4.09 Rear Gear Set

[Lucky7s]

I plan on installing them when I remove my rear cradle to do the bushings. But id still rather 1/4 mile the car first to see what the trap rpm is.

Why not just use the gear ratios based on speed and tire size to figure out how fast you would be going at say 117MPH at the end of the 1/4 mile?

3.73 Gears = (8250-6312/143-110)*End Trap speed(117)= 6871 RPM

4.09 Gears = (8250-6312/131-101)*End Trap speed(117)= 7558 RPM

Now this also assumes you gained zero speed with the gears. Based on what we know about the torque curves you would probably be trapping closer to 121 MPH so RPM would be 7816. Heck being almost 1000 RPM higher in the rev band sounds good to me, would translate to possibly .3 to .4 in the 1/4 mile.

my 2 cents!

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[Voodooo]

I understand what you mean but I'm now running a 305/30 tire. Not the 305/35. On my fox body I run a 28" tire and a 5.14 gear. Through a Jericho 4 speed. Once I trap the car I'll have a better idea of what gear ratio I can run.

 

[Lucky7s]

Tire diameter vs stock.

305/35-19 = 27.405 inches diameter, 305/30-19 = 26.204 inches diameter. Therefore your tires is 95.6% of the stock diameter tire.

So at 100 MPH your are actually doing 95.6 MPH on your Speedometer. (unless you have recalibrated it in software).

so at 117 MPH (end of 1/4 mile trap on 3.73) you would be doing 111.9 MPH. If you move up the trap speed to 117 then your have to move your RPM's up as well.

so at 117 MPH on 305/30-19 you would be turning 122.4 MPH on stock tires, which equates to:
(8250-6312/143-110)*122.4 MPH=7188 RPM.

Now with 4.09 Gears you get this.
(8250-6312/131-101)*122.4 MPH=7907 RPM.

Top speed 4th gear, 305/30-19 with 3.73 gears is 136.7 MPH. (which is 143 MPH with 305/35-19's), Which gives your more to run out in 4th,
Top speed 4th gear, 305/30-19 with 4.09 gears is 125.3 MPH. (which is 131 MPH with 305/35-19's) traps you around 11.7's running out of gear.

 

[superman07]

Very interested, This would seem to be very helpful for those that spend more time as a DD. 5th and 6th gears have to be at high speed to be in the sweet spots. Even 4th is best at like what, 60?

 

[Lucky7s]

Well considering the torque really comes on at 3500 RPM, with stock tires, you would be turning 60/143*8250=3450 RPM... right where the torque comes on with 3.73.

Now with 4.09 gears, 60/131*8250=3780 RPM which is a bit higher.

In cruising gears, 3.73, 70(cruising)/178(Top speed in 5th)*8250=3244 RPM
In cruising gears, 4.09, 70(cruising)/162(Top speed in 5th)*8250=3564 RPM

Which means if ya punched it in 5th at 70, you are right where you wanted to be with 4.09 gears.

In cruising gears, 3.73, 70(cruising)/282(Top speed in 6th)*8250=2050 RPM
In cruising gears, 4.09, 70(cruising)/258(Top speed in 6th)*8250=2240 RPM

Not a big difference in 6th

 

[superman07]

thanks, that is helpful. for me cruising at 70 ish dropping to 5th to pass it might make a difference, Although usually closer to 75-80 dropping down would put me around 4k with 4.09 and be "pull pull pull"

I don't have many miles so maybe I need to open it up a bit more but it seems to start pulling more at like 3600

Does this ECU adjust itself to your driving habits like some of the others?

 

[Lucky7s]

3600 is the beginning of the jump in the torque. If you look at the torque curve it jumps 100 ft/lb's from 2500-3500 RPM's

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