Gas mileage w 4.88's or 4.56's

I'm looking at a used DCSB, 4x4, v6, auto, with 285's. It's at a dealer about 500 miles away from me. I think the PO re-geared it, but the dealer was not aware when they took it in. The dealer said that at an indicated 60 it was turning about 2100, which would mean 4.88's if the PO re-calibrated the speedo. I'm wondering if anyone could share their real world mileage with 285's and 4.88's? (or 4.56's?). Also, if I were to re-gear it back to 4.56, 4.1, or stock, what would I need in addition to the ring and pinion gears?

Thanks

 

My gas mileage with 4.56 is are around 13-15. I get about 250-60 a tank.

I am running on 315/70/17.

 

Gears front/rear

You need Master install kit for front/Rear

solid pinion spacer Front/Rear

 

You are trying to determine your gearing from your MPG? Good luck.

A GPS will usually give you reliable real speed, if the speedo is in question.

 

If it has been regeared and you want to change it back to stock honestly I would just look for a different truck. It's a pretty big expense to do it right. Unless you happened to know someone local who would swap 3rd members and front diff with you. I did that once years ago with a guy who had 5.29's and just did a V8 swap on his 88 toyota. My 86 4runner on 35's was much happier with 5.29's than 4.10's. If the gearing is correct for the tire size the speedometer will actually be accurate too. And mileage on a truck with larger tires should be better than it would with stock gears. Another thing you could verify with a GPS plugged in.

 

This^^

 

LOL. Trying to get close. Hopefully the sales guy will use his phone speedo app to get an accurate speed. I'm trying to figure out the answer before they do. Helps negotiations if they are a little panicked because they didn't know the truck was re-geared.