Gas mileage issues after rear end replacement
 

I have a 1999 F-150 2WD with the 4.6L engine. I recently bought a salvage 3.08 rear end to replace the 3.55 I had in there (which was TOTALLY shot). Since putting the new 3.08 in, my gas mileage has gone down severely. I was getting about 300 miles to a tank of gas, now I'm getting about 265.

Also, I don't think my speedometer is reading correctly. I'll get up to 70 (that's the highway speed limit here in NC), set my cruise, and FLY past people on the highway. 70 feels more like 80. Could the new rear end have anything to do with this?

Thanks in advance for any help!

My oh my!
Where to begin?
You can't just do that on these computer controlled systems like an old carbed vehichle.
The computer known as the PCM has all the into in the program for what the stock gear ratio is, tracking speed, when to shift, speedo display, and how to handle various functions.
With such a high ratio you foot is closer the the floor now trying to move the truck.
You just threw that all out the window with such a radical change.
Put a 3:55 back in.
Good luck.

the change from 3.55 to 3.08 is about .47 ... which is about 12-15% or the original 3.55, so you are going 12% faster on MPH and the RPM is 12% less which is screwing up your milage. When you set the speedo at 70, you are actulally going 12% or 9-10 MPH faster so 80 MPH... YES.

Bluegrass, what you said is exactly what I was thinking. I figured there would be some computer issues, seeing as the truck was used to a 3.55. Is there a computer reprogram that can fix these issues?

Steve, thanks for the confirmation that I'm going faster than what my speedometer says.

Now to figure out how to deal with this mess...

You might find a dealer who is willing to make a change in the program to accomadate that rear ratio since they came with that gear as a stock build.
Don't know at this point if there is a change in shifting sked.
If it seems to upshift at what you 'feel' are normal speeds vs throttle opening, then your ok.
Often it depends on how the speed hardware is set up year to year and for the build.
Some setups sample the rear gear road speed from the ABS sensor some from the trans output shaft.
They are two different sample locations so you can see there could be an issue just from that.
Good luck.

not sure how much you want to invest, but a tuner will let you select the ratio and make these adjustments for you. you will get some added benefits as well.