4.10 gears

[TotalyHucked]

Bringing this thread back to life. I don’t have the pro codes in this truck, but according to vin decode, 350/th400, the gears are 4.10s. The rpm’s are really high, even at low highway speeds. Wish there was a way I could verify.

There is, you put the rear up on jack stands (put wheel chocks up front), put it in neutral (not running) and spin the tires by hand. Mark the driveshaft so you have a reference point and count how many times the driveshaft spins vs one revolution of the tire. 4 driveshaft revolutions to 1 tire revolution would be 4.10. 3 3/4 driveshaft revs would be 3.73 and so on

 

[Dano500]

@totallyhucked that’s awesome, I’m going to try this. Thanks!

Can you even travel on the highway with 4.10s? Like aside from poor mpg, anything to worry about? It has 265/75/16s on it.

 

[Ricko1966]

Bringing this thread back to life. I don’t have the pro codes in this truck, but according to vin decode, 350/th400, the gears are 4.10s. The rpm’s are really high, even at low highway speeds. Wish there was a way I could verify.

There are ways to verify. Pop the cover count teeth on ring gear,count teeth on pinion gear,divide solve for x. Put a mark on a tire put a mark on the drive shaft push the truck forward,counting driveshaft turns. There are more ways but these are dirt simple. You can tape a piece of string with a weight to the driveshaft to count drive shaft turns,watch where the tape is when done in relation to where it started.

 

[77 K20]

@totallyhucked that’s awesome, I’m going to try this. Thanks!

Can you even travel on the highway with 4.10s? Like aside from poor mpg, anything to worry about? It has 265/75/16s on it.

Some stock trucks even came with factory 4.56 gears. "Highway" speeds means many different things to people in different states. If you are in Texas with the speed limit of 85 mph and where people do 90 mph... then not so much.

 

[DoubleDingo]

@totallyhucked that’s awesome, I’m going to try this. Thanks!

Can you even travel on the highway with 4.10s? Like aside from poor mpg, anything to worry about? It has 265/75/16s on it.

You drive on the highway, you just won't go fast without turning more than 4000 rpms

 

[77 K20]

RPM, speed, gear ratio, tire size calculator

Spicer has a calculator for this (click link above). It even estimates in torque converter slippage.

A 265/75R16 tire is 31.7" in diameter. But the tire will squish down due to weight. Isn't going to be a true 31.7".
Lets say 31". With 4.10s you will be running around:

2738 rpm at 60 mph
2966 rpm at 65 mph
3194 rpm at 70 mph
3422 rpm at 75 mph
3650 rpm at 80 mph
3878 rpm at 85 mph

I have 4.10s on my 77K20. When it was stock and even when pulling trailers I'd just run at 65 mph. It seemed happiest at 60-65 mph. It could go faster- but it really seemed to suck down the gas and was just noisy and more uncomfortable at those speeds.

 

[Dryriver1]

Very good information. Thanks for bringing this back!

 

[TotalyHucked]

@totallyhucked that’s awesome, I’m going to try this. Thanks!

Can you even travel on the highway with 4.10s? Like aside from poor mpg, anything to worry about? It has 265/75/16s on it.

Yeah, you can. You'll just need to stick to the slow lane lol. Your truck will likely be most comfortable between 60-65mph. I've driven our '76 shortbed C10 6cyl, 3 on the tree, 3.73 with much shorter tires (225/75/15) on the hwy plenty of times. 65 is about as fast as she wants to go. She'll do 70-75 but she's letting you know she's not happy

 

[DoubleDingo]

That's why the speed limits were 55 back in the day. Only a small percentage had overdrive or gears to let the engine turn lower rpms. Heck, when I first got Mean Green in the late 80s, I had to turn 4,000 just to keep up with the traffic and the speed limit then was 60 I believe, so everyone was going 65-70. That 283 and 4.57 rear made for a screaming engine. Now we have cars that cruise 80 while turning 1,800-2,000 rpms and get 30 mpgs doing it. Oh wait, we're talking about trucks, yeah, trucks are still built with better gearing nowadays than they were way back when...lol...

 

[Rusty Nail]

Not great for gas mileage, but meh.

That was pretty funny.

 

[Rusty Nail]

Bringing this thread back to life. I don’t have the pro codes in this truck, but according to vin decode, 350/th400, the gears are 4.10s. The rpm’s are really high, even at low highway speeds. Wish there was a way I could verify.

1. Jack both the rear wheels off the ground.
2. Getcha wheel chock and install it.
3. Take a piece of masking tape about 1 inch square and stick it on the driveshaft so you can see it through the well well.
4. Put truck in neutral.
5. Spin one wheel one complete revolution WHILE you count how many times the tape goes around the driveshaft.
That number is your gear ratio.

Easy peasy.. Take pix.

 

[Ricko1966]

Multiple people have said to raise both rear wheels. There is a small problem with this method. You need to raise both wheels,spin one while watching the other. 1spins the opposite direction means open diff put 1 tire back on the ground and proceed. Wheels turn same direction its posi proced as directed. This is why I just check them on the ground. It's less time consuming.

 

[Dano500]

Multiple people have said to raise both rear wheels. There is a small problem with this method. You need to raise both wheels,spin one while watching the other. 1spins the opposite direction means open diff put 1 tire back on the ground and proceed. Wheels turn same direction its posi proced as directed. This is why I just check them on the ground. It's less time consuming.

I don’t get it, does the open diff alter calculating the gear ratio with spinning the tire?

 

[Dano500]

RPM, speed, gear ratio, tire size calculator

Spicer has a calculator for this (click link above). It even estimates in torque converter slippage.

A 265/75R16 tire is 31.7" in diameter. But the tire will squish down due to weight. Isn't going to be a true 31.7".
Lets say 31". With 4.10s you will be running around:

2738 rpm at 60 mph
2966 rpm at 65 mph
3194 rpm at 70 mph
3422 rpm at 75 mph
3650 rpm at 80 mph
3878 rpm at 85 mph

I have 4.10s on my 77K20. When it was stock and even when pulling trailers I'd just run at 65 mph. It seemed happiest at 60-65 mph. It could go faster- but it really seemed to suck down the gas and was just noisy and more uncomfortable at those speeds.

The tires are fairly dry rotted, although they keep air. I was going to replace them, so by that calculator, if I go to 33” it looks like it will drop RPMs to 2900. I wonder if that’s even significant. Lol

 

[Ricko1966]

I don’t get it, does the open diff alter calculating the gear ratio with spinning the tire?

100 percent. If it's open diff you need to keep 1 wheel from spinning. If you mark a tire and roll the truck posi or not you'll get your reading. Mark your driveshaft , mark a rear wheel at 6 o'clock, pull forward to 4:30, look at your drive shaft, pull forward to 3: , look at your driveshaft pull forward to 1:30, look at your drive shaft when your tires back at 6 position the number of drive shaft turns = x x to1 is your gear ratio. If you use a piece of string on the drive shaft,it will be string wraps plus or minus where ever your mark lands = x. X to 1 is your gear ratio. He[[ you could even make 4 marks with white out on the driveshaft 1 dot at 6. 2 dots at 3. 3 dots at 12. 4 dots at 9 to make your calculation easier.Dot 1 started at 6 ended at 9 thats 3/4 of a turn .75 So 3 string wraps plus .75 = 3.75 to 1. We know the closest ratio to that is 3.73 to 1