From what I can see from cutting and messing around on these cars is the engine is the same other than an update to a better computer that is more tolerant of not having a perfect engine. The old obd1 computer had a fuel table and a spark table but no trim tables. So the newer computer can make up for an injector that is slowly getting clogged or a dirty air filter or any other maintenance type of issue without going out of tune. The old ones had to rely on the fuel table and the O2 sensor to keep it right with no way to adjust the fuel table over time.
The obd2 cars are heavier and have heavier tires so I think that is where all the difference in mileage is. You should edit the wikipedia entry and put the proper info in there
It would be an interesting experiment to try to see what the difference in mileage is between the 145/12 tire and the stock 13 incher. The reduction in engine rpm might not offset the extra load from spinning heavier tires maybe.
To correct the speedo to do the experiment is easy:
The speedometer drive plastic gear with 18 teeth is the same on all(fixed on diff.).The differents are only on speedometer driven plastic gear
speedometer ratio (faster) 18 x 16(brown) for wheels 14" (GT,GTi)
speedometer ratio (slow) 18 x 17(white)for wheels 13" (GA,GL,GLX,GS)
speedometer ratio (slower)18 x 18 (yellow/beige) for wheels 12"