rts9364

Me, too, maybe. I'll be installing a supercharger soon, for which I am still trying to sort the tuning. So there is a good chance I will incorporate the gear ratio changes into the PCM tune, but since I already have a Hypertech unit I will use that in the meantime (if not indefinitely...). I think that the PCMs are smart enough now that it doesn't really matter which way you do it, but I have yet to see that confirmed.

mraz72

I have a couple of ignorant questions

1. what is the benefit of 410's over say 331s? I see the OP is getting better gas milage, is that also the case with towing? Growing up with friends that liked to talk about 1/4 mile times, it was thought that a 4:10 rear was good for racing but might rev too high for a daily driver.
2. What is the downside of having 4:10s?
3. Another member mentioned that he thought the rear made less of a difference with the 10 speed trans, what do you guys think of that?

(I didn't want to start a new thread just for those questions)

Apples

One benefit is when you change your wheels and tires to something a lot bigger.

If I have a stock truck with a 3.31 axle gear wearing Michelin LTX 245/70-17s, that tire's diameter is 30.5". What happens when I bolt on a set of 35" tires?

Well, I've changed the tire diameter about 4 inches... which results in a big change in the truck's effective final drive gearing... switching to a 4:10 would more than the gearing required to get the effective final drive gearing I had with a lil' tiny 30.5" tire.

A 3.73 would do the trick... but then I'd still have a somewhat "tall" *effective final drive* gearing just like when the truck was stock, so jumping to the 4:10 gear would be beneficial to both acceleration and perhaps cruise, since our trucks have double overdrive. Be like having a 3.55 with the tiny stock tires.

SilverSurfer15

1) yes, BUT that was before overdrive transmissions. At least most of that theory. In most cases, its hard to have too much gear. You could put 4.10s in a stock f150 and be totally fine. Especially if you dont drive over 70, the O/D ratios are pretty aggressive.

2) the downside is higher hwy rpm which *can* lead to worse gas mileage. But again as in #1, if you currently run 1400rpm at 70mph, and then you did a gear change and ran say 1700rpm at 70mph, your mileage wouldnt change that much. A good rule of thumb, if your highway rpm is under 2k you are fine. You wouldnt want to regear to where you are running 2300rpm at 70 or something that extreme.

3) So the 10spd, you get more available gears to work with so you can always be in the right gear, so to speak. Theres some truth to this, it allows for more tolerance but you still need the rear end to be correct.

A lot of these trucks are undergeared from the factory IMO, they should be offerring 3.55/3.73/4.10 instead of 3.31/3.55/3.73. You want the more aggressive gearing for acceleration, towing, or just general moving resistance (wind, hills, etc).

The only downside is the increased operating RPM at all speeds, which can mean less fuel economy. But its not as black and white as that, if you are undergeared and always putting a heavy load on the engine at a lower rpm, your mpg will suffer and its terrible for the motor/transmission to be lugged down that hard all the time.

using the 10th gear (.63 o/d) the rpm would be estimated as follows at 70mph:

3.31s: 1608
3.73: 1812
4.10: 1991

This is using a 30.5" tire as mentioned. None of the rpm is that crazy, the 4.10 is probably too much for someone who never loads down the truck or tows, but if you do, or you live in say CO or TN with the giant elevation changes, more gear is nice to have. the 1600rpm from 3.31 gears is just too low for anything less than an ideal road, IMO. I mean thats less RPM than a modern powerstroke turns at the same speed.I guess the idea you downshift to accelerate, but i doubt it stays locked in O/D that low with ANY real resistance. And if its always dropping in and out to 9th, whats the point.

rts9364

^
^
Exactly, and especially this:

If you are running comparably lower RPMs, but are having to downshift on every little hill you come to, then your mileage is going to go to hell. So, while, say, 4.10s are going to have you running slightly higher RPMs at cruising speeds, that "cost" is offset by the mechanical advantage you are providing for the engine.

I had 3.55s in my truck with a 5.0/6-spd, and after going to 34" off road tires, I basically can't use 6th gear anymore except on the flattest of roads (or down hills). I'm going to 4.10s which should let me pull up most hills in 6th gear again.

Newfordguy29

a 5.0L with the lift and 35s with the 6 speed, rolling with a 3.31 rear end is drive able but a damn dog. It's driveable yes but highway drivin is only good when you lock out 6th so your able to pass people or your constantly down shifting. Too low of rpm for the tires, with lower gears (4:10) your able to raise the rpm and push the weight (tires) effectively to where your engine runs the way it should. Mpg will suffer if you have a heavy foot

SilverSurfer15

yea theres a sweet spot, and it will be different for everyone based on their truck parameters and location and use and all that. But in general, its better to slant toward to the too much gear side vs the too little gear side.

Newfordguy29

4:09s dropped In today, truck feels great. Smooth accelerating and sport mode awakens the coyote from a 35 inch tire sleep lol

Toprat

My 4.09's are in the garage awaiting install. Going out of town and will set up install after I get back. Have you seen any improvement in mpg?

Apples

Hahaha... outstanding! And, poetic.