74 k10 going 55 mph is at 3,500 rpm

[Bextreme04]

So... just looking at a basic calculator here. What size tire do you have? According to this calculator: https://spicerparts.com/calculators/engine-rpm-calculator with a 31" tire and 3.73 gears, you would be at 2283RPM. With a 28" tire and 4.56 gears, you would be at 3090RPM. Which is right about what you are getting.

 

[Trucksareforwork]

According to this the axle is a 3.73 gear stock axle, how do I go about figuring out wether someone changed the gear ratio?

Couple ways:

1. Open up the rear and start counting teeth on the ring and pinion.

2. Jack up one side of the rear, put the transmission in neutral, and turn the wheel that is off the ground by hand 2 full revolutions while counting turns of the drive shaft. With one wheel turning (I'm assuming it's not a posi traction, by the way) you should get roughly 3 and 3/4 turns of the drive shaft per 2 full revolutions of the tire. If it's a posi, jack up both sides of the rear and just turn the tires one full revolution while counting drive shaft rotations.

The second way has more words but is way easier than the first unless you were already planning to service the rear end.

 

[Bextreme04]

If you pull the diff cover, you can see what ratio you have. If you haven't done that already, I would highly recommend it to at least make sure you have clean gear oil in there. You will see two numbers stamped into the side of the ring gear. Usually something like 41 10. That is the number of teeth on the ring gear and pinion. Gear ratio is the big number divided by the small number.. In this case 41 10 equals 4.10 gear ratio. I'm just a little ways away down in Salem

 

[granthendrickson]

If you pull the diff cover, you can see what ratio you have. If you haven't done that already, I would highly recommend it to at least make sure you have clean gear oil in there. You will see two numbers stamped into the side of the ring gear. Usually something like 41 10. That is the number of teeth on the ring gear and pinion. Gear ratio is the big number divided by the small number.. In this case 41 10 equals 4.10 gear ratio. I'm just a little ways away down in Salem

Sounds good, I’m gonna go get some gear oil and a gasket and change on that while I check the ratio

 

[granthendrickson]

Couple ways:

1. Open up the rear and start counting teeth on the ring and pinion.

2. Jack up one side of the rear, put the transmission in neutral, and turn the wheel that is off the ground by hand 2 full revolutions while counting turns of the drive shaft. With one wheel turning (I'm assuming it's not a posi traction, by the way) you should get roughly 3 and 3/4 turns of the drive shaft per 2 full revolutions of the tire. If it's a posi, jack up both sides of the rear and just turn the tires one full revolution while counting drive shaft rotations.

The second way has more words but is way easier than the first unless you were already planning to service the rear end.

I got 3 3/4 turns on the driveshaft with two tire revolutions. How do I find out the gear ratio based on that?

 

[Bextreme04]

Sounds good, I’m gonna go get some gear oil and a gasket and change on that while I check the ratio

Its likely to be a truck 12 bolt axle. I can't imagine it came stock wit ha 4.56, but if someone had big tires on it at some point they might have swapped them in. Going that big would have called for a carrier change, so they might have put a limited slip or locker in there at the same time. While it's apart, take some good pics of it and check to see if you have a gov lock, aftermarket locker/lsd, or an open diff. The numbers you are looking for will look like this:

You must be registered for see images attach

The 13 40 are the gear numbers in this example from a 10 bolt.

 

[granthendrickson]

Its likely to be a truck 12 bolt axle. I can't imagine it came stock wit ha 4.56, but if someone had big tires on it at some point they might have swapped them in. Going that big would have called for a carrier change, so they might have put a limited slip or locker in there at the same time. While it's apart, take some good pics of it and check to see if you have a gov lock, aftermarket locker/lsd, or an open diff. The numbers you are looking for will look like this:

You must be registered for see images attach

The 13 40 are the gear numbers in this example from a 10 bolt.

Sounds good I’ll post what I find

 

[Old Guy Bill]

oops, already posted.

 

[gmbellew]

The tach is up by around 220 rpm according to my timing light

I bet the RPM difference is proportional to engine speed. 220 at idle might be more at higher RPM....

 

[Bextreme04]

I bet the RPM difference is proportional to engine speed. 220 at idle might be more at higher RPM....

Doing the math on that... If it is 220 off at 1000rpm, that is a 22% offset. The gear calculator with a 3.73 rear end and 28" tire at 55mph has the expected RPM to be 2528. A 22% error at that RPM would read ~3,084RPM. It's also worth noting that this is almost exactly the error you would expect if you have a tach set for 6 cylinder running on a V8. The 6 cylinder setting is expecting 3 pulses per rev and the V8 is producing 4 pulses per rev, which is going to give you a 25% error on the high side.

 

[Trucksareforwork]

I got 3 3/4 turns on the driveshaft with two tire revolutions. How do I find out the gear ratio based on that?

That's your 3.73 ratio (i.e., 3 and 3/4 turns of the driveshaft for one rotation of the total axle... an open diff spins half as fast when one wheel is stalled, I should have explained that). You have a 3.73 open diff by the looks of it.

 

[gmbellew]

I bet it is off about 25% too high bc of the tach being not set up for a v8.

3500 x 0.75 is 2625 RPM

 

[granthendrickson]

That's your 3.73 ratio (i.e., 3 and 3/4 turns of the driveshaft for one rotation of the total axle... an open diff spins half as fast when one wheel is stalled, I should have explained that). You have a 3.73 open diff by the looks of it.

Awesome thank you for the information!

 

[granthendrickson]

Doing the math on that... If it is 220 off at 1000rpm, that is a 22% offset. The gear calculator with a 3.73 rear end and 28" tire at 55mph has the expected RPM to be 2528. A 22% error at that RPM would read ~3,084RPM. It's also worth noting that this is almost exactly the error you would expect if you have a tach set for 6 cylinder running on a V8. The 6 cylinder setting is expecting 3 pulses per rev and the V8 is producing 4 pulses per rev, which is going to give you a 25% error on the high side.

So the tach is probably just in a six cylinder setting and my rpm at 55 is about where it should be?

 

[squaredeal91]

Cool, now we're getting somewhere lol. Go team! Now on to the 8mpg, Guessing you don't have a quadrajet on it.