Go to Astro performance warehouse and get the 2.95 Super Alloy gearset which includes 1st and 2nd and 3d gears, new input shaft, and a cluster. Makes the box Z rated to boot. astroperformance.com is where I found them and at 5.0 Mustang.
I went to 3.73 and hated the highway rpm's. I am using near stock diameter tires/wheels (245/45/16) so I went with a 2.95 gearset and love the results. Great acceleration and a large step down in rpm's on the highway. Almost the best of both worlds for me.
IF GAS MILEAGE IS A FACTOR- Hope this helps.
I just drove my car to NY from FL. It has the 3.73's. I got better gas mileage at 80mph than I did at 75mph. Anyway, gas mileage shouldnt be a factor because I got 23 mpg on the hwy at 80mph.
I've had 3.73's and 4.10's.
The car has been a daily driver and track toy with both gear sets.
Personally, I like the 4.10's much better.
If I was freeway driving at 80+mph most of the time, the 3.73's would be more livable.
With 3.73's my car went 13.18 @ 102.5mph stock motor, stock suspension full weight.
With 4.10's my car went 12.96 @ 104.0 with same stock motor, stock suspension full weight.
That's 2/10ths and 1.5mph gain, doesn't sound like running out of gear to me.
As far as cutting out at 100mph... um... I've pulled 5500rpm in 5th gear with the 4.10's... that is dang handy to 150mph... and it got there quick.
Maybe it's the car. My buddy's seems to act like it's not pulling hard anymore with the 4.10s in it once he gets to 100 or higher, where mine starts to pull harder.
4th stays at 1:1 but 5th drops from a .68:1 to a .63:1 ratio. The over all effect is like going from 3.73s to 3.45s if you were to keep the same .68:1 overdrive.
I have 3.73s and it runs 3000 rpms at 75mph in 5th gear which really sucks. After playing with the math that works out to the .79:1 overdrive. I just got the car this summer so must be the tranny has been reworked to get that overdrive ratio