How many REAR ENDs are there on LR3s?
 

Highway MPG on the other LR3s is around 18, and the one we just got pulled an honest 20MPG on the drive home. Is it possible that LR3s come with a choice of rear ends, or is there another explanation?

Same type and speed of driving, and same fuel type (premium ethanol).

air filter, state of tune, humidity, atmospheric pressure, plugs, brand of fuel, tire size, tire inflation, weight of SWMBO (don't ever ask!), number of golf clubs in back, spare tools in back, etc.). The LR3 shop manual just mentions 3.73 ratio. A 10% increase is nice, no matter how you did it.

Yep, an increase is great, and having the same rear end in all the LRs makes it even more puzzling, though. Each truck was driven from NE to Alabama over 1000 miles empty (not even a fat chick to share the ride) :D so the only change I can put a finger on is the miles on the clock. This one is in the 40s, the others are 80k+

Land Rover produces 3 gear ratios for the Discovery III. They are as follows;

Item Specification
Reduction ratio:
V6 Diesel engine - Manual transmission 3.07:1
V6 Diesel engine - Automatic transmission 3.54:1
V6 Petrol engine - Automatic transmission 3.73:1
V8 Petrol engine - Automatic transmission 3.73:1

So 3.73 for a NAS gasoline, and if you have a diesel, your are a lucky dude, but 20 mpg might be low....

Guess you could use a scanner to read engine RPM at known speed (GPS or time with Rolex for 60 seconds between mile markers) and compare between trucks. Certainly Herrod Pithaway II could have fitted the wrong ones on the production line whilst wreckless eyeballin' of new lass in nearby assembly line. This would mean one bloke with a diesel is not getting the mpg he expected.

IMHO the fresher engine is the difference.