I'm probably going to upgrade my diff to a 4.11.

What is the size of the stock diff?

Also when it's fitted is that all I need to do? I've heard about incorrect odometer readings?????

Also and most importantly how will this effect fuel economy?

This should be in the Gearbox, Suspension, Brake & Driveline Workshop forum.

how can you make the call "im going to upgrade my diff for 4.11" if you dont know half of the problems associated with doing so?? what makes you think this is a good idea??

i felt the need to bag you on here aswell but ill just be civilised about it...

what car? (to determine stock diff)
to make sure your speedo is not out you need to buy the speedo gears which go into the transmission. ford dont make 4.11 gears so you either need custom, or get an electronic speedo corrector unit from jaycar. but thats probably too complicated for you to assemble.

and least importantly your fuel consumption will go up if you do highway km's and down if you drive around town alot. and you guessed it, somewhere in between if you do a mixture.

why bother with 4.11 on your pile of crap car anyway? i thought you said you didnt wanna make it into a performance car or something...

Do a search!