Whats my gear ratio now?

[RJM03Cobra]

I just put in a Detroit locker, almost all the MM IRS bushings, and 4:10 gears, took out the stock 3:55 and diff, went from 26 inch tires to 28 tall MTETs. Whats my net gear ratio change? Up 3 gear sizes and up 2 inches of tire, anyone know. I hope to trap 125- 135 mph in 4th. Thanks, how much speedo diff?

 

[Black2003Cobra]

Changing to the 4.10 from the 3.55 will increase torque at the rear wheels by about 15.5% (in any gear, and at any rpm). Increasing the tire diameter doesn't change torque, but it does reduce the motive force applied to the ground at the contact patch, (F = 2*rwTQ/Dtire). This has the opposite effect on acceleration as the gear swap (accel in G's = F/w). The "loss" from the tire change works out to about 7.1%, so your overall gain acceleration-wise is only about [(4.1/3.55)*(26/28) - 1]*100% = 7.2%. That said, the primary reason to go to a taller tire is for the added sidewall, which aids traction.

 

[RJM03Cobra]

Sounds good, so what would that equate to in a gear ratio if I was still on the 26 inch tire, I am trying to guess if I will hit the limiter in the traps in 4th now!

 

[Black2003Cobra]

Your RPM through the traps will depend on your trap speed, as well. Here's a table RPM vs MPH for the 4.10's and 26 or 28 inch tires. It should help you if you have an idea of what your trap speed might be. For your power level, you should probably stick with the 28" tire (depending on where your rev limiter is set).

	RPM	RPM
MPH	26	28
120	6361	5906
122	6467	6005
124	6573	6103
126	6679	6202
128	6785	6300
130	6891	6399
132	6997	6497
134	7103	6595
136	7209	6694

 

[RJM03Cobra]

Your RPM through the traps will depend on your trap speed, as well. Here's a table RPM vs MPH for the 4.10's and 26 or 28 inch tires. It should help you if you have an idea of what your trap speed might be. For your power level, you should probably stick with the 28" tire (depending on where your rev limiter is set).

	RPM	RPM
MPH	26	28
120	6361	5906
122	6467	6005
124	6573	6103
126	6679	6202
128	6785	6300
130	6891	6399

You are the man! thanks thats exactly what I waas looking for, the only question needs to be answered now is if I put on the whipple or KB as planned will the 4:10 still work?

 

[Black2003Cobra]

Sorry...forgot to answer your other question, too. Compared to the stock tire (Dtire = 25.6") and stock gears, with the 28" tire and 4.10's your speedo will read about 5.3% lower than you're really going.

 

[Black2003Cobra]

Depends on how much your power curve changes. To first order, your trap speed would go up as the cube root of the increase in power. I think maintaining traction might become an issue. (Such a problem, right?!)

 

[RJM03Cobra]

I instruct cardiologists on heart medication therapy options but you are losing me on the cube root theories! You must be an engineer, I think I will hook it will I trap before 4th gear redline is the question.

 

[Black2003Cobra]

Yes, I'm an engineer. Good guess. Any way, if you tell me how much boost you plan to run and on which blower, I can estimate your power output, and from that, your trap speed. Then we can answer your question about rpm. But I need to sign off so I'll have to check back. So if I ever have any heart problems, then I'll be asking the questions...deal?

 

[RJM03Cobra]

Right now I am making about 550 rwhp and hoping to break 11 seconds, If I go KB or whipple I will go with the 2.2 and should make 600 rwhp on the motor and 650-675 with a 80-100 shot, hope the stock pistons/block last a season and maybe break 10?