I recently bought a 1996 Firebird LT1 with the 6 speed, I am gettign ready to replace the clutch and would like to do the rear gears to wake the car up, it is a little sluggish at low RPM. I am thinking the cam needs more gear. I was looking at 390 and 410 at summit and the Motive 410 (MGR-GM7-5-410A) is $136 and the 390 (MGR-G875390) is $333. not sure why there is such a big difference between them but I am also not familiar with the brand at all. I cannot find a 390 at Summit close to the price of the 410

There is a Richmond EXCell 390 (RMG-GM75390) for $226
Looking online I found a 390 (EG GM7.5-390) from Ron's Machining Service for $210 but I have never heard of this place or brand of gears.

The car is built with:
Downgraded to OBD1 (1994) and tuned
LE custom Grind Lunati cam 231/239 @ .050, .571/.587 with 1.6 self aligning rockers
36# injectors
Ported and polished heads
XS power long tube headers.

My goal is mostly street and some track probably a 90/10 split. I have read a few posts on suggestion's on the 3.90 vs 4.10 but either I am overlooking it or have not found an overall preference. I am concerned the 4.10 would be shifting too much or soon in traffic but also realize there is very little difference between the two and I have no real world experience with either. I would definitely like some input from people that have used either or both or can direct me to posts that I may have overlooked detailing the difference and preference.***Update*** I just pulled the rear cover and found that the car has a 3.23:1 instead of the 3.42:1 gear so at some point it was swapped out with an auto rear end.

 

OP,

That cam with a T56 and 3.23s definitely needs more gear. Swapping while you’re doing the clutch makes sense. Both 3.90 and 4.10 would be a big improvement, but the best pick depends a bit on your rear tire size and how much highway driving you do. If you’ve got that info, we can help dial it in better.

Motive Gear 4.10 (MGR-GM7-5-410A)

• Budget-friendly OE-style gear
  
• Strong enough for street/strip use
  
• One-year warranty
  

Motive Performance 3.90 (MGR-G875390) and 4.10 (MGR-G875410)

• Made in Italy from a high-end alloy
  
• Fully CNC machined and easier to set up
  
• Quieter and more refined
  
• Two-year warranty
  

The big price gap is mostly product line, not the ratio. The Performance gears cost more because they’re made to tighter specs and are easier to install cleanly. If noise and long-term durability matter to you, they’re worth it. Otherwise, the standard 4.10 is still a great bang-for-buck choice.

Let us know your tire size and how you use the car, and we’ll help you dial it in.

 

Than you for the reply! Currently the car has 315/35R17 drag radials that need replaced and are on the list to be replaced, there will be some highway driving just not often and probably not more than 30- 40 miles.

 

That size tire I would go 390s.

I have an 01 Z06 (700whp and 700 wtq) w the M6 gearing (RPM built) w 390 rear on a 26" tire. The car was setup for 60-130. I'm at 3900 in 3rd at 60 and 6500 in 4th at 130.

I love the m6 gearing w the 390s.

 

OP,

No problem, and you just got some solid first-hand input from @lafnatu. Based on what you’ve described, 4.10s are still a strong option. It sounds like this will be mostly a street car with light highway use and the occasional trip to the strip.

We put together a couple of RPM charts to show where you'd be in each gear from 35 to 70 MPH. This is with the T-56 ratios and 26" tires accounted for. Obviously, you're not cruising in first at 70. However, the charts give a clear side-by-side look at how 3.90s and 4.10s compare across the board.











With your 26" tires and T-56, the average RPM difference between 3.90 and 4.10 is about 186 RPM. In the cruising gears, that’s just enough to help the engine feel more responsive and get into the cam sooner, without making highway driving uncomfortable. So, you can see we're not talking about much of a difference between the two ratios.

At the end of the day, it comes down to whether you want to go with a budget-style gear or something more performance-focused. Let us know what you're thinking and if you'd need help lining up an install kit. We'll be happy to help!

 

3.90 works better than 4.10 in these cars. You really gain nothing with 4.10 besides limiting your top speed in 4th gear.

The 2.97 geared trans is really the way to go.....then use 3.90s.

Unless you're turning over 7k rpm then you may want more gear.

 

I would absolutely agree with this for cars 450rwhp or less. I had the 2.97 mn12 gearing and 390s when I was na and it was awesome!!!! Once I went with the blower and then bigger motor it was too much imo.

The mn6 and 390s are good for higher rwhp cars imo.