another escort diesel

[johnboy]

my 1994 escort 1.8 TD, is using a lot more fuel than my other car (same car, different shape, same engine + same turbo). Ive thought of changing the air filter, cos common sense tells me that if the fuel dnt get enough clean air, then its not gonna give me optimum fuel consumption. however, when i looked at the air filter in my other car, its full of crap. Its almost black, its that dirty. But i still get brilliant fuel consumption from it. Why wnt my newest car give me the same good fuel consumption?
Any ideas welcome please.
Cheers, john.