tank of fuel: @240 Miles
Link to the websitehttp://lrdg.hegewisch.net/lrdgvehicles.html