What gear ratio?

J Kuchel · 11-03-2008, 09:58 AM

I thought I would change my gear ratio for next year. I need help with picking the right ratio. The car weighs 3200# with driver, BBC, turbo 400 with a brake and Dana 60, a 14-32 tire with a 101 rollout. The engine made 621 HP.@ 6100 RPM and 621 TQ. @ 4500 RPM. Also can someone shed some light on what rpm I should be shifting at? Right now I am shifting at 6400 rpms. The car runs 10.02 @ 132 Mph. with a 4:10 gear. I hope this is enough information thank you for the help.

Owen S Quirion · 11-03-2008, 10:22 AM

What do your RPM's drop to at each shift and what is your trap RPM?

Fred Popov · 11-03-2008, 10:49 AM

Assuming that you have a standard TH400 with 2.48, 1.48, and 1.00 gears, according to my calculations you should run 4.56. That should have you crossing the finish line right at about 6400 RPM. (Right now you are crossing it at about 5700)

SLR 11.3088
Gear Ratio: 4.56
Tire Diameter 32
Shift RPM 6400
Weight 3200

RPM after shift

Shift RPM 1 - 2 2 - 3 3 - Max
2000 1194 1351 2000
2500 1492 1689 2500
3000 1790 2027 3000
3500 2089 2365 3500
4000 2387 2703 4000
4500 2685 3041 4500
5000 2984 3378 5000
5500 3282 3716 5500
6000 3581 4054 6000
6500 3879 4392 6500
7000 4177 4730 7000
7500 4476 5068 7500
8000 4774 5405 8000

MPH 53.9 90.3 133.6

Fred Popov · 11-03-2008, 11:07 AM

Also, I think that I would try to make the 1-2 shift at 7000 and the 2-3 at 6400. Look at your HP and TQ curves and see where your RPM's end up on the curves after the shift using that "RPM after Shift" table that I posted.

11-03-2008, 09:58 AM	#1
J Kuchel Banned Join Date: Apr 2005 Location: Sterling,Illinois Posts: 111 Likes: 0 Liked 0 Times in 0 Posts	What gear ratio? I thought I would change my gear ratio for next year. I need help with picking the right ratio. The car weighs 3200# with driver, BBC, turbo 400 with a brake and Dana 60, a 14-32 tire with a 101 rollout. The engine made 621 HP.@ 6100 RPM and 621 TQ. @ 4500 RPM. Also can someone shed some light on what rpm I should be shifting at? Right now I am shifting at 6400 rpms. The car runs 10.02 @ 132 Mph. with a 4:10 gear. I hope this is enough information thank you for the help.

11-03-2008, 10:22 AM	#2
Owen S Quirion Junior Member Join Date: May 2005 Location: Captiol District of New York Posts: 71 Likes: 0 Liked 0 Times in 0 Posts	Re: What gear ratio? What do your RPM's drop to at each shift and what is your trap RPM? __________________ BF/S 1985 Grand Am 3.0 V6

11-03-2008, 10:49 AM	#3
Fred Popov Junior Member Join Date: Sep 2007 Location: Chesapeake, VA Posts: 64 Likes: 0 Liked 0 Times in 0 Posts	Re: What gear ratio? Assuming that you have a standard TH400 with 2.48, 1.48, and 1.00 gears, according to my calculations you should run 4.56. That should have you crossing the finish line right at about 6400 RPM. (Right now you are crossing it at about 5700) SLR 11.3088 Gear Ratio: 4.56 Tire Diameter 32 Shift RPM 6400 Weight 3200 RPM after shift Shift RPM 1 - 2 2 - 3 3 - Max 2000 1194 1351 2000 2500 1492 1689 2500 3000 1790 2027 3000 3500 2089 2365 3500 4000 2387 2703 4000 4500 2685 3041 4500 5000 2984 3378 5000 5500 3282 3716 5500 6000 3581 4054 6000 6500 3879 4392 6500 7000 4177 4730 7000 7500 4476 5068 7500 8000 4774 5405 8000 MPH 53.9 90.3 133.6 Last edited by Fred Popov; 11-03-2008 at 11:22 AM.

11-03-2008, 11:07 AM	#4
Fred Popov Junior Member Join Date: Sep 2007 Location: Chesapeake, VA Posts: 64 Likes: 0 Liked 0 Times in 0 Posts	Re: What gear ratio? Also, I think that I would try to make the 1-2 shift at 7000 and the 2-3 at 6400. Look at your HP and TQ curves and see where your RPM's end up on the curves after the shift using that "RPM after Shift" table that I posted.