Gear Swap

[M1ck3y]

Can someone explain how gear ratio affects driveshaft rotation?

 

Google says a lower (higher numerically) ratio reduces driveshaft speed. Other websites state that it’s the opposite and that the drive shaft spins more for every rotation of the engine. 
 

I’m trying to determine how a gear swap would affect the drive shafts critical speed, and or the trucks top speed..

[davester]

To go a given speed (say 30 mph) on your tires, a rear diff w 3.73 ratio, the driveshaft would rotate slower, compared to the same truck w a 4.10 rear diff.

 

However, your top speed may not change a lot, because these trucks are flying bricks.  While your driveshaft may spin slower, the engine may not have enough power to go wot, or the transmission may downshift.

[M1ck3y]

Is a gear swap worth it? Better acceleration?

[MikeBMW]

9 hours ago, M1ck3y said:

Is a gear swap worth it? Better acceleration?

I suppose it could be if you're looking for quick acceleration and worse gas mileage.
Not sure what you have now, but mine is a 3.42.
I just went the route of tuning for a 91+ octane for being quicker and worse gas mileage (with the benefit of more expensive gas). 

[M1ck3y]

42 minutes ago, MikeBMW said:

I suppose it could be if you're looking for quick acceleration and worse gas mileage.
Not sure what you have now, but mine is a 3.42.
I just went the route of tuning for a 91+ octane for being quicker and worse gas mileage (with the benefit of more expensive gas). 

I have 3.23s with 33” tires so it’s really around 3.10 now.

 

Im just not sure if a move to 3.73s would be enough to be noticeable..

[MikeBMW]

1 minute ago, M1ck3y said:

I have 3.23s with 33” tires so it’s really around 3.10 now.

 

I'm just not sure if a move to 3.73s would be enough to be noticeable..

It would be definitely noticeable.
When I was much younger, the entire aspect of our camaros and corvettes was getting 4:11s!
Horrible for highway driving but drag racing was great!
My old Chevy truck with 40s, on 4:11s, though, worked well. 

[Buckwild27]

My 91 has 4.88's on 37's with manual 5 speed. I had no complaints 

[swathdiver]

On 9/22/2025 at 7:53 PM, M1ck3y said:

I have 3.23s with 33” tires so it’s really around 3.10 now.

 

Im just not sure if a move to 3.73s would be enough to be noticeable..

 

You will certainly notice the change.  I could even tell the difference between 3.42s and 3.73s.  

 

You might consider 4.10s with the 33s.

 

Do you have the 8-speed transmission?

[M1ck3y]

8 hours ago, swathdiver said:

 

You will certainly notice the change.  I could even tell the difference between 3.42s and 3.73s.  

 

You might consider 4.10s with the 33s.

 

Do you have the 8-speed transmission?

Yeah, 8l90e

 

So you had to change the carrier for 3.73s?

Edited September 27, 2025 by M1ck3y

[swathdiver]

15 hours ago, M1ck3y said:

So you had to change the carrier for 3.73s?

 

I'm using a different axle in a different truck and incidentally, no, didn't have to change carriers as it started with a 3.42.

 

In both cases, the carriers are smaller for 3.23 and lower numerical numbers but a spacer is made to accommodate the higher numerical numbers but it is not as strong and many shops won't use them.  A carrier change is required to do it right.