Rear diff help

[Lonewolf1990]

I have 77 620 with a ka24e engine and I'm running a stock rear end. But I'm using very large wheel and 295 50/15 , and when I drive I have trouble going at high speed ,truck seems like it has trouble keeping up, so which rear diff would I need and where do I get one from 

[RyanC]

What gearbox?

[Crashtd420]

I think the bigger question would be what differential is in the rear end?

 

I think with your tire size you might be about 10% bigger than stock so would that mean a 10% decrease in the differential is needed?

 

If 4.37 you would want 3.90.... 

 

You need to know what's in there first to determine what you need...

 

[thisismatt]

14 minutes ago, Crashtd420 said:

I think the bigger question would be what differential is in the rear end?

 

I think with your tire size you might be about 10% bigger than stock so would that mean a 10% decrease in the differential is needed?

 

If 4.37 you would want 3.90.... 

 

You need to know what's in there first to determine what you need...

 

I think you have that backwards - he needs lower gearing, not taller. A 295/50R15 is about 1.24x bigger in circumference than a 195/50R14 if my math is right.

[Crashtd420]

4 minutes ago, thisismatt said:

I think you have that backwards - he needs lower gearing, not taller. A 295/50R15 is about 1.24x bigger in circumference than a 195/50R14 if my math is right.

Isn't 3.90 lower than 4.37 or am i thinking of it wrong?

Or are you saying he'd have to use 4.88 gears?

 

(Final decision needs to be based off current gears...)

Edited August 4, 2023 by Crashtd420

[datzenmike]

The '77 had a 4.375 so no matter what, you should be more than capable of going 70. The KA has about 145 hp or about 50% more than the original L20B and it could easily go 70. In fact a 620 should be capable of over 90 in 4th gear.

 

The 295/50R15s are just over 26" which is the close to the 25.5" 195/75R14  stock diameter but twice as wide.

 

 

There's no telling what pinion gear is in the KA transmission. Go on the highway and drive 10 miles using the mile posts and then back. How does it compare to your odometer??? Should be under 5% or 1 mile per every 20.

 

If you have trouble going the speed limit then it's the engine. Maybe the timing, maybe the engine is just high mileage and tired. Do a compression test.

 

 

[Crashtd420]

I'm realizing now stock 521 and 620 tire size are different....

 

[Lonewolf1990]

If I go lower gear I should have a high acceleration ?

[EDM620]

A "lower" gear is numerically higher, that's why it is often confused.

 

To explain, a 3.90 gear means the driveshaft turns 3.9 times for one rotation of the tire. A 4.88 gear means the driveshaft turns 4.88 times for one rotation of the tire.

Another way to consider it, with a 3.90 gear, for 1000 turns of the driveshaft, the tire will rotate 256.41 times. A 4.88 gear will result in 204.91 rotations.

 

So, a numerically lower gear will turn fewer times over the same distance. This equals lower engine RPM to go the same speed (also gives lower fuel consumption). Overall or top speed is therefore potentially higher.

 

Torque is affected in the opposite way. A numerically higher gear results in more torque at the wheel, giving you a stronger launch but sacrificing speed.

 

The Math:

Your 295/50R15 tire has a circumference of 83.6" which means it turns 758 times per mile. With a 3.90 gear, the driveshaft would rotate 2,956 times to go a mile. A 4.88 gear would require the driveshaft turning 3,699 times - 20% more revolutions over the same distance.

[datzenmike]

4 hours ago, Crashtd420 said:

I'm realizing now stock 521 and 620 tire size are different....

 

 

I think both were 600:14 6 ply sidewalls with the early rear 521s 8 ply.

 

521s were 4.375 differential

620s were mostly 4.375 with the exception of the '73 automatics and the '79 standards.

[banzai510(hainz)]

shit with a KA this should be killing it!!!!!!!!

[datzenmike]

Yup 50% more power more than makes up for any tire choice or gearing.

[Crashtd420]

26 minutes ago, datzenmike said:

 

I think both were 600:14 6 ply sidewalls with the early rear 521s 8 ply.

 

521s were 4.375 differential

620s were mostly 4.375 with the exception of the '73 automatics and the '79 standards.

But you said the stock 620 tires were 25.5 diameter....

In other posts which is why I chose the tire I run on my 521 to claimed the 521 had a stock diameter of 24 or something close to that.....

Edited August 4, 2023 by Crashtd420

[Crashtd420]

[Lonewolf1990]

1 hour ago, banzai510(hainz) said:

shit with a KA this should be killing it!!!!!!!!

It's a huge difference then the stock for sure but when I jump on the freeway I feel like I'm slower then other cars 

[Lonewolf1990]

So where can I find a rear diff I seen I'm able to pull it off A d21 ?

[datzenmike]

14 hours ago, Lonewolf1990 said:

I have 77 620 with a ka24e engine and I'm running a stock rear end. But I'm using very large wheel and 295 50/15 , and when I drive I have trouble going at high speed ,truck seems like it has trouble keeping up, so which rear diff would I need and where do I get one from 

 

7 hours ago, datzenmike said:

The '77 had a 4.375 so no matter what, you should be more than capable of going 70. The KA has about 145 hp or about 50% more than the original L20B and it could easily go 70. In fact a 620 should be capable of over 90 in 4th gear.

 

The 295/50R15s are just over 26" which is the close to the 25.5" 195/75R14  stock diameter but twice as wide.

 

 

There's no telling what pinion gear is in the KA transmission. Go on the highway and drive 10 miles using the mile posts and then back. How does it compare to your odometer??? Should be under 5% or 1 mile per every 20.

 

If you have trouble going the speed limit then it's the engine. Maybe the timing, maybe the engine is just high mileage and tired. Do a compression test.

 

 

 

 

 

 If it can't get to 70 with a 4.375 and lots of torque how in the wide wide world of sports will it do it with higher gear like a 4.11 or 3.90 or even a 3.70????????? It's not the differential! Your KA should make 50% more power than the L20B. If you can't go 70 then there is something wrong with it.

 

Stock L20B in a '77 620 goes 95 at 5,500 RPMs. In 4th gear too!!!!

[banzai510(hainz)]

Ask  or PM Ichouse on here . he had a King cab 620 with a KA.  I think he went just fine but maybe he had a 390in there

Steve(redeye) own the truck now and its a 4x4

 

 

I assume this truck has a 5 speed also from the KA

 

 

you wrote 295 50 15 tire. this is considered a low profile tire still.  might be slightly taller cause of the 295 if such a tire. maybe you meant 195. which is close maybe even less than stock height.

but i think 5 speed 70 should be ez to do. I releaze the KA is a long stock motor and maybe be harder with such a low gear ratio in the top end

Edited August 6, 2023 by banzai510(hainz)

[datzenmike]

Switching to 3.90 or even a 3.70 is just because the KA can pull it and easier on gas too. If L20B can go well over 80 then KA should be able too as well and more. Tune the engine up, check compression, check the timing. Does it have EFI or was it converted to carburetor? Maybe secondary not opening? There's lots of things than can cause lack of power and going to the trouble and expen$s of changing the differential won't fix it.