Maybe it's just a regional thing, but over the years we have had may OBS F250's and all but 1 had 4:10 TTB's. I have a 94 out back with a 4:10 TTB D50 and another GUTO 4:10 TTB D50 in the barn I'm saving for hard times. If the OP is really wanting to do an upgrade, he might want to consider a SAS and a RSK. That would work well with those big tires and gears, no matter what ratio, would be easier to source.

 

Again, awesome, it has the ZF ratios in a drop down, I put in his figures for kicks, I suggest you all do it to.

 

Think of the axle/ tire combo in terms of simple levers, there are two in action here one is the R&P ratio and the other is the ring gear to tire diameter ratio. When totaling these the R&P ratio is always positive and the ring gear to tire diameter is always negative. The OP reduced the mechanical advantage of the final drive system when he installed larger tires and that will reduce torque at the ground in all trans gears. Do the math... it's not hard

 

You say it in a convoluted way, but no this isn't wrong, and IF the truck had one gear then you'd be absolutely correct, but it doesn't it has 5. Going from a 235/85R16 to a 35/12.5R16 is a 9.4% change in revolutions per mile. So at any given speed the difference is a 9.4% reduction in RPM and the need to drop to a lower gear 9.4% sooner, that is all.


The math says his current setup is doing about 1800RPM at 70MPH, this is generally a little on the low side for a 5.8 but fair for a daily driver where MPG is a concern. Going to 4.10s would bring that number to about 2100, better for power. But more to my point, so would just dropping to 4th gear when needed, 4th @ 70 is about 2350RPM. If it won't pull the hill at 2350 then why would it pull it any better at 2100? Inability to maintain speed up a hill has nothing to do with gear ratio in this example. It's either a lack of power or a lack of driver. Going to a lower axle gear only removes the option of cruising at a low RPM hurting MPG engine life, noise levels, etc.


Now if the OP never intends to go over 55, and/or tows a large wind drag of a trailer a lot then maybe it's worth it. But the OP said he "drive(s) the truck everyday and don't want to lose to much mpgs"


This says it all right here, the truck is a daily and mileage is a concern. Just need to dispel the myth that "power" can be gained with a gear change, it can't. A lower gear can give the illusion power by providing more off the line axle torque, that is all.


The answer to the stated problem of the wish for more power and..
Is that it doesn't have enough power, not the gearing.


Power doesn't have to mean supercharger, could be as simple as a tune up, fresh injectors, bumping the timing, etc. This is the place to look to better pull a hill without loosing MPG.

 

I don't know why you say that.
They are everywhere around here.

Agree with everyone else that they are kinda small for 35's in a 351 truck.

 

Here is the truck. It's got an 87 F350 front axle. And the rear is posi. I remember when I switched my mustangs gears 3.27 to 4.10s. It made a huge diff. My buddy who would put car lengths on me (to many to count). Was behind me by a fender through the top of 4th after the gear change. I know you don't gain any power from the gear change, but the truck is now geared to high bc of my tires. 70-75 mph is around 2,000 (I have to check it again bc my speedo is off). It is hard to stay in overdrive on a two lane road bc to rpms are so low. When I say the truck has good power I am comparing it to my buddy's automatic. I certainly don't mean that it is fast lol.

 

If it wasn't for the speed density. I would change the HCI. I need to get this truck switched over sometime.

 

So yeah you truck would benefit from some power adders like longtube headers and GT40 heads, and it would be a good idea to recalibrate the speedo so you know how fast you are driving and how far you are gone. The truck is in no danger of damaging itself with the current gearing but it's now got the equivalent of 3.20 gears which isn't a performance setup by any means, and OD isn't of much use except on flat roads at high speed. So should you do a gear change? It would be expensive and would take a long time to pay for itself, but you know from first hand experience what difference it makes.

 

If you do the gear swap, I want to buy your 3.55 stuff. PM me.

 

Ill let you know. Might be a little while gotta refresh the 89 mustangs 302 this spring. I should take the ported gt40's off of it for the truck.