Gear Swap

Reply Subscribe

Thread Tools

Search this Thread

Jan 20, 2019 | 03:08 PM

#46

RandyinTN

Fleet Mechanic

Joined: Jun 2018

Posts: 1,463

Likes: 537

From: Wasilla, AK

Quote:

Originally Posted by cob1181

I just got back from an 80 mile trip hauling my 3500 lb flatbed trailer. My truck weighs 7700 lbs so I was around 11,200 in combination, nothing heavy at all. It was some in town driving but mostly interstate with a lot of steep grades. The truck felt a little better towing but not $2200 better! I reset the fuel gauge when I got on the interstate and it averaged 10.1 mpg. The last time I drove this route with the same trailer, I got 11 mpg. This is all from the computer, not hand calculated. I'm wondering if I would get better mileage with different tires. I have stock 245/75R17. It seems that the people who gained MPG's have the 275/70R18. Once my travel trailer is done getting warranty work completed and my gears are broken in, ill post my results. If my towing mileage doesn't improve, ill probably buy a set of the larger stock take off wheels and tires from Craigslist.

Also, at times, it fells like I have a parachute behind me when I'm driving. Do you think that this is a result of the gear ratio or the fact that the gears are still new?

Sounds like you have another issue going on. I would of thought your truck would be more “peppy”. Maybe the shop messed up something in your braking system?

Jan 20, 2019 | 03:30 PM

#47

cob1181

Senior User

Joined: Sep 2018

Posts: 157

Likes: 2

My truck feels better every time I drive it. It is "peppy", I have to be careful pulling out, not to spin the tires in 1st gear. I towed 11,000 lbs the other day and it felt great. I averaged about 8 mpg in town over 30 miles. I then pulled a 6000 lb trailer on a 100 mile round trip and averaged 9 mpg. On the hills at 65-70 MPH, it holds 6th gear and still has enough power to slowly accelerate; when I had the 3.73's it would drop into 4th or 5th to accelerate.

At 65 MPH with the 3.73 gears, my RPM's were 1840. At 65 MPH with the 4.30 gears, my RPM's are 2056.

I am happy with the gear swap and I would do it again but with the 245/75R17 tires, Im wondering if this is a bit overkill on the highway while unloaded. I am going to buy a set of factory take-off wheels and tires in 275/70R18 to see if they are a good compromise between the power and fuel mileage. From what I read, this will be similar to having 4.10 gears.

Jan 20, 2019 | 04:16 PM

#48

Super Duty TMac

Mountain Pass

Joined: Apr 2018

Posts: 143

Likes: 3

My truck already has the 275-70-18 factory. I was hoping the higher you get in the gears that the less difference in rpm versus 3.73.

Jan 20, 2019 | 04:17 PM

#49

2017-350

Tuned

Joined: Aug 2017

Posts: 344

Likes: 21

From: Galveston, Texas

I have 37'' tires...if i go to 4.30 in gears, what kind of mileage performance should i expect? Im also tuned and deleted...are swapping gears worth the money for me? i think i have 3.73 now

Jan 20, 2019 | 05:14 PM

#50

cob1181

Senior User

Joined: Sep 2018

Posts: 157

Likes: 2

Im assuming that you have a Powerstroke based on the tuned and deleted comment? We are talking about the 6.2 gas. I doubt that you would have to re-gear based on the fact that you have double the power and a turbo. This is a site that I used a lot in deciding on gears for my truck. https://tiresize.com/gear-ratio-calculator/

Jan 20, 2019 | 05:21 PM

#51

cob1181

Senior User

Joined: Sep 2018

Posts: 157

Likes: 2

Quote:

Originally Posted by Super Duty TMac

My truck already has the 275-70-18 factory. I was hoping the higher you get in the gears that the less difference in rpm versus 3.73.

If I get my tires this week, ill let you know what my 6th gear RPM's are at 65 MPH. If you get the 4.30 gears, you can get 37-38" tires and it will have the equivalent RPM's as the 3.73