Rear end gears, gauges

[D-93-b]

I have no complaints at all with how it runs, actually runs a lot better than I expected when I bought it. I just don’t have much knowledge with different gears and the affects they have. I don’t claim to be a mechanic but I’m learning as I go


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[D-93-b]

I have no complaints at all with how it runs, actually runs a lot better than I expected when I bought it. I just don’t have much knowledge with different gears and the affects they have. I don’t claim to be a mechanic but I’m learning as I go


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[D-93-b]

It’s got a white cluster in it, makes me think someone’s taken the needles off and the rpm needle didn’t get set back in the right place. Guess the easiest check would be aftermarket rpm gauge right? Just plug it up and compare with what’s in the dash? It’s more less a peace of mind thing honestly


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[D-93-b]

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Someone’s put one of them white paper films in the cluster. I bet that explains the rpms seeming high. Probably didn’t get the needle set back in the right place


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[D-93-b]

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Someone’s put one of them white paper films in the cluster. I bet that explains the rpms seeming high. Probably didn’t get the needle set back in the right place


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[sewlow]

You can get a GPS/Speedometer app for your phone.
A lot of them have a 'Heads-Up Display' (HUD) option.
Most are free.

 

[D-93-b]

You can get a GPS/Speedometer app for your phone.
A lot of them have a 'Heads-Up Display' (HUD) option.
Most are free.

That’s how I figured out my speedometer is 10 fast. I think that’s caused by the different gears, I just wanted to check my rpms because they seem high. Like when I put it to the floor the rpms will blow past 6k and shift probably around 7k


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[stutaeng]

The speedometer on my K3500 also reads about 8 mph too fast @ 70 MPH. I think it's the cluster on mine. When I hook up my scanner, the live data shows within 1 mph @ 70 MPH compared to my phone's GPS. Basically, the ECU is right, but not the cluster. I thought I may had aftermarket gears for a while, but I confirmed they are stock 4.10s on mine.

You can easily figure out the ratio by jacking the rear and counting the number of driveshaft turns to one wheel turn....

 

[1989GMCSIERRA]

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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if that’s your truck there is no way you’ll get 2600 rpm with 3:73 with that size tire. You either have 4:10 or 4:56 installed.
3:73s are not considered low gears. That’s basically a optional stock factory gear ratio.
The quick and dirty way to get a idea of your gear ratios
Jack up the rear of the vehivle
Place car in neutral
Put one mark on the tire at 12:00
Put one mark on the bottom of the driveshaft
Now move the tire one full revolution starting abd stopping at 12:00.
count the amount of times the driveshaft moves. Each full revolutions count as 1. Anything less is fractional. So if you get 3-1/4 it’s a 3:08, 3-1/2 turns it means 3:55 gears. 4-1/4 turns means 4:10 and 4-3/4 turns is 4:56 or around there.
These are approximate as you are basically getting a rough estimate. If you want to know true ratio pop the diff cover abd spin the axle. The ratio will be stamped on the ring gear edge.

 

[JeremyNH]

Your spedometer is set by the VSS which measures transmission output speed then multiplies by the tire circumference and differential ratio. Yours is likely set for the stock 3.73 which at 65mph is 2716rpm in OD. So when the transmission out is 2716rpm your spedometer will read 65mph. If you have 4.10s instead then at 2716rpm your speed is 59mph but the spedometer still reports 65. If you have 4.56s then at 2716rpm your speed is 53 but the spedometer reads 65. Tire pressure plays some role in it too. Sounds to me like you have 4.56s which was the typical ratio for a differential mated to a 4L80e. That will give you higher than stock low end torque multiplication but it will cost you fuel at higher speeds. It will chew up your 4L60e too if you're heavy on the pedal.

 

[b454rat]

Doubt it's 4.56s. I haven't seen many (read NONE) with 4.56s. I know there out there, but just because it's a 4L80 don't mean crap. Prolly get 3.42s if wanted...