Hi,

I just put 305/65/18 BFG AT KOs on my 2006 5.4L 4X4. The tires are 33.7" diameter and my stock gears are 3.55. I do notice some performance loss compared to my stock 32" tires...mainly the truck won't hold OD like it used to.

Which gears would be appropriate if I upgrade? I drive a lot of highway so I'm thinking 4.10 because 4.56 would be way too much gear for a tire less than 34".

If I don't upgrade will I strain my tranny too much having 33.7" tires and 3.55s? Thanks for any input!

Pat

 

4.10's with that tire will just darn near about put you back to stock performance. IMHO, it should be mandatory to budget for gears any time you budget for tires...the performance impact is amazing. But, you've already figured that out. It's not worth living with.

Don't forget 4.30 is an option.

No, it will not strain the tranny...just the performance will suck butt. The tranny doens't know any difference, besides the torque converter locking and unlocking...BTW, is what you're experiencing. It's most likely not dropping in and out of OD.

Bear in mind, your current configuration has cost you some fuel mileage as well. 4.10 gears, and probably even 4.30s will at a minimum return that, if not improve slightly.

 

4.30's are not an option with 4x4.

 

not gonna hurt the trans running 33.7's and 3.55's

i ran 36's on 3.55's for a year no issue

but if you want to regear then 4.10's but its not worth it. Once you break the 37" range regearing is almost mandatory to keep your trans in good shape.

theres no 4.30's for the 8.8r (front axle)

 

Ha!! Did not know that about the 8.8, thanks guys.

No flames, but I could not disagree more with 4.2trimble. I went from 3.55 to 4.10 with my 34's and it is hands down the best money ever spent to improve the performance of my truck. It fixed everything....acceleration, fuel mileage, towing capability, everyday driveability, tranny performance (converter locking/unlocking), you name it.

As I stated earlier and still maintain, a gear change should be factored into ANY tire purchase that increases diameter more than two inches or so. ESPECIALLY if you're starting with 3.55s. If you already had 3.73s you'd probably be OK depending what you do with your truck, but the 55s suck.

 

dont really want to argue but i drive a 4.2L 3.55ls 36" tires and offroaded it every weeked. and YES gears help but they arent a must have when the tires are so small. Its not in everyones budget to go out and drop $1500 on gear swap.

Now i still have the 4.2L 5.38ls and 46's and can roast them. i know the benifits from gears. But if he changes from those 33.7's to a 35-38 then he effectively wasted money. Cause he will need to yet again change gears. Got to know whats best for EACH member. Not just you.

 

No arguing, just stating my results and going to 4.10's would fix the problems he's asking about. Doubt he'll go to 35-38" on a stock lift truck, but that's a careful assumption. Im sure he's a big boy and can weigh in that decision about future plans for his own truck.

 

Thanks for all the info guys! I had already considered the need for a gear change before changing tires but I figured I would do the tires first and see how it felt driving to decide if I really wanted gears. I have heard mixed opinions on if the gear change was necessary so I figured I would try it with out first. For me to be happy driving the truck I've convinced my self that the gear change is necessary.

I guess my question comes down to...is 4.56 too much gear for a 33.7" tire? I'm going to have these tires for the next 50,000 miles and when I replace them I won't ever go bigger than 35". I drive about 75% highway and I don't want to be screaming the RPMs and burning too much gas. That is why I thought 4.10's would be appropriate with these tires. I just don't want to regret spending $1500 and not going to 4.56's if the gas mileage loss compared to 4.10 is negligible.

at 70 mph in OD I calculate about 2000 RPM with 4.10 gears
at 70 mph in OD I calculate about 2230 RPM with 4.56 gears

How much worse is the gas consumption cruising at 2200 - 2400 RPM vs 2000 RPM?

I'm guessing I will be best off with 4.10's based on my driving but any other comments would be appreciated.

Thanks!

 

i think your RPMS will be lower than that in OD @ 70 with either gear sets. Ive nver had good luck with the calculators/

what do you use your truck for? Give us alittle run down?

 

The truck is mainly a daily driver...I have a 60 mile round trip commute to work that is about 75% highway. That commute is likely to change soon though and become closer to 50/50 highway/city. I do use the truck on occasion on my parents farm to tow hay/corn wagons or haul a load of hay or fertilizer in the bed. The towing or hauling only happens 10-12 times a year or less and it is for pretty short distances. I think you already know this but keep in mind this is also the 5.4.

 

ok well IMO since you do tow with the truck even though its not frequent i say OVER gear it will help. the gas mileage between the 2 wont be much

 

If it healps I have stock 410's and 295/70/17s (33.1) and I run 2030-2050rpm at 70 unloaded. My mpg's are right around 12-13 on the highway I think 456's are to mutch

 

did you special order 4.10's they arent a standard option

3.55 and 3.73 are your standard options

 

410's are standard on the heavy duty's or 8200gvwr f150's as well as larger sterling rearend. Normal f150 is 7200gvwr or less!

 

That explains why your mileage is not that great. You have alot of weight on OP's truck.