3.90 gears...

[jonrpick]

Anybody run 'em?

It's one of those that you never hear about, but may just be the perfect balance (for somebody) between 3.73's and 4.10's.

A friend of mine has 3.23's and was going to throw in 4.10's. I told him that may be a bit much for a street-driven/DD truck, but... He has the 4.8L Vortec ('07 model) motor, and 24" rims on low-profiles... He says it's a dog with the slightly-larger-than stock tires.

Anyway, I got to thinking and remembered the 3.90 gearset. It seems everyone bypasses it and jumps from 3.73 to 4.10 as if 4.10 is the next level.

 

[4X4HIGH]

Are you sure there is even a 3.90 gear set for our truck axles? I know that ford uses them and i think Chrysler also but never heard of GM using them before. They certainly aren't listed in any place i've seen.

 

[jonrpick]

Are you sure there is even a 3.90 gear set for our truck axles? I know that ford uses them and i think Chrysler also but never heard of GM using them before. They certainly aren't listed in any place i've seen.

Apparently so...

http://www.ringpinion.com/ProductDetails.aspx?ProdID=5675

Edit: don't see them for the 14BFF though... so maybe I spoke too soon.

But since they are available for the 10B, I'll run my friend's tire size through the calculator and see what would be best.

 

[4X4HIGH]

Did you see where it says "(USE OEM 4 Impala & CAPRICE)" AFAIK the car axle is not the same as a truck axle.

 

[jonrpick]

Did you see where it says "(USE OEM 4 Impala & CAPRICE)" AFAIK the car axle is not the same as a truck axle.

Even the internals? I can see width, shock and spring mounting locations, etc, being different, but it seems like a waste to make an entirely different axle with the same diff cover.

 

[4X4HIGH]

I'm not 100% sure but i know for a fact that the 12 bolt car and truck are completely different so i don't see why GM wouldn't do the same with a 10 bolt axle.

 

[jonrpick]

I'm not 100% sure but i know for a fact that the 12 bolt car and truck are completely different so i don't see why GM wouldn't do the same with a 10 bolt axle.

Interesting...

 

[toomany]

I think they'd work...for one of our trucks. If you look at the compatible with models link, it covers allot of models/years. What year is your friends truck? The site says they only fit the 10b until '01.

 

[jonrpick]

I think they'd work...for one of our trucks. If you look at the compatible with models link, it covers allot of models/years. What year is your friends truck? The site says they only fit the 10b until '01.

Actually, then it wouldn't work... didn't notice the year. His is an '07.

 

[YZEATER]

i've never even seen a set of 3.90 gears. someone a couple weeks ago on here said they had some in his truck???

 

[irocvette355]

That would be me...havent ordered yet...may go with 4.10's for the 35's.

 

[6.2Blazer]

Are you familiar with the term "splitting hairs"? That's what you are getting into since there is such a small difference between 3.73 and 3.90, and 3.90 and 4.10.

 

[jonrpick]

Are you familiar with the term "splitting hairs"? That's what you are getting into since there is such a small difference between 3.73 and 3.90, and 3.90 and 4.10.

True... But why were similarly small differences offered, like 3.08's and 3.42's?

 

[Stomis]

A car 10bolt is 7.65 ring. The truck 10bolt is 8.5in ring gear. Huge difference.

 

[Hogback Fabrication]

True... But why were similarly small differences offered, like 3.08's and 3.42's?

The OEM's spend a lot of time and money to get their fleet fuel economies up by very small amounts. Its very unlikely that any of us can guess the right gear ratio close enough that .20 or less difference in ratio will make any difference.

A car 10bolt is 7.65 ring. The truck 10bolt is 8.5in ring gear. Huge difference.

There are tons of older cars that had 8.5" 10 bolts.

 

[dyeager535]

True... But why were similarly small differences offered, like 3.08's and 3.42's?

I'd also offer up that the difference between 3.08 and 3.42 or even 3.42 and 3.73 are not "small", certainly not like 3.73 to 3.90.

Think about the difference that 3.08 to 4.10 makes, and the difference between the commonly offered later (80's+) truck ratios, almost all of which have differences that are .25 or better.

You won't notice much difference from 3.73 to 3.90, but you do from 3.08 to 3.42!

Those "closer" ratios are good when you know exactly how fast you want the engine turning at a specific vehicle speed in a specific gear, like racing, and they allow you to "tune" that engine speed.

BTW, 8.5" rears were offered at least up into the 90's in cars, since many of the full size RWD cars had them. (and even some of the mid-size stuff, G-bodies like the Grand National, Hurst/Olds, and 442 all three of which are obviously quite rare compared to the full size care 8.5's) I believe ALL were 28 spline though, no conversion to 30 spline like the trucks.

 

[6.2Blazer]

True... But why were similarly small differences offered, like 3.08's and 3.42's?

Others beat me to it, but the % difference between 3.08 and 3.42 is double that as compared to 3.73 to 3.90.

You will find that most normal gear ratios (3.08, 3.42, 3.73, 4.10...etc...) are at least 10% different, where as there is a 5% or less difference with a 3.90 compared to either a 3.73 or 4.10. I've driven lots of trucks (both personal vehicles and for work) that have the normal "one step" (10% different) ratios between them and it's really hard to tell the difference at 10%. I don't think there is anyway most people could tell the difference of only 5% (exception being fine tuning race cars or the like).

 

[irocvette355]

Others beat me to it, but the % difference between 3.08 and 3.42 is double that as compared to 3.73 to 3.90.

You will find that most normal gear ratios (3.08, 3.42, 3.73, 4.10...etc...) are at least 10% different, where as there is a 5% or less difference with a 3.90 compared to either a 3.73 or 4.10. I've driven lots of trucks (both personal vehicles and for work) that have the normal "one step" (10% different) ratios between them and it's really hard to tell the difference at 10%. I don't think there is anyway most people could tell the difference of only 5% (exception being fine tuning race cars or the like).

I cant wait to feel some 4.10's, it will be like a new truck!