Mpgs w/ 4.10s

So I am thinking of going to 4.10 gears to go with my 31" KM2s. Now I had a buddy let me borrow his rear axle for a few weeks (about a year go) to see if I like the gearing. I liked the power but my MPGs dropped drastically. I know Mpgs change with the gear ratio. But I thought that with 4:10s and 31" tires you would be somewhat close to factory. Is there anything else I should do with the gears? All Mpgs were verified with my gps. Also over the last few months I have done precision injectors, K&N cold air kit, and a banks header. Would it really be worth it to change my O2 sensors? My 2000 XJ has cali emissions so i have 5 O2 to change. I believe in if it ain't broke don't fix it. Any help would b great thanks.

The ones pre-cat would be good to change, wouldn't worry as much about after-cat sensors. But you can test the sensors to confirm they are working.
Are you sure you weren't a little heavy on the skinny pedal with the 4.10s?
If you even THINK you want to go to a 32-33, then get the 4.10s.

I admit I was heavy on the skinny pedal sometimes. But on a drive to New Hampshire (220 miles one way) that I could do with just over 3/4 of a tank took over a tank. And that was cruising 65-70mph.

Depending on terrain, altitude, humidity and a number of other factors -
you really *can* get better mileage with factory gearing on 31s. In the city, my XJ brings the suck. In the hills, it brings the suck. On flat highway - at 65-70 - I get better than stock fuel economy.

Get the 4.10s, but don't assume it is going to pay for itself any time soon.

My mpg on flat ground went down when I switched to 4.56. Which is a bummer, but over all, I think mpg is better, and there's less strain on the drivetrain.

Gotta ask. Did you change the speedo gear when you changed ratios?

No I didn't change the speedo gear when I changed ratios. I verified everything with my GPS. I know it wasn't the best idea.