I would use the 26city/32highway/28combined rating. You said you don't have the 4A-GE, so its not that one. According to fueleconomy.gov the 20/26/22 is for a Corolla with AWD (see the right most in the image below).
Also, I'd be very surprised if your 89 corolla was actually carburated (no offense). Toyota probably still used the 4A-F in 89, but I doubt in this country. If it is a 4A-F, I'd guess the engine had been put in at some point. If it was offered here for that year it would be a selection on the fueleconomy.gov site since the EPA would have to know if it was sold here.
Congrats on the very nice mileage too!
