I have a 90 BRONCO II. I took out my twin cat converters, knocked the catalyst out of them and re-installed the pipes. This got me an extra 5-8 mpg. ****Warning**** check to make sure your state does not perform emission checks prior to doing this.
Upgrade to the 2.9L EFI Motor. 1986.5-1987/newer V6. The 2.8L 2bbl carb just wasn't a power horse. The 2.8 & 2.9L shared so much of the same, the displacement was slightly bigger (NOT MUCH), both engines were almost the same other than the 2.9L was fuel injected which = better fuel mileage, better performance and reliability.
I owned a 86.5 Bronco II, 2.9L V-6 5 spd manual 4x4. I bought the truck with 170K on the clock, and drove it daily to work at 130 miles a day for 2 years. All I installed was 1 battery, starter and a clutch.
After retiring it as a daily driver, I lifted it with a 6" suspension kit & a 3" body lift, and ran 35-15.50-15 Super Swamper TSL. It became my offroader/trail machine. I did sell her after many years and miles. When I sold her, the clock had turned over 3 and half times and still ran flawlessly.
The only problem it did have, which I forgot to include, was a pretty big oil leak. It leaked bad.. It was near the oil filter. It was near the block to filter housing. After all the miles I never did fix it, I kept just adding it. So basically the leak was so bad by the end of a month, it was like changing my oil every month.
It was a good truck, and to this day I wish I hadn't sold it. It wasn't a power house, didn't have a lot of room compared to today's standards. BUT it was a real SUV and not like a car cross overs of today. It drove like a truck and you knew it when you drove it... But was really great, dependable, and I have to say fun to drive. It was a very capable truck, even in stock trim.