Gear Ratio Help

Codyj · Mar 10, 2015

Hey guys, Im running a 6" lift with 38x13.50R20 Toyo's. Im looking at changing my gears to get me closer back to stock and wanted to see what you would suggest. I do tow about 15% of the time, usually around 10,000 lbs and up to 16,000lbs.

From what i have read so far people have been running 4.30's or 4.56's, does anyone have experience with these?

sootie · Mar 10, 2015

talk to Jared @ River City Diesel.

powerSmokin · Mar 10, 2015

I have a buddy with 4.56's and he loves them I also have a buddy on 35's with 4.30's that says it's perfect. You are kind of in between so it's your choice, 4.56's you will have a little better takeoff for towing.

Codyj · Mar 10, 2015

Cool, i sent an email off to RCD. I think 4.56's are the way to go.

Martin #6 · Mar 11, 2015

I'm on 34's with 4.10's and have been happy with the results.

TD-5 · Mar 12, 2015

Ask yourself a few questions , How much highway vs town driving do you do, when your on the highway what speed do you cruise at?, how much towing do you do? Again highway or city? What RPM do you want to cruise at on the Highway?

Plug some numbers into the following formula and see what you get. Also measure your tires, A lot of the time the actual working diameter of the tire is smaller than what's printed on the side wall.

rpm = (speed x (axle gear x od gear) x 336) / tire diameter

OD Gear for the 5R110 = .71

4.30:1 are a good all around gear for the 37", 38" tire, that being said I went with 4.10 in mine with 37" tires, actually 36.3" tall. I got a good buy on the OEM gear sets other words I was looking at 4.30"s. I highway drive 600km every couple of weeks so I wanted to keep my RPM at or below 2000 at cruising speed. The 4.10 also allow me to drop a tire size & still not be geared to deep.

Codyj · Mar 12, 2015

TD-5 said:
Ask yourself a few questions , How much highway vs town driving do you do, when your on the highway what speed do you cruise at?, how much towing do you do? Again highway or city? What RPM do you want to cruise at on the Highway?

Plug some numbers into the following formula and see what you get. Also measure your tires, A lot of the time the actual working diameter of the tire is smaller than what's printed on the side wall.

rpm = (speed x (axle gear x od gear) x 336) / tire diameter

OD Gear for the 5R110 = .71

4.30:1 are a good all around gear for the 37", 38" tire, that being said I went with 4.10 in mine with 37" tires, actually 36.3" tall. I got a good buy on the OEM gear sets other words I was looking at 4.30"s. I highway drive 600km every couple of weeks so I wanted to keep my RPM at or below 2000 at cruising speed. The 4.10 also allow me to drop a tire size & still not be geared to deep.

Thanks for all the info. I probably highway drive about 40% of the time and when I tow its mainly on the highway. My highway speeds are usually around 120km - 130km

With that equation i came up with:

(70x(4.56X.71)x336)/37.7= 2019.85. @ 70mph or 112km
OR:
(77x(4.56x.71)x336)/37.7 = 2420 @ 77mph or 124km

I lifted my truck and put 38's on it a week after i bought it, so i am unsure of what the stock rpms are at on highway speed. What do you think about the 4.56? is 2450 to high rpm?

Codyj · Mar 12, 2015

Or 4.30 gears look like:

(70x(4.30x.71)x336)/37.7 = 2045 @ 70mph
Or
(77x(4.30x.71)x336)/37.7 = 2282 @ 77mph

This may be a good choice. How would 4.30's tow?

TD-5 · Mar 12, 2015

With 4.56 it will pull like a freight train, but 2400 rpm on the highway for hours at a time is going to consume fuel like a freight train too.
4.30 wouldn't disappoint either, they just won't pull quite as hard out of the hole, but these engines have so much torque the bottom end pull won't feel much different,
One think to remember, Ford dropped the factory gearing in 2010 to 3.55 with optional 331 to try & improve fuel consumption at highway cruising speeds. These things like to burn fuel at higher RPMs
Running 4.56 with your 37.7" tire is the equivalent of 4.11 with a 275/65/20 (34") tire. This would be similar to a F350 dually with 4.10's

Running 4.30 with your 37.7" tire is the equivalent of 3.87 with the same 34" tire.
Being your truck is an 2008 it most likely has 3.73 factory gearing, so the 4.30/tire combo would give you a slightly deeper equivalent ratio than what your truck came with from the factory in stock trim.

Either gear ratio would be fine with your tire.

"(70x(4.30x.71)x336)/37.7 = 2045 @ 70mph" this is actually 1905 RPM

Codyj · Mar 13, 2015

TD-5 said:
With 4.56 it will pull like a freight train, but 2400 rpm on the highway for hours at a time is going to consume fuel like a freight train too.
4.30 wouldn't disappoint either, they just won't pull quite as hard out of the hole, but these engines have so much torque the bottom end pull won't feel much different,
One think to remember, Ford dropped the factory gearing in 2010 to 3.55 with optional 331 to try & improve fuel consumption at highway cruising speeds. These things like to burn fuel at higher RPMs
Running 4.56 with your 37.7" tire is the equivalent of 4.11 with a 275/65/20 (34") tire. This would be similar to a F350 dually with 4.10's

Running 4.30 with your 37.7" tire is the equivalent of 3.87 with the same 34" tire.
Being your truck is an 2008 it most likely has 3.73 factory gearing, so the 4.30/tire combo would give you a slightly deeper equivalent ratio than what your truck came with from the factory in stock trim.

Either gear ratio would be fine with your tire.

"(70x(4.30x.71)x336)/37.7 = 2045 @ 70mph" this is actually 1905 RPM

Thanks for all the information. I defiantly gives me something to think about, I think the 4.30's would be the best way to go then, keeps my fuel consumption down and sits better than i am now.