3.73->4.56 Regear Results

[parker_rowe]

Regeared my truck a few weeks ago.

I thought I would start a thread to let people thinking of doing the same what to expect. I've definitely seen the question asked a few times in the past, and on every forum I've been a member of the usual response is:

"You'll shoot your eye out kid" or something along those lines.

The truck we started with was an 136K mile 1996 Suburban K1500 5.7 Vortec.
It has the tow package, so it already came with an external cooler, 3.73 gears, and larger rear brake drums.

I've already upgraded the brake master cylinder to a 2001 model, and installed 3/4 ton calipers. Also has 17" wheels with E-range tires from a new Nissan Titan. Also has helper springs in the back.

We tow a 2015 Starcraft 239TBS, 5200 "dry" on the yellow sticker, 6500lbs GVWR. Not huge for most people but plenty for this old half ton.

When the axle seals went, repeatedly, I decided it was time for new axles and a regear.

We installed 4.56 gears and I had the computer retuned for the new gear ratio.

After the gears, it cruised at about 2200rpm in OD at 65mph, with 31.5" tires.

PERFORMANCE

Tows great, big improvement. Before the gears, OD was useless. It would unlock the converter or downshift at the slightest slope or gust of wind.

Now, it will stay firmly locked in OD for most hills. A downshift to 3rd was enough for MOST steeper hills. On our test trip, we did a ~2700 down through NC, TN, GA, then back up to DE Via the Chesapeake Bay Bridge.

We did hit a few hills in TN/NC on RT26 that required 2nd gear...I looked them up later at they were an 11% slope. Not sure what that is in grade.
It did get hot (230F) on those climbs, with 90+ air temps and the AC running on max. I will be addressing the cooling system next to fix that.

FUEL ECONOMY

First two legs of the trip, with the hills, resulted in 8.9mpg and 8.5mpg. My normal mpg on flatter trips has been a BEST of 8mpg. So even with what most would consider crazy 4.56 gears on almost stock size tires, I gained MPG towing in worse conditions. Not sure about unloaded mileage because I mostly use this truck for the camper.

I will try to get some flat land towing numbers this weekend, if I bother to refill the truck when we get home.

Overall, I am very happy with the regear. No power mods I could have done would have resulted in this much improvement. The cruising RPM doesn't bother me, it is smooth and quiet at 2200 with plenty of passing power.

[BenK]

Missed the fan shroud thing...sorry...

Another thing to consider is going to a 180*F thermostat. Above the set point to go into closed loop and provides more head room to the first limp mode set point to reduce power allowed. I like to drill a 1/32" dia hole in the flange to change the hysteresis in opening the stat to wide out

If you are still using DeathCool...make sure the old radiator is rebuilt and if going to new, DO NOT allow O2 into the system. The overflow bottle is NOT pressurized like the newer systems. So it gets O2 in there just by breathing...

I've switched to American Green around 1998-99

[parker_rowe]

mountainkowboy wrote:

Nope...460 and it has the full Banks powerpack kit on it already, headers, exhaust, intake, and program. If I did that I would add a GV OD for better gear selection.

Should work great then!

BenK wrote:
On your truck, that era had three different fan clutch's available in the after market world

Standard duty, HD duty and "SEVERE" duty. They all have a min of about 15% coupling when not turned fully on by the clutch assembly

When they turn on fully....Standard duty couples a max of around 50% shaft RPMs to the fan blade. HD ups that to around 60%-70% and severe duty couples up to 90% of water pump shaft RPMS....IIRC, but that is the approx range to give you and idea of which you will decide to get

The severe duty clutch ROARs when it kicks in and can see the temp needle drop like a rock.

But...you have a small block 5.7L, so no where near the amount of heat a big block needs to reject. Consider going to electric fans. Get the one sized for a big block (they don't recommend towing with it, but is fine for a small block towing heavy)

Breathing is one of the biggest bangs for the buck on any 90's small block. If you do, make sure to investigate ceramic coating the headers if you don't know of it.


This is the NAPA HD radiator for a big block and will fit your small block engine bay (they are the same between big block and small block). It also referred to as "the short neck", because the ATF internal cooler is larger than the small block radiator (long neck)....got photobucket to work again and hope these image links works...

Could go with a big block radiator, but I would need a new upper shroud and radiator mounts. BB Radiators are 2" taller (34"x19") than mine, and a 2 core. It would bolt in if I find those parts. I have a 34"x17" single core now.

I'd have to troll some junkyards for those parts, I've had a hard time finding them online. These things are old enough that even in junkyards I don't see the 7.4 trucks that often.

I can only find the 1500 upper shrouds new.

That would be the best solution though. I want it to control the temp even climbing in 2nd.

If I had the money, I would throw on a diesel bumper and the matching air deflectors too.

[mountainkowboy]

parker.rowe wrote:

mountainkowboy wrote:
I've been mulling around the idea of going to 4.56 gears in Red from 4.10's for better cruising...it always has the TC on it and is at 11,000lbs 95% of the time not towing anything.

Is your F350 a dIesel? My only concern with a diesel truck would be the smaller RPM range they have to operate in.

If it is a 460 EFI, we did headers and exhaust in my FIL early 90's 460 EFI motorhome. Made a big difference in power and fuel economy.

Nope...460 and it has the full Banks powerpack kit on it already, headers, exhaust, intake, and program. If I did that I would add a GV OD for better gear selection.

[EnzoColorado]

I went from 3.08 to 4.10 last year. Loved it. Unloaded highway cruising mpg went down by 1, but when towing, the mpg is about 0.5 to 1 higher. Due to the percentage change, overall it didn't use any more gas than before. But so much easier to tow and drive in general.

[dodge_guy]

parker.rowe wrote:

mkirsch wrote:
WTH are you talking about? NOBODY responds "You'll shoot your eye out kid," with regards to gear changes. In fact, the majority SUPPORT and ENCOURAGE gear changes, regardless of whether they're necessary or not.

Thinking about a gear change? It's more like, "Go for the 5.30's kid."

Ha! I agree completely.

But, it's often "Don't bother, it's still not a 1 ton diesel kid".

On other non-rv sites, they tell you the truck will suddenly start getting 10mpg on the freeway unloaded. And you engine will wear out super fast from turning "so much" RPM.


Like they don't use the same gas engines in boats that cruise around at 4k+ their whole lives.

Right. And there are those that tell you that just running in 3rd gear does the same thing! Lots of people with opinions that I/we shake our head at.

[BenK]

On your truck, that era had three different fan clutch's available in the after market world

Standard duty, HD duty and "SEVERE" duty. They all have a min of about 15% coupling when not turned fully on by the clutch assembly

When they turn on fully....Standard duty couples a max of around 50% shaft RPMs to the fan blade. HD ups that to around 60%-70% and severe duty couples up to 90% of water pump shaft RPMS....IIRC, but that is the approx range to give you and idea of which you will decide to get

The severe duty clutch ROARs when it kicks in and can see the temp needle drop like a rock.

But...you have a small block 5.7L, so no where near the amount of heat a big block needs to reject. Consider going to electric fans. Get the one sized for a big block (they don't recommend towing with it, but is fine for a small block towing heavy)

Breathing is one of the biggest bangs for the buck on any 90's small block. If you do, make sure to investigate ceramic coating the headers if you don't know of it.


This is the NAPA HD radiator for a big block and will fit your small block engine bay (they are the same between big block and small block). It also referred to as "the short neck", because the ATF internal cooler is larger than the small block radiator (long neck)....got photobucket to work again and hope these image links works...





PS...that is my home made 'reverse flush' kit...



Consider modifying the spark plugs...'cut back' ground electrode. I use copper core...not the platinum they list for my big block




.....this might be too small, but that is the way of photobucket these days...if they don't work...will find time later to fix that












Also, the OEM intake filter box can hold the HD filter. It is about 1-2 inches larger in dia, so more filtering surface area and LESS restriction

[parker_rowe]

mkirsch wrote:
WTH are you talking about? NOBODY responds "You'll shoot your eye out kid," with regards to gear changes. In fact, the majority SUPPORT and ENCOURAGE gear changes, regardless of whether they're necessary or not.

Thinking about a gear change? It's more like, "Go for the 5.30's kid."

Ha! I agree completely.

But, it's often "Don't bother, it's still not a 1 ton diesel kid".

On other non-rv sites, they tell you the truck will suddenly start getting 10mpg on the freeway unloaded. And you engine will wear out super fast from turning "so much" RPM.

Like they don't use the same gas engines in boats that cruise around at 4k+ their whole lives.

[mkirsch]

WTH are you talking about? NOBODY responds "You'll shoot your eye out kid," with regards to gear changes. In fact, the majority SUPPORT and ENCOURAGE gear changes, regardless of whether they're necessary or not.

Thinking about a gear change? It's more like, "Go for the 5.30's kid."

[parker_rowe]

BenK wrote:
Thanks...love GMT400's too

When new, the 7.4L would occasionally get +15 MPG and hoping to get that or more after the rebuild. With a 42gallon fuel tank..kidneys would dictate a rest stop before adding to refuel

Key is go have enough torque down low in the RPM curve to take advantage of that 0.5 OD

Not saving anything in money/ROI...just having the ability to choose wnen/where to stop for gas

There is a savings in not spending the kinds of money new vehicles cost these days, and not into LAGs

Plus, with a tuned big block with 5.1's...going to be sooooo much fun !!!

Big block with gears like that would be awesome.

I love the 42G tank. Even towing the camper, a tank of fuel lasts longer than I do.

I plan to add a later model 11 blade fan, or maybe a duramax fan, and a 2 core radiator to address the cooling problems. Maybe a bigger external trans cooler, they have a (pretty much) bolt in model that is about 5" taller than stock. Already have an HD fan clutch.

After that, I will be replacing the exhaust h-pipe. They are extremely restrictive on the 1500 5.7L trucks. 2" pipes with crush bends down to 1 7/8". The 3/4-1 ton 5.7L had 2 3/4" pipes. I think that would help power a bit and help get some exhaust heat out of the engine.

If I ever replace it, it will be with an 8.1L Suburban/Yukon...if I can find one with decent miles that the rockers haven't rusted off of.

[BenK]

Thanks...love GMT400's too

When new, the 7.4L would occasionally get +15 MPG and hoping to get that or more after the rebuild. With a 42gallon fuel tank..kidneys would dictate a rest stop before adding to refuel

Key is go have enough torque down low in the RPM curve to take advantage of that 0.5 OD

Not saving anything in money/ROI...just having the ability to choose wnen/where to stop for gas

There is a savings in not spending the kinds of money new vehicles cost these days, and not into LAGs

Plus, with a tuned big block with 5.1's...going to be sooooo much fun !!!

[K-9_HANDLER]

Thanks for the post. Sounds like you had great results. Also good to see an older rig still earning its keep.

[parker_rowe]

ramyankee wrote:
Our 2002 with the 8.1 has the 3:73. Always wondered how it would effect our fuel and all the other factors if we re-geared...And was curious as to cost?

It was just the cost of parts for me. I have a friend who has a lift and can set up gears.

Roughly $500 in parts to replace both axles, the gears, and all bearings/seals.

BenK, I always love seeing your truck, I have a soft spot for GMT400's. This is not my first.

Funny, I was just looking at gear vendors this morning, but I think it would be overkill for my application. If I have a 3/4 Sub with a big block, it might be worthwhile.

mountainkowboy wrote:
I've been mulling around the idea of going to 4.56 gears in Red from 4.10's for better cruising...it always has the TC on it and is at 11,000lbs 95% of the time not towing anything.

Nice write-up thanks for the info.

Is your F350 a dIesel? My only concern with a diesel truck would be the smaller RPM range they have to operate in.

If it is a 460 EFI, we did headers and exhaust in my FIL early 90's 460 EFI motorhome. Made a big difference in power and fuel economy.

[dodge_guy]

Sounds like my experience. I went from 3.73 to 4.30. Performs much better and I gained .5mpg! I wish I would’ve went to 4.56’s. I don’t think I would’ve noticed the extra .25 in ratio.

Enjoy!

[mountainkowboy]

I've been mulling around the idea of going to 4.56 gears in Red from 4.10's for better cruising...it always has the TC on it and is at 11,000lbs 95% of the time not towing anything.

Nice write-up thanks for the info.