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]

Congrats on your updated Suburban !!!

Many time...going to a higher numeric diff ratio will gain MPG when towing heavy. A lot depends on the drivers style, terrain and resistance (wind, weight, etc)

If you have the budget (OP and others noodling) consider a GearVendors OD. It has a 'gear spliter' function that will insert itself between each tranny gear. Turning your 4 speed auto into an 8 speed with 8th gear a double OD




Mine is a big block, 7.4L, with 4.1's with 31.6 dia tires. Similar RPM at 65MPH as your new gearset

Click For Full-Size Image.



Am noodling going to 5.1's for the rebuild/update of my 1996 GMC K3500, 7.4L, 4.1 Suburban for those who have similar and considering keeping these towing beasts and rebuilding/updating

rv.net Gear Vendor/Gear splitter, page 3

Gear Vendor 0.5 OD is on my list of stuff going to do when time to rebuild the 7.4L...and IMHO...germain to this discussion

Either the 4.88's or 5.13's will have lots more torque multiplication than my current 4.11's...torque multiplication works both ways...

Chart in below quote is for a GearVendors 0.7, thinking of going to 0.5 so that the final double OD to the axles will be a higher gear (lower numeric) than stock

Should I keep my old truck? This is that thread quote below is from

BenK wrote:


With the cost of new vehicles today...going to save some coin rebuilding my 1996 GMT400 K3500, 7.4L Suburban with all new interior (seats, dash stuff, etc)

8 speed tranny with double OD vs stock of 4 speed and single OD....1,000 HP rated rebuilt 4L80E that has a 3 year warranty...diff ratio either 4.88's (will be 0.7 GearVenders) or 5.13's (will be 0.5 GearVenders)....etc, etc, etc


Posted on another members thread where he is noodling stuff for his rebuild or not. Hopefully he found this of interest and helpful in making up his list of stuff to do

gear ratios, page2

BenK wrote:

Similar to my nooddling plans for my 1996 7.4L K3500 Suburban. Stock is 4.1's and noodling going to something north of 4.56's. Currently think 4.88's will be it, but can't find enough info on that config's strength. IIRC, the pinion might be too small (both face sq/in and tooth root section...cantilever strength)

First, gear boxes are for a change in direction and/or torque multiplication (some times just a change in RPMs)

Here are the listed ratios for my 4L80E:

Gear Ratio
First 2.48
Second 1.48
Third 1.00
Fourth 0.75

Here is just one link to a +1,000 HP 4L80E and cost is similar to a plain Jane rebuild locally vs the 750HP version. The +1,500HP version just a couple thousand more...all warranted


Stage 2 – Warranted 750 HP...Stage 5 - Warranted +1,500HP


The Gear Venders OD noodling and is whether an 0.7 or 0.5 ratio that will split (insert itself) between each 4L80E gear. That would turn my 4L80E into an EIGHT forward gearbox and if you count the XfrCase...16 forward gear ratios...and...a lower 1st gear ratio than the 4L60E...AND...a similar to potentially higher OD final...

Here are the resultant gear ratios




Then switch from LT265/75R16E's to LT255/85R16E's for a LOWER rev's per mile and still not lose much in width...just 10mm, or 5mm each side of rim center.

Toughest part is finding a HO crate 7.4L that will pass SMOG. Most found to date are offroad and/or carberator. No Vortec to be found.

The guy who built the L88 for my Vette has passed on and no one even understands what an L88 was/is... Sure an L88 was a hot rod engine, but most of it can be for a TV...all in the cam...

[ramyankee]

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?

[garyp4951]

Awesome job, and you didn't shoot your eye out.

[theoldwizard1]

Big step up ! I am glad it worked !!

[parker_rowe]

time2roll wrote:
2200 at 65 seems low for OD to hold but if it cruises fine you are going to really appreciate the change. Even 4.88 would seem reasonable for that set up.

Twelve years ago I got about the same response as I swapped 3.55 to 4.10 and it has been working great all these years.

That was my first thought after we got out in the open and the cruise was set. "Man, maybe I could have gone with 4.88's"

Time will tell. I was turning around 2400 towing strictly in 3rd gear before, but had less options for downshifts when I hit a grade. Since it seems to be keeping the converter locked, the computer seems happy with the load it is seeing.

4.88's would have been livable, I think. Although cruising at 70 unloaded is so effortless now, it may have been a bit much with 4.88's.

rjstractor wrote:
Good writeup. I'll bet you're glad you went big with your gear swap.

I am. No regrets!

[time2roll]

2200 at 65 seems low for OD to hold but if it cruises fine you are going to really appreciate the change. Even 4.88 would seem reasonable for that set up.

Twelve years ago I got about the same response as I swapped 3.55 to 4.10 and it has been working great all these years.

[rjstractor]

Good writeup. I'll bet you're glad you went big with your gear swap.