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School me! need new gears with 37's?

14K views 26 replies 14 participants last post by  M3NNIS 
#1 ·
ok enlighten me please. I tried to search but not sure what to search for. if you can answer or direct me to a thread that has already answered.

I am putting 6 inch and 37's on my 2012 F250 short. Do I need to change gearing?
I will tow a boat and a camper. I also daily drive the truck.

along with that. please tell me what the different gearing numbers reference.
thanks.
 
#4 ·
I have the 3.55 gears and 37's. I tow an 11k lb trailer with no issues. I am normally turning around 1600 RPM at 80 w/ no load. (note....the speedo says ~73 when I am doing 80).
 
#8 ·
If you were running 20" rims from the factory then that is tallest the computer will accept. Mine came with 18's, so when I bumped up to 295's, I had the dealer set the truck for 20" wheels and 275/70/20's and now I am only .5 mph off.
 
#9 ·
I think the max they can set the tire for in the software is 34" :(
 
#11 ·
When regearing the thing I learned was every inch of tire you go up you add .10 to your gearing. Example would be if you had 30" tires with 3.55 gears you then put on 37" tires you would add .70 to your gears resulting in a 4.25 gear.

Then you figure whats the closest gear to that which i believe is 4.30. It should help in getting the speedo closer but wont be as accurate, but better than if you didnt.
 
#12 ·
I do know that H&S's site specifies 34", just read it today.

I wonder if you could change the gear ratio (in software) to compensate for the tire being bigger (set it to 3:31, though they are really 3:55) or if that would mess up the active systems
 
#13 ·
Upgrade to 4.30s and truck will be a lot more fun and pull better in the hills. I understand the 6.7s are torque monsters, but 37s and 3.55s is a lot of strain on the drivetrain.
 
#15 ·
I wanted 37s too, but with the stuffed look. Can be done with the 12.5 wide and minor trimming. You shouldn't have any trouble with a 6" Lift.

Problem I ran into was that the 37" AT tires drop to a D rating. And almost none with the 12.5 width.
If you are going to run a mudder, there are a lot more options.
I'm hoping that by the time I wear these out they'll have a AT in the size I want.

I ended up with a 35x13.5x22 w/ a +10 offset on a 22x10 wheel. Stuck them on there the other day and they look perfect cleared everything even w/o the leveling kit. They stick out 1.75" which is perfect for the bushwacker flairs.

The bumper bracket is what you'll hit w/ bigger tires. Easily fixed with a cutoff wheel.

Good luck.
 
#17 ·
I run 37 inch trail grapplers with 3.55 gears and I have no issues with it. I also had 37s on my 6.0 that had 3.73 gears and it wasn't that bad either. These 6.7s have a real low first gear in transmission which makes it just as easy taking off.


Sent from my Autoguide iPhone app
 
#19 ·
Your still losing tons of power with those high gears. The same truck with 37s and 4.10s or 4.30s would be much more fun, much more pulling power, better fuel economy, and less strain on drivetrain. Especially mountain pulling.

I know gears are expensive and it's the main reason people shy away from it.
 
#20 ·
Also, I think jetjock15 is running 4.56s with 37s with f/r Detroit lockers. Sick rig.
 
#21 ·
Is there a link to somewhere that explains the gearing in more detail?
Im still unsure.
so 37 with 3.55 will work but its putting a lot of strain on the truck? right?
and 37 with 4.10 would be like normal? would it hinder hwy performance when not towing?
and 35 with 3.55 would be fine? like normal?
man this sucks. I dont want to wear anything out pre maturely out of reason that is.
maybe no 6' lift and no 37's
hmm 35 and 4"
 
#22 ·
This link below will give you an idea, it's not 100% accurate for our trucks due to the 6-speed 5r110 gearing, so adjust accordingly..

http://www.quadratec.com/jeep_knowledgebase/article-26.htm

37s look awesome, but it's a big heavy tire, and will change the dynamics and parasitic loses of the truck, regardless of those who say "it runs fine" or "no problems"...Ford went with higher gearing for these trucks strickly for fuel economy, knowing that the 6.7s are torque monsters and could make up for the lack of gearing...

Personally on the 6.7s, 35s with 4.10s and some detroits would be heck of a pleasure to drive:D
 
#23 ·
My expereince...

I put 4.56 in my 7.3 for 37's because when on the highway towing my boat (27' baja ~8k pounds with trailer) It wouldn't shift into overdrive - but that was a 4spd transmission.

Once I upgraded the gears - I didn't even know that boat was back there. It was awesome.

It was awesome till I decided to put the truck back to stock. Stock tires and the 4.56 were way too much! Luckily I ended up just swapping axles with someone so that I could go back to 3.73 but the entire ordeal was a mess and my 4.56 swap cost me $1800 with a new locker in the rear!

In hindsight, I wish I had tried tuning first to fix the shifting issue rather than "waste" $1800 on the gear swap.

I also would've gone with 4" and 35's over 6" and 37's. Don't get caught up in bigger is better. As soon as I get my 6.7, it'll be leveled and 35's and DONE. I might add lockers to front and rear, but won't change the ratios.
 
#24 ·
I'm not sure where the speed sensor is for f250's but in my 2006 f150 when i regeared to 4.88's it greatly changed the speedometer. Of course i could change it with my programmer, but before i did i could run about 80mph and it would say i was going 95+.
 
#26 ·
I'm going through the gear issue right now too on my '12 crew cab 4x4. I have the 3.31 rear and my truck came with the 18" wheels and 275/65/18's (32"). I was getting an average of 19.7 mpg's on the highway at 70-75 mpg and around 16 in the city. I put some 20x9 XD wheels and 295/65/20 (35") nitto trail grapplers. I know my speedo and trip are not correct so it is hard to get an accurate mpg rating but it's looking to be around 15.5 mpgs on the highway. I make a run to my parents house and it is about 145 miles so I fill up at the station next door to them and hand calculate it. I noticed that at 70 mph I am running about 8-10 lbs of boost. Stock at 70 barely registered any boost.
 
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