I found UK Jeep wrangler MPG and used the 1.2 conversion factor to convert from UK to US gallons.

The UK 3.7L
Combined = 24.4 mpg = 18 US MPG
highway = 32.1 mpg = 26 US MPG :confused:
City = 17.2 mpg = 14 US MPG

The UK 2.8L
Combined = 28.5 mpg = 23 US MPG
highway = 34 mpg = 28 US MPG
City = 22 mpg = 18 US MPG

Two things ... the UK highway mileage of 3.7 at 26 MPG (US) seems much higher then the US Highway mileage of 21 MPG (US) .
this seems higher than test methodology caused differences.

If these number are true, then base on the current diesel prices, the 2.8L will not save much if any money at the pump.

but the 2.8 will get you 302 lb ft @ 2000-2600 rpm
174 bhp @ 3800 rpm

and the 3.7 gives you 232 lb ft @ 4000 rpm
Maximum power 196 bhp @ 5000 rpm


here is the UK Jeep MPG site
http://www.jeep.co.uk/jeep/versions.aspx?ID=422,2,174

 

I think that your numbers are off. Figure out what the consumption is and then divide by 4.54L /Imperial Gallon and then 3.78 US Gallon. If you say the 3.7L gets 24.4 combined that is a usage of .189l per mile. The vehicle should have the same fuel usage rate regardless if the measurement used (US/UK). That means that the US MPG Would be 20mpg . (3.78L /.189l per mile = 20mpg) You have it listed as 18.:confused:

For highway you list UK 32.1mpg = 4.54l / 32.1miles = .141l per mile.
US would = 3.78l /.141l per mile= 26.8 miles on a US gallon

Seems correct to me.