he's talking about one of these:

 

Check engine light? I would check the fuel pressure and exhaust back pressure. Of course it can be a lot of different things but most of the time when I get a "low power" complaint and no check engine light it is from a partial plugged Cat. What RPM are you at @ 90 MPH? Is the car going into 4th gear?

 

the fiesta i meant was the euro spec one similar to this but a 2006 silver 5door.



I suppose at some point a gearset change may have happened. I currently have the rear end off. How do i check which they are. will it be on the casing or the gears themselves?

as for speedo accuracy it is pretty much correct. MY stanav has a speed reading on it which is only 3mph different to the dials.
The car is running standard 16inch alloys with standard Pirrelli tyre sizes.
Once its all back together and back on the road ill get it on a dyno and have a power check.
If its not a power or gear issue then would a slipping clutch be the next problem to have tested?

 

Your ring gear should have numbers stamped into the end. Divide the smaller number into the bigger number. Or, count the number of teeth on the ring & pinion gears and divide the small into the large. ex: 41 ring / 10 pinion = 4.10's.

 

ok thanks for the advice. much appreciated

 

Sorry ive just noticed i didnt answer questions left by GORN. The cat does rattle a lot and the exhaust sounds rough when its revved up. The car has been off the road a few months now and im unsure what revs are like at 90mph off the top of my head. I do remember however that 65 in 5th was only around 1800rpm. The car easily does around 75 in 3rd then pulls a little in 4th then 5th is just a completly dead gear as far as acceleration was concerned it just drops the revs right down and doesnt pull at all. Its good for fuel econ when cruising though. Ill have a look at the rear at weekend when i have time and see what gears are in. I think id have trouble finding another set in the UK with what little money i have left for this project though so if power is ok ill probably just have to deal with having a slow thirsty car.