alotawatts

I have a 97 EX automatic with 86000 miles that gets 20 mpg in town at best. I purchased it two years ago with 78000 miles. I have done the following:
Timing belt & tensioner
water pump
cam and crank seals
valve adjustment
Front/Header O2 sensor
Distributor cap & rotor
Plugs and wires
Trans fluid flush
Fuel filter
The car starts and runs great. The transmission shifts smoothly up and down gears with the exception of some slipping when engaging D-rive to take off. I does have a pronounced whine in Reverse but I think thats normal.

It appears the auto trans may have been swapped earlier in life as there are hand painted / non factory numbers on the trans case in ink which may indicate a rebuild ?
I know an EX will suck gas / VTEC but I know I should have better mpg and I'm wondering if it's worth the cost to have diagnostics done on the trans ? Or maybe a Cat/Conv check ? My next part swap will be the rear O2 sensor.
All thoughts appreciated
CH

sicones

any check engine lights? automatics get shitty gas milage