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hi low gearbox or just change the gearing in the rear axel?

S

sweed

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Hello everyone! I have a 95 C2500 suburban 6,5 with x-over pipe, E prom, turbomaster and all the good stuff from heath diesel. i just have to change the exhaust and injectors and add waterinjection.
Anyway my issue now is the rpm at 55mph it's 1900rpm in od and that is a little high for me i would be happier with 1200-1400 range.
I do live in Sweden so the sub is towrated for 4500lbs.
I do know thise beast can tow a lot more but that is not gonna happen.
Does anybody know of a compact hi low gearbox to fit behind the 4L80E that hold up for a 6,5 slightly tuned?
 
Search Gear Vendors overdrive. They are add-on OD units.

Peak torque happens right around 2000 RPM with these engines. Stray too far away from that and efficiency will suffer.
 
I second the idea of the Gear Vendors overdrive unit. It will give you another .78 overdrive in addition to what you have now, not to mention that the rpm drop between the other gears will be much less, which keeps the truck closer to the power band. Assuming you have 30.5" tall tires, 3.73 rear gears, 4L80E OD transmission, at 70 mph you are turning about 2070 RPMs. Adding the Gear Vendors Unit will drop that to about 1600 RPMs. This unit is rated at 1200 hp.

Possible cheaper option...What size tires do you have on the truck? Going up in diameter 1.5" can drop engine speed about 100 RPMs.

But, like 3500GMC said get too far away from the peak, the efficiency will suffer.
 
i have 245 75 16 tires on the suburban. Right now it runs close to 2400 rpm at 70 i think i have 4.10 in gear ratio in the rearend
 
are we talking mph or kph. i have a gearvendor, 4l80, push 1900 at 70 mph. i love it.
 
sweed said:
i have 245 75 16 tires on the suburban. Right now it runs close to 2400 rpm at 70 i think i have 4.10 in gear ratio in the rearend
Click to expand...

A quick look at the RPO codes on the door of your glove box will tell you what gear ratio you have. This might help:

GT1 : AXLE REAR, 2.56 RATIO
GT2 : AXLE REAR, 2.29 RATIO
GT4 : AXLE REAR, 3.73 RATIO (DUP WITH 5 X 1)
GT5 : AXLE REAR, 4.10 RATIO (DUP WITH GT8)
GT7 : AXLE REAR, 3.33 RATIO
GT8 : AXLE REAR, 4.10 RATIO (DUP WITH GT5)
GTR : Opulent Blue Metallic; CADILLAC
GTY : AXLE, WIDE TRACK
GU1 : AXLE REAR, 2.41 RATIO
GU2 : AXLE REAR, 2.73 RATIO
GU4 : AXLE REAR, 3.08 RATIO
GU5 : AXLE REAR, 3.23 RATIO
GU6 : AXLE REAR, 3.42 RATIO
G80 : Rear Locking Differential

As far as the tires go, jumping up to 235/85/16s will drop you down about 100 RPMs. I have 255/85/16s (winter time) on my dually and run about 1900 RPMs at 74mph, but I have the 3.73s.
 
If you are interested in the Gearvendors overdrive there is an earlier version that was used in 80s era Volvo turbo cars. The OD used was the Laycock unit. There are two versions, the J series and the P series with the P being the heavier duty unit.

Gearvendors bought out Laycock and going by what I searched out the only change made to the P series was to put in a larger diameter output shaft. GV advertises that their OD can be used in motorhomes and other high stress situations so I'm thinking that the P series would be strong enough for your Suburban since you don't plan to do any heavy towing.

So if you are able to do a bit of fabrication you might be able to find a Volvo P series Laycock in a junkyard there in Sweden. I don't have the Gearvenders web address at the moment but it shouldn't be hard to find. Do some researching of your own since all the info I have is only secondhand.

Don
 
that sticker on the glovebox door has been removed by P.O. So no help there but i do have 1900rpm at 90kph 56MPH in od.
 
sweed said:
that sticker on the glovebox door has been removed by P.O. So no help there but i do have 1900rpm at 90kph 56MPH in od.
Click to expand...

I have 4.10 gears, 265/75/16 tires and am going 62 MPH at 2100 RPM in OD. I'll have to try my pickup at 56 MPH tomorrow to see where it is running at. We're close but tire sizes can make a difference.

Don
 
jack up the rear, both wheels, chalk mark line on drive shaft and tire, get someone to rotate wheel one revolution, count your revolutions. it will be close enough to tell if you are loaded with 4.10's or not.
 
sweed said:
that sticker on the glovebox door has been removed by P.O. So no help there but i do have 1900rpm at 90kph 56MPH in od.
Click to expand...
Using the RPM calculator, it would appear you have the 4.10s. Tires are probably not gonna put you in the RPM range you want, unless they are really tall. The 235/85/16s will help a little and 255/85/16s will help some more, but you will still be over 2100 RPMs at 70 mph. The Gear Vendor Unit will put you at 1750 RPMs at 70 mph...give'er take a bit.
 
gearvendors.com for a brand new unit, or look on craigs for a used/new/rebuilt unit like i did. it is an extra OD, bolts in place of the tail shaft. it adds to the trans, not a replacement.
 
You most likely have the 4.10 gears. Yesterday I compared my speed to RPMs. At 56 MPH, in OD and with the TC in lock mode, I was at 1900 RPM. That's with 4.10s and 265/75/16 tires. I was using my Tiny Tach for readings. It reads injection pulses and doesn't rely on the alt (alt could easily be inaccurate due to wrong size pulley if the alt is not original).

I've checked out the GV website some time back. The new GV units are $3000 and up in cost. Watching e-bay will often get you better prices. Still more than I can afford at this time.

Don
 
handcannon said:
......I've checked out the GV website some time back. The new GV units are $3000 and up in cost. Watching e-bay will often get you better prices. Still more than I can afford at this time......Don
Click to expand...
true. i felt the same way. i bought all my parts, sight unseen from craigs from 2 diff people. one had a rebuilt unit, other had all the adapters. paid $800.00 total. did the drive shaft work myself.
 
it happens so my uncle have a volvo manual 4sp with od laying around from a old project that has been cancelled... due to back injury, smashed 3 discs... is the od unit marked with P or J?
 
To reiterate, the Gear Vendor unit is more than just a second overdrive, in addition it splits the gear shifts of the other gears as well. This part is often overlooked and the benefit of maintaining the engine rpm from gear to gear. The unit essentially turns your 4-speed auto into an 8-speed auto with double overdrive, i.e. 1st gear, 1st-over, 2nd gear, 2nd-over, etc. to 4th-over. It will also make the truck seem like it has more HP when really it is just keeping the engine closer to peak performance RPM. There are some cautions to this application too, for example you must shift the unit out/off when deccelerating down a hill...no grade braking. A gear Vendor rep told me this will shorten the life of the unit....quickly. The unit can be wired to work in sync with your auto trans and shifts itself requiring you to only turn it on or off as needed.
 
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