Rear end gears, gauges

[D-93-b]

So I’ve got a 96 with a 5.7 vortec, 4l60e transmission (motor and trans are not original if that matters) and the rearend has low gears. I’m told 373, I think lower. It wasn’t a rearend swap from a 4x4, because it’s still 5 lug. My speedometer is now about 10 mph fast, would that make my rpm also incorrect? I can tell my the tone my highway rpms are higher than normal, but the rpms read around 2600 at 70mph. Can anyone share previous experience with this?

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[stutaeng]

What is your RPO code and what is what you actually have?

Same for the tires?

3.73 are considered "high" gears (numerically.)

 

[mattillac]

http://www.grimmjeeper.com/gears.html

Using the tool at the link above to calculate.

4L60E + 4.56 + 275/65/15 = 2600RPM @ 70MPH

4L60E + 3.73 + 275/65/15 = 2600RPM @ 86MPH

(picked up tire size from another post)

 

[sewlow]

3.73 are considered "high" gears (numerically.)

I prefer to think of gear ratios in the terms of 'Tall' or 'Short'.
Tall (high) gears have lower numerical numbers such as 2.79, 2.90, 3.00, 3.25. Short (low) gears have higher numerical numbers, such as 4.11, 4.30, 4.56, 4.88, 5.13, 5.36, etc.

D-93-b...
Your truck's OBD II. You can adjust the speedo via that.
Once you know the tire height & the rear gear ratio, most hand-held tuners can re-calibrate the ECU after following a few prompts.
The tuners are kinda expensive (HyperTech, DiabloSport, Edge, etc.) for what they do, but if you know someone that has one, there's a way to un-lock their tuner from their truck's serial number so you can do what needs to be done to your's.

GT4 -- 3.73
GT5 -- 4.10
GU4 -- 3.08
GU5 -- 3.23
GU6 -- 3.42
G80 -- Gov-Loc limited slip
HC4 -- 4.56

 

[D-93-b]

What is your RPO code and what is what you actually have?

Same for the tires?

3.73 are considered "high" gears (numerically.)

Rpo? I’m not sure what your referring to. Tires are 275/60/15.


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[Nad_Yvalhosert]

RPO: Regular Production Order. 3 digit codes on a sticker in your glove box, that when decoded, list out the options equipped on your truck from the factory.

 

[b454rat]

Sounds like OD aint working. I have 3.42s with 29"-ish tall tires and at 70 its touch over 2k RPM.

 

[D-93-b]

Sounds like OD aint working. I have 3.42s with 29"-ish tall tires and at 70 its touch over 2k RPM.

Od is working. I If I knew how to upload a video on here it would probably help a bit. With the gears and posi track This thing runs like a felon with warrants. It’ll smoke tires no problem from 35 mph and below


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[sntrym]

If you use that calculator, 4.10s will give you 75mph at that tire size. So you probably have 4.10 gears in it. Could have been swapped in or the rearend was installed from another truck and the axles swapped.

 

[sntrym]

Also, RPM is calculated at the engine while speed is on the transmission. Changing gears and tires will throw the speedo off but not the RPM. RPMs will be higher with the 4.10s than the 3.73s though.