Smidgy

It was a bad guess, but you get the point. I didn't realize he had an 03, I was assuming a '69 which had like a 2.78:1 or 2.32:1. 32.1 tires vs 35 tires are like 10% different in circumference, or like removing 10% of gear, by adding 4.56's you are adding like 19% back in. If you go straight to 35's with no gear change you truck will feel like a stock truck with 3.31 gears. If you want more gear to each their own, but I would think you would want something like 4:11's not 4:56's

JLTD

From a start, you are correct. But the top gear needs to be considered. The earlier model was .70 in 4th, and the current is .69 in 6th so he should go with the 4.56 IMO to maintain highway performance; if not, 6th might be unusable or at least the trans will be hunting and downshifting all the time.

Smidgy

Well I agree totally that you need to look at all gears, but what your saying is wrong with 35" tires. Like I said the difference is about the same as 3.31 to 3.73, is 6th with 3.31 gears unusable? Obviously, No it is not. You would be making 1st less useful imo going with 4.56 gears, I would look for less of a jump. But to each their own, through 4.56 tears in it if that makes you happy. If this was 37 tires then it would be different.

JLTD

Not to start an argument (but rather a discussion) but isn't the goal to maintain performance at the level it was out the dealer's door? If it's not then the ratio makes no difference. If it is, then you want to have the engine at the same (or close) rpms at a given speed, in this case on the highway. Inasogard has stated he had 4.56 on his '07 with the same transmission top gear ratio as OPs '15 (.69 vs .70) and it was nice.

I guess my point is that 4.56 with 35s has been verified as good by a member, whose truck has the same transmission gearset in top gear. We could discuss forever and throw math at each other but it's OP's desires that matter. If he's looking for a factory rpm level on the highway then he could go 4.56. If he's happy with the highway performance then leave it well enough alone. His tach, and the amount of shifting the transmission does, will help make the decision.

I prefer to gear up, as it takes stress off the transmission and also helps keep the transmission in 6th on the highway but that's just me. Although lower rpms on the highway aren't necessarily a bad thing. But how low is too low is up to OP to decide.

Smidgy

You would be gearing up an additional ~10% over stock in 6th with 4.56's and 35's, and 1% over his last truck in top gear with 35's and 4.56's, but you basically ignoring all other gears in the transmission and designing the truck for highway speeds. To make it like stock you would need about 3.73 × 110% (difference in 35's from 32.1)which would be 4.10 gears.

You would basically go nowhere in first before it shifted, Like I said to each their own, it is your truck so do what you will with it, but the truck is already geared heavy with the 3.73's (steepest available stock), so going 10% over that to 4.10's is going to keep that geared up feel. 4.56's is double the correction needed for 35's, but would be close to perfect for 37's. A tranny with a 4.17 first and a 3.4 reverse is taking a lot of the strain already, once it in motion most of the strain is gone.

Steve1774

If the axle code on the door is L9 it is a 9.75 Axle. In my case its a 3.55
Elocker. Part of the Max trailer tow package

xlttune

You can't just use the difference in height when figuring gear ratio changes, that number gives you a starting place, you also have rotational mass to think about. I don't know what motor he has but if it is the 5.0, he will need the 4.56 to get factory like performance out of it.

MGD

Yep.

Rotational inertia (the silent killer that increases massively with velocity)
Static mass.
Rolling resistance.
(and a tad poorer aeros).

We do same in Russia. Big meat, same gear. BUT - No worry when have no power to begin with - nothing to lose.



Salut!

WTF150

This might be a stupid suggestion, but what about just locking out 6 gear and running in 5th gear on the highway which would essentially give the gearing of 6th gear with 4.56's.

JLTD

Amen to most of that; but the lower gears will still be used as designed and as controlled by the PCM....but as mentioned before, the 4.56 would make the truck much more responsive than the 3.73...4.10 is certainly a good compromise. But if he wanted to go to 37s he would already be set.


Not stupid and makes sense from a ratio standpoint. But if you are going to re-gear anyway then simply go to 4.56 and use the entire transmission. IMO having to lock out a gear every time you are on the highway is a tedious adventure.