The Fox V8 version of the T5 is absolutely a compromise. Like most OEM designs the considerations under which it was designed are different than those enthusiasts have. The Mustang was built under CAFE (Corporate Average Fuel Economy) standards where a manufacturer's fleet had to meet certain fuel economy criteria set by the Federal Government. This meant that in order to get good highway mileage they needed to run a low rear gear and use an overdrive ratio to reduce it even further. This is why the stick cars got the 2.73 gears, and automatics typically had 3.08s, though under some circumstances 3.27s were optional (quite rare too).
The quandry this presented was that the car still had to run an drive like a performance car in the first 4 gears. The only way to accomplish that was to use a tall 1st gear, so the overall 1st gear ratio was still acceptable. A 3.35 1st gear coupled with a 2.73 rear results in an overall 1st gear ratio of 9.15, which is pretty good. I find in most V8 applications 10:1 is about perfect.
That was great, but then you still had to get down to 1:1 in two steps. The engineers figured most spirited driving involved shifting between 2nd and 3rd gears. They therefore cheated those ratios closer together, rather than just create an even gear drop between all gears. This is why there is a larger RPM drop on the 1-2 shift than on the 2-3 or 3-4 shift.
The 4-5 drop wasn't seen as something about which to be concerned. Most vehicles of this era would not pull top gear; it was used solely for fuel economy. It was assumed that you would downshift on the highway if you needed to pass. The only production T5 that created a good spacing between 4th and 5th was used by GM in the 1LE Camaros. These cars used a 0.8:1 5th gear. They did this to allow the cars to pull long straights in stock class racing. Those T5s are exceedingly rare (I rebuilt one once almost 2 decades ago), and the 0.8:1 5th gearset being highly sought after is akin to hens teeth now (I had a set years back I got $500 for on eBay). There are aftermarket companies that sell different ratio 5th gearsets like G-Force, so it is still possible, but they are not cheap either.
Consequently, the Z-Spec T5 is a better choice, especially when pairing with a performance rear gear. I used to be able to buy Z-spec gearsets for $450, but they are NLA. There are aftermarket choices here too like G-Force and Astro, but these are not cheap, and unless you need the added strength you're better off to just buy a new Z-Spec trans rather than try to upgrade a stock T5.
Though, its all somewhat academic. I find the Mustang V8 T5 paired with a 3.50 rear gear and a warmed over A code 289 makes for a fun to drive combination. Its not perfect, but I paid $0 for the trans plus about $450 to rebuild it, so it was considerably cheaper than buying a new Z-spec trans. I can usually find used T5s for $250 or so. I always rebuild them, but again, that's about $350 in parts if they need nothing hard (gears, etc.), but even those parts are relatively cheap now. OEM parts are mostly NLA, but there are aftermarket parts coming out of India that are very high quality. I've used these for years and never had a problem. Side by side you'd be hard pressed to tell them from stock, and the metallurgy must be decent because I've yet to have anything fail either.
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50vert wrote:
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