When you get a chance, drop down to the 6.2L V8 forum here 6.2L V8 - Ford Truck Enthusiasts Forums and browse around. Several folks on here have the 6.2 with the 3.73 and 4.30 gears. If you'd like, post this question there as well and you should get some good responses.
The 6.2 forum sees a lot of action too so between the two, you should get some pretty good info.
Also, I haven't tried just standing on mine in 5th gear but I have pulled out in 2nd many times in manual mode and when you stand on it, it stays in that gear until you change it yourself. It doesn't downshift on its own in manual mode....or at least mine never has.
I have a 2013 with the 6.2/4.30 gears and at 65mph the tach is just shy of 2000rpm, probably around 1800 by my guess since the tach only has marks every 500rpm. I have also checked the speedo with a GPS and it is right on.
go with 4.30 gears, Ive read alot of post with guys owning the 6.2/3.73 combo and they cant stand them. Anything from gutless to constant shifting up and down, for the added power of 4.30 gears and the fact that they get the same MPG as 3.73 id get the 4.30 gears
2004 F-350, XLT, SC, SB, FX4, 5.4, 4R100 w/ .472 line mod valve & TransGo HD2 reprogramming kit, 4.30 gears w/ Detroit locker, zoo dad mod w/ STOCK intake, Gibson stainless headers, SPD y-pipe, 51509 cat, Borla 3" exhaust, 1" rear AAL, dually upper and lower overload spring, 08 diff cover, ProComp 22210 springs, more.....
Go with the 4.30's, you'll never regret it. As an example, I had a 2002 V10 F250 6 speed manual 4x4 truck standard cab with 3.73's (35" tires) and never broke 14 mpg. It was wrecked and I bought a new 2004 V10 F250 6 speed manual 4x4 truck Extended cab short box and have no problem getting over 14.5 mpg cruising. I have more power, way better torque, having to shift less and a useable overdrive at speeds below 60. Rpm's at 65 are just shy of 1650 with my 35's.
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