Well First of all, Late Congratzz on Buying a beautiful Beast, to me the corrola's big body is meant for these Big Engines! Your Ride is very Great, Just add Original Fog Lights to it, & keep your ride Full-time Maintained & Grippy, Cuz if you don't care of Especially a diesel Engine, it sounds Really Bad after some years of Non Caring Use!
As for your query, Don't have great knowledge about Corolla's diesel engine, but know this for confirm
1.Scientifically, Diesel engine gives much more output than petrol engine, Consumes 35%-38% of fuel & don't contains spark Plugs,while petrol engine just 25-26% So I think its great average
2. Coming to facts, comparing average to Counterparts:
Corrola 1.6 Dual VVTi (11-11.5km) Avg
Civic 1.8 i-VTEC. (10-11km) Avg
Civic Hybrid 1.3Ltr (19-20.5km) Avg
City 1.3 iVTEC (15-16Km) Avg
Dont have idea about, Xli Gli, Cuz none of my Cuzns or friend have it!
So to my point of view, it is giving you a very great average!
Well,
thats what I can tell you buddy!
*edited