gear calculations

[maneval69]

http://www.chargerforums.com/forums/...=2859#post2859

The following was a responce I posted to another forum but in my fact finding I got some interesting results. This may have been covered but still food for thought.


I just hope to have a chance find out.
I know there is only one overdrive my point is you have smaller steps between gears because you have 4 gears from 3.59:1 to 1:1. Like the tremec 6 speed.
The tremec gear ratios are 1st=2.92, 2nd=2.07, 3rd=1.43, 4th=1

The stock LX R/T 5 speed actually has a lower final drive ratio in 1st (2.82x3.59=10.12) than the 6 speed Camaro’s that have 3.42 rear gear. (3.42x2.92=9.9
But I forgot one thing Tire Diameter. The LX has a 28.6 inch tire and the F body has 25.7 which works out to a 0.116:1 reduction in gear ratio for the LX making it a 10.004 for comparison. (please check my math)
True not much of a difference but a lot of people think the stock R/T gear is way too high for a street performance car, when in fact they hit the nail on the head. See quote.

Because transmissions are comprised of several gear choices, the transmission allows the vehicle to accelerate quickly with lower gears and to maintain a cruising rpm using higher gears. In the '60s and '70s, most transmissions offered three or four gears with a 1:1 high gear. Using a TH400 as an example, First gear is 2.48:1, Second gear is 1.48:1, and Third gear is 1:1. Multiplying the 2.48 First gear by the 4.10 rear axle results in a final drive ratio of 10.16:1 (2.48 x 4.10 = 10.16). For most street performance applications, a 10:1 final First gear ratio is usually considered optimal. The disadvantage of operating a 4.10:1 axle ratio on the street with a 1:1 high gear is excessive freeway engine speed

Reference site http://www.chevyhiperformance.com/te...148_0208_gear/

My only experience is running a best 14.560 @ 95 mph in my 72 Cutlass with 2.73 rear gears, changes to 3.43 and ran 14.003 @98.94mph. That’s a change of 7 tenths in gear and a gain of 5.6 tenths in the ¼ mile. So, I’m pretty confident that 2 tenths is a good number for gains going from 2.82 to 3.06 gears. OK maybe 1.92 tenths, but a far cry from the 0.1 tenths you would believe.

Don’t get me wrong I would like 3.23 or 3.43 but nothing higher. The lower the gear the bigger a 2 tenths change in ratio makes. For instance you will see a bigger gain going from a 3.0 to a 3.25 (using round numbers for ease of calculation) than you will from a 4.0 to a 4.25.
.25/3,0=8.33% change
.25/4.0=6.25% change

If this is wrong please give me the correct calculations. It cost a lot less to figure it out on a calculator than swapping parts.


Here is another way to look at it.
Here are the shift points in mph for each rear-end gear ratio for both R/T and SRT.

R/T shift rpm 5400
-----------2.82------3.06-----3.42------3.73-------3.92

1st gear---45.8------42.2-----37.8------34.6-------32.9
2nd gear---74.4------68.6-----61.4------56.3-------53.5
3rd gear---115.6----106.5-----95.3------87.4-------83.2
4th gear---------------------134.0-----123.2------117.3

SRT-8 shift rpm 6000
-----------2.82------3.06-----3.42------3.73-------3.92

1st gear---50.9------46.9-----41.9------38.5-------36.6
2nd gear---82.7------76.2-----68.2------62.5-------59.5
3rd gear---128.4----118.4----105.9------97.1-------92.4
4th gear--------------------------------136.9------130.3

Looks like 3.06 would be good for the R/T and 3.42 for the SRT8.

what do you think?

[noworries]

Hi maneval69 - Haven't had a chance to review your numbers yet, but looks like you are on the right track. I have an '05 RT AWD. I ordered the AWD for a couple of reasons, one of which was to get a better rear gear ratio - 3.07 vs the RWD's 2.82. At any rate, this gear should be a drop in for the RWD and improve performance a bit. I don't know if larger gears will fit in the stock rear housing, but this is clearly one of the best ways to improve performance. I'm with you as far as getting something in the 3.20/3.40 range when they become available. Keep up the good work!

[MCaesar]

Good work.

I think the 3.42 with a high revving engine like the 6.1 are a good match.

[BobF's 6POINT1]

I think the diff between the LS1 6 speed and our car is the Clutch can be released at higher engine speed and in my opinion, the LS1 had better low-end torque.

[maneval69]

The ls1 has more horsepower and low end torque.
I am saying 3.06 for the RT's because of shift points. 2.82 works but you’re only using 2 gears. 3.42s don't work because you run out of 3rd gear right at the end of the 1/4. So, you take the lost time of the shift and don't get to use 4th long enough to make up for it.
3.06 gears allow you to use third gear through the highest part of the torque curve. This is why I want to take my brothers charger to the track. I think these 5.7 lx cars would run faster E.T.'s if you could shift at 5000 rpm rather than 5400. The only lx capable of this is the Charger RT. (because DCX lied about the autostick feature in my Magnum)

It also looks like 3.92 would work out for an RT. Traction would make a big problem if you used 3.92. If you could hook up out of the hole and firm up the shifts then 3.92s would be worth having for ¼ mile times. But 2700 rpm at 71 mph would be hard for me to live with.

[MCaesar]

good info