ORIGINAL: Fallstar01

I have some gear questions.

First, how much does it cost to get them installed and is this a possible DIY? usually 150-200, and its not a DIY unless you've got an experienced friend with you that can show you the ropes. This isnt as simple as an alternator change, as this requires possibly some setup and measurement work - much like building an engine.

Second, when deciding on gears, it's apparent that highway vs. city driving matters
but how much? What kind of MPG loss/gain can someone expect from stock? It seems to me that this gas milage dispute is only important for your top gear cruising. Is the bit about city driving (in higher gears) improving gas milage significant? You'll likely get better gas mileage around town but get worse on the highway, but were only talking about a couple mpg either way.

Third, when deciding on gears, should someone with an automatic go higher or lower than
someone with a manual. yes, only slightly. 4.10 is a good gear for an auto for those that don't wish to rev too high - 4.30's would be even better

Fourth, should someone with a manual 5speed (T5 or 3650), choose different than someone with a 6speed (T56) as found in a cobra. And on that note, does the 6th gear allow for better gas milage or is it that the 5 speed's gear ratios are just distributed out among the 6 gears? (like is 3650 in 5th equivelent to T56 in 6th) No I wouldnt choose a different gear based on being a 5 or 6 speed, through the dragstrip you only use the first 4 gears. But... with a Cobra or Mach1 with a higher rev limiter I would go with a higher (numerically) gear. You want to ideally be as close to redline as possible in your finishing gear at the dragstrip. So, usually 4.10's are being run in 4.6 GT's and 4.30's in DOHC 4.6's

Fifth, I hear that ECU programming becomes neccessary. How neccessary, and how cheap can it be done? Its necessary to correct the speedo, a chip or ECU reflash is used. On m/t cars a speedcal can be used, and on pre 99 auto (I think) cars a speedcal can be used.
Sixth, I noticed that Saleens and Cobras seem to have 3.55, Is there a reason that they found that this upgrade was some sort of ideal/siginificant number? Cobra's use 3.55's too, that gear is slightly higher than the stock 3.27 and provides some acceleration over stock gears and not really any decrease in mileage. Its not worth it to swap 3.27's to 3.55's though IMO - if your gonna swap then go at least 4.10's or you'll likely regret it

Just a tip, I talked to some guys at a dealership and they said get the actual ford part, (not motorsport). They are apparently the exact same part (ford box with motorsport sticker put on it) but motorsport charges more. Thats usually the case.