Gear Ratio Recommendations

[oldschoolyamaha]

I have a 2008 F350 4x4 with 37 inch tires, it came stock with 3.73 gears. My question is what gear ratios should I be running for optimal performance. We are in the process of getting a 40 ft. 5th wheel toy hauler so the change is a must. I've been told 4.56 is about right but i've heard 4.30 is right any help out there would be great. All the off road shops i've talked to go back and forth between the two.

By the way any luck on those spartan tuners

[mattebury]

OK, here's my take on it.

At 60 MPH with stock tires (~31.5" tall LT245/75Rx17E) and a 3.73 rear-end (final drive ratio = 2.65:1), engine RPM ~ 1,700

With 37" tires and a 4.30 rear end, RPM ~ 1,665
With 37" tires and a 4.56 rear end, RPM ~ 1,765

Here's the formula that I used, so I could have made a mistake:

5,280 / (Diameter (in feet) * PI (3.14159)) = number of revolutions of tire in one mile

Final drive ratio = 0.71 (5th gear on the auto tranny) * rear axle ratio

RPM at 60 MPH = number of revolutions of tire in one mile * final drive ratio

So:

5,280 / ((37/12)*3.1459) = 545.1 revolutions of a 37" tire in one mile

0.71*4.30 = 3.053 final drive ratio

3.053 * 545.1 = 1,664 RPM @ 60 MPH

So, if your goal is to keep RPMs around the original speed, the 4.30 will keep your 60 MPH RPMs closest to stock.

[oldschoolyamaha]

Thanks for the info..... in your opinion would the 4.56 gears tow the 40 ft trailer load easier than the 4.30 gears, sounds like the 4.30 gears are better for everyday driving as well.

[Stymie]

did he have the 245's or the 275's?

[oldschoolyamaha]

hey stymie, my truck is identical to yours 37's 6 inch lift 3.73 gears trying to find the right gears to pull a 40 ft trailer plus every day driver

[Stymie]

Originally Posted by oldschoolyamaha

hey stymie, my truck is identical to yours 37's 6 inch lift 3.73 gears trying to find the right gears to pull a 40 ft trailer plus every day driver

personally i'd go with the 4.56's, it'll keep you closer to 2000 on the tach, and keep the tranny from shifting as much. I have pulled a trailer that weighed in over 10K and weven with my 3.73's i was never hurting for power.

[torino73]

I'd have to go with Stymie. Get the bigger gears. I am a firm believer in the theory that pulling with these trucks at low rpm causes alot of the egr, dpf, and intake coking problems we always hear about. My complaint is- 4.10 gears not available stock on my truck. With my stick and 35" tires, it makes common speed limits hard to maintain. At 25 mph, it screams in 2nd or lugs in 3rd. At 35, screams in 3rd, lugs in 4th. At 50, it teeters between 5th and OD. It's a little inconvenient, but not worth the cost or the surgery required to change gears. If I were to get the 4.10, I'd get a locker in the rear to replace the USELESS limited slip rear. Mine has never worked, and neither did the one on my '05.

[oldschoolyamaha]

So with the 4.56 gears that would still be ok for an every day driver will it affect the mileage much better or worse? (IT PLAIN SUCKS ANYWAY) I think the trailer we are getting wil be 14 to 15K lbs empty probably 18K loaded. That is why i contemplated the 4.30 gears because of the everyday use and occasionally once a month maybe twice a month hauling the trailer

[torino73]

I may have to reverse my earlier recommendation. My truck has 35's with the 3.73 rear. That puts my actual ratio well below stock, and I pulled a 14K trailer with no sweat. If your pulling will be a rare occurrence, I'd go for the 4.30 to save you from bending over at the pump any more than you have to.

[mattebury]

Originally Posted by oldschoolyamaha

So with the 4.56 gears that would still be ok for an every day driver will it affect the mileage much better or worse? (IT PLAIN SUCKS ANYWAY) I think the trailer we are getting wil be 14 to 15K lbs empty probably 18K loaded. That is why i contemplated the 4.30 gears because of the everyday use and occasionally once a month maybe twice a month hauling the trailer

I'd go with the 4.56 gears towing that much weight, especially if you get anywhere near a mountain. 18k pounds loaded will be right at your towing limits.

Yes, you will take a mileage hit with 4.56 gears. How much, I can't say.

I get 12.5 MPG in mainly city driving and 15 MPG on the highway. My overall towing mileage runs around 10. I have stock tires, duals, 4x4 and 4.10 gears and my fiver is around 10,000 lbs loaded. In the mountains, I just pull away from the guys that have 3.73s. They hit the grade and bog down.