If you put in the XR auto Trans, then No, the speedo with be Correct ONCE you install the diff.

THANK YOU verry verry much at least someone is reading what i am asking and giving me a correct answer...

I had to re-read a few posts,

basically all clusters no matter v8/6cyl are the same.

Providing you have original equipment then no problems.

However if you had retained the fairmont box, it would most likely had the speedo drive for a 3.23:1 diff.
Making you car slower than the speed reported (meaning no fines!)
Also making the gear changes (auto only) not optimum for the diff ratio.

XR ECU's have a slightly more aggressive shift at different rev's so if the car had a diff ratio of a different ratio, it wouldn't benefit the ecu.

(i hope that makes sence)



But as you've changed the whole lot BUT the cluster, you can drive with a smile!



The only way to be certain is to check, You May not know if the donor car has had a reco box or not, if so, there's a chance the car MIGHT have the incorrect driven gear.
Just double check everything, I mean if you've sourced parts from various cars, there might be some slight differences, I've seen XR's with 3.23:1 /3.08:1 diffs. (obviously changed at some stage)

Most of these GPS's have pretty good speed calculators, test with that or even the old' look in another car/ partners car test/

sorry to confuse you there

thanks heaps the best help i could possible get you are a champ thanks again..

My post wasnt a question, I was stating a fact that i learnt from ford mods?
Maybe if people read their posts a bit better, the answers might be there in front of them.

Correct, I had read 3 posts all the same each with different titles!

Hell, I had to read a few more times because some sentences with question marks on the end of them weren't questions!

To my knowledge all falcons have wheels close to around 25" in overall diameter. Basically they try to keep them all the same, and they are so similar as to be the same, whether 14", 15", 16", or 17". I think once you go to 18's or 19's, and certainly 20's, the diameter pretty much necessarily increases, an inch or maybe two in the case of 20's.