gear ratios

[tinkerman56]

The truck is a 1988 R30 Silverado dually four door weighing in at over 6600 lbs. The horsepower will be 375 hp, 400 lbs. torque with a Turbo 400 3 speed. Towing a 30 ft Airstream at 7800 lbs. Would like to change the 4.10 gear ratio in the differential down to 3.54. Is that too much? Opinions please.

[BenK]

Oh forgot...my 7.4L's 410 ft/lbs starts at about 1,400 RPMs and flat till about 3,800 RPN's. So it should be able to hold 65MPH in double OD when used as a car.

I seldom tow in OD in the hills. Sometimes on the flats, at sea level, but not often. MPG is NOT on my have to have list...just nice to have

Shooting for +500 Ft/Lbs on the rebuild and still pass SMOG.

More HP if can find the valve train that will allow higher RPMs

[BenK]

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

Click For Full-Size Image.


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...


Click For Full-Size Image.

[dodge_guy]

blt2ski wrote:
BIG issue you have is the really gal first gear in trans. IE a 2.48 which equates to a an approx 4-1. Which equates to around a max piling grade of 22-14% @ 20k lbs. A Muncie 4 sp with 3.42s I'm pumpkins can do 30+% due to the 6.8-1 granny low.
So a 3.55 and th400, else suck from a takeoff standpoint. My 89 R3500 flatbed with a stock 4bbl 454, th400 @ 4.10 gears cold not go up very many steep grades, blew up grannies every 30k like clockwork!
Another option is 2st two gears from a th350/700r4. Or as mentioned, best is a built 700r4. Then you have a 3.1 5-1 with tc engaged, and a.71 I'd to slow engine Dow. 4.33s or 4.56 gears and a 700r4 would be a sweet setup.

Marty

Yep. If the OP goes with 3.55’s then there will be trans problems, lots of strain put in it with the higher gearing. If it were me I would install a built 700r4 with the proper gearing.

[blt2ski]

BIG issue you have is the really gal first gear in trans. IE a 2.48 which equates to a an approx 4-1. Which equates to around a max piling grade of 22-14% @ 20k lbs. A Muncie 4 sp with 3.42s I'm pumpkins can do 30+% due to the 6.8-1 granny low.
So a 3.55 and th400, else suck from a takeoff standpoint. My 89 R3500 flatbed with a stock 4bbl 454, th400 @ 4.10 gears cold not go up very many steep grades, blew up grannies every 30k like clockwork!
Another option is 2st two gears from a th350/700r4. Or as mentioned, best is a built 700r4. Then you have a 3.1 5-1 with tc engaged, and a.71 I'd to slow engine Dow. 4.33s or 4.56 gears and a 700r4 would be a sweet setup.

Marty

[tinkerman56]

yeah the rear end will be the last project just to see how the 410 will work. Basically putting a 1974 chevelle in a 6600 lb crew cab

[DaveF-250SD]

I am around 2,800 rpm at 65 in my '78 GMC one ton SRW standard cab pickup with a 4.10 rear. Right in the power band, and it does great. Use it to pull a car hauler with a car, GVW of trailer is 7,000. Not much wind drag like you will have with a travel trailer though. Mine is stock 454/TH400 combo., 250 horsepower and 395 ft. lbs. torque, factory dual exhaust and catalyst exempt due to it's 9,000 GVW rating. Honestly, I think you would be fine with either gear, or split the difference and go with a 3.73. Before I got the new tires for the GMC, I used a friend's F-350 to move a car here from out of town. His truck is a 1996 crew cab long bed SRW with 460 and 3.54 gears. His truck did the job well, but was noticeably slower to accelerate than mine carrying the same load. In it's defense, the F-350 is heavier than mine due to being a crew cab, but the difference is very noticeable.

[tinkerman56]

Great advise thank you. I am old timer with the KISS mentality. This truck can be taken apart with 10 different wrenches. I can fix every thing. Florida for the winter would cost me 500.00 or more in fuel from NH. I can live with that

[time2roll]

tinkerman56 wrote:
right now its racing at 60 its top peak power, no rpms just a big fuel gauge. Not to mention 16 225/75 tires

That is a 29.3" tire and should put you at 2821 rpm at 60mph.

3.55 ratio should put you at 2442 RPM at 60 mph. For a BB that might even be a bit high. If mostly towing at 60 probably fine but if you run empty much or prefer 65/70 mph you might even go to a higher ratio.

www.4lo.com

[donn0128]

There is not much other than a new motor will get you beyond maybe 9MPG. Those old school big blocks were simply gas hogs. You do realize that all the work your doing to the motor can only make matters worse. To get HP you need fuel.
You might be better switching the old inefficient carbuetor for electronic fuel injection.

[tinkerman56]

Being a Dana 70

[tinkerman56]

Yeah Dodge Guy thats what I wanted know. So a 373 would at least be better

[tinkerman56]

I can change gear ratios a lot cheaper than a transmission. If I can get 10-12 mpg. Ill be happy right now with 180,000 mile its getting 5 mpg.. Good for dump runs!

[dodge_guy]

Go for the O/D trans. 3.54 gears will make it work too hard and the trans will run hot. You say it’s racing. What is the RPM at 60? If your at 2800-3300 that will be normal for a 3speed gas truck. My conversion van 5.9L with 3.90’s in 3rd gear would spin at 3300 rpm. Did that all the way to Glacier NP and back!

[tinkerman56]

right now its racing at 60 its top peak power, no rpms just a big fuel gauge. Not to mention 16 225/75 tires