I have a 2002 mondeo zetec s 2.5 with a five speed box. Can I change it to a six speed box from say a ST 2.2 diesel or 3.0 petrol. If I can will it make much difference to the mpg as I do 65 miles a day at motorway speeds and only get 30 mpg.

Thanks

I'd guess, not much (and it is a guess). How many rpm are you doing at your typical cruising speed?

You might get a similar amount out of more economical tyres (of the existing size), or, if you can put up with a mis-reading speedo, a change in tyre size.

BTW, I'd have expected slightly more than 30 mpg in mostly-motorway driving (err, at legal-ish speeds). Maybe there is a fault or it needs a service?

Edit:

Of course, even if you do only save a small amount every day, and keep on doing it, it can mount up. A first guess might be that you could save 30 - 60 p per day, which sounds trivial, but if you do that for 5 days per week and 50 weeks per year, that's £75 - 150 per year, sand that sounds a lot more like it, if you are intending keeping the car.

Edited November 9, 201411 yr by BOF

any fuel improvement would be negligeable IMHO, certainly when you offset it against the cost of replacing the gearbox int he first place.

If the gearbox is on its way out then its worth considering more, but bear in mind gearbox mounts and clutch as a bare minimum need to be upgraded. But if your just going to fork out £600+ on a gearbox for the sake of saving some money, then its not likely going do you any favours.

Pretty sure that the top gear ratios are the same between the 5 & 6 speed boxes

I don't think it's a viable proposition. Just about everything is different on the six-speed box. Even the gearchange cables are likely to be different as the 6 speed has the turret mounted towards the front. I would have thought 30mpg was probably about right for a 2.5. That's the price you pay for a v6 engine!

Pretty sure that the top gear ratios are the same between the 5 & 6 speed boxes

I think the 6 speed has a higher top-gear ratio, and a latemodel(6-speed) ST220 would have the parts/ linkages etc (not sure about the speedo, though) but i agree with others that its not worth the hassle/ cost

Perhaps a more economical car? or an ST220?