Changing rear diff gear ratios

DocBar · Jun 17, 2012

I'm kicking around the idea of changing my rear gear ratios from 3.73 to 3.55.

I plan on keeping stock lift, 16" wheels and I've been running 285/75/R16 tires since '04. I tow a 12.5K# 5er a few times a year (from job to job) but that might be 2K miles at a time, through any part of the country.

One of my biggest questions is what to do about my t-case gears and front diff. since I have a 4X4.

I've never attempted anything like this, so any thoughts, ideas or opinions are greatly appreciated.

JoeDaddy · Jun 17, 2012

Just FYI. I went from 4.10's to 3.55's and love it withe 265 tires. My truck is super light. I noticed yesterday that it doesn't like to tow in O/D below 70mph (1900rpms). In drive it does fine but is turning 2500rpms. It's kind of an akward gear. I would suggest staying with the 3.73's. Your mileage will also suffer if your truck is heavy.

But the answer to your question is the T-case doesn't change. Just swap out the front and rear gears.

DocBar · Jun 17, 2012

JoeDaddy said:
Just FYI. I went from 4.10's to 3.55's and love it withe 265 tires. My truck is super light. I noticed yesterday that it doesn't like to tow in O/D below 70mph (1900rpms). In drive it does fine but is turning 2500rpms. It's kind of an akward gear. I would suggest staying with the 3.73's. Your mileage will also suffer if your truck is heavy.

But the answer to your question is the T-case doesn't change. Just swap out the front and rear gears.

That's very interesting. My truck is 8,150#'s with me in it and mostly full of fuel. I was thinking of the swap to increase fuel mileage, but that might not be the case?
I don't understand the bolded.

DocBar · Jun 17, 2012

Delete. Double post.

farm boy · Jun 17, 2012

I believe he is saying that with the 3.55's, he has a hard time towing in OD under 70mph. In 3rd, towing at 70 mph is putting him at 2500 rpm's.

Since my speedometer is re calibrated for the 35" tires, its like having a higher gear ratio. Cruising in OD, 2000 rpm = 75mph. So most of my heavy towing is done in 3rd gear. Gotta keep them RPM's up...

I wish I could find someone to swap me some 4.10 gears for my 3.73's.

DocBar · Jun 17, 2012

farm boy said:
I believe he is saying that with the 3.55's, he has a hard time towing in OD under 70mph. In 3rd, towing at 70 mph is putting him at 2500 rpm's.

Since my speedometer is re calibrated for the 35" tires, its like having a higher gear ratio. Cruising in OD, 2000 rpm = 75mph. So most of my heavy towing is done in 3rd gear. Gotta keep them RPM's up...

I wish I could find someone to swap me some 4.10 gears for my 3.73's.

That's what I thought he meant, but wanted clarification. I prefer to keep my RPM's up when towing, so fuel mileage doesn't matter a ton to me, since I don't tow often and when I do, I get paid for it.

So is 8,100#'s considered heavy for 3.55's? 99% of my driving is to and from work at normal speeds.

farm boy · Jun 18, 2012

DocBar said:
That's what I thought he meant, but wanted clarification. I prefer to keep my RPM's up when towing, so fuel mileage doesn't matter a ton to me, since I don't tow often and when I do, I get paid for it.

So is 8,100#'s considered heavy for 3.55's? 99% of my driving is to and from work at normal speeds.

I don't think that's too heavy.
I drive mine around at #9300 lbs on 35's and 3.73's. I'm sure you will be fine.

Cat_rebel · Jun 18, 2012

Not enough of a change between a 3.55 to a 3.73 in a 8100lb brick on wheels to warrant the cost of the swap in the savings of fuel. Seen some gear swaps make no change in mpg, higher gear is soooooo much nicer empty but loaded down the lower gear shines. I'd stick with the 3.73s & run a 285/75-16r & call it a day. About the perfect gear/tire combo for most 7.3s to have mpg & power.

DocBar · Jun 18, 2012

Cat_rebel said:
Not enough of a change between a 3.55 to a 3.73 in a 8100lb brick on wheels to warrant the cost of the swap in the savings of fuel. Seen some gear swaps make no change in mpg, higher gear is soooooo much nicer empty but loaded down the lower gear shines. I'd stick with the 3.73s & run a 285/75-16r & call it a day. About the perfect gear/tire combo for most 7.3s to have mpg & power.

That's what I have, so I guess I can just call it a day. I really don't want the expense of adding something like a gearvendors unit.

JoeDaddy · Jun 18, 2012

All the above.

There's a fine line in economy bettween less RPM and more load on the engine. My point was that in my 6100 lb truck 3.55's are perfect. I can tow fast at 75mph w/o zinging the engine and tow super heavy at 65-70 with the truck in power.

With your truck I'd guess that 3.73's are spot on for the same outcome. 3.55's would put undue stress towing and empty and I'd bet your mileage would suffer.

DocBar · Jun 19, 2012

Thanks to all of you for the responses and good info that is probably keeping me from wasting money. Much appreciated.

Cat_rebel · Jun 19, 2012

A gear vendors would be a good investment if you were running say a 4.56 or 4.88 gear but wanted to keep the low gear for pulling trailers off the line like say in a F450. But for what your doing the invest won't reward you for a long time & adds another weak link to a high hp driveline.

DocBar · Jun 19, 2012

Cat_rebel said:
A gear vendors would be a good investment if you were running say a 4.56 or 4.88 gear but wanted to keep the low gear for pulling trailers off the line like say in a F450. But for what your doing the invest won't reward you for a long time & adds another weak link to a high hp driveline.

I agree. I've had a truck with that on it and it worked great but is not needed in my current application. I think I'll just stick with what I've got and be happy with it.