Confused about gear ratios (long)!
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Cadillac CTS First Generation Forum - 2003 - 2007 Discussion, Confused about gear ratios (long)! in Cadillac CTS Coupe, Sport Sedan and Sport Wagon Forums; I am confused about the gear ratios in my new '04 Manual Trans CTS. The stats on the GM site ...
  1. #1
    BlueMalibu's Avatar
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    Confused about gear ratios (long)!

    I am confused about the gear ratios in my new '04 Manual Trans CTS. The stats on the GM site show the following information for the Manual trans Non-V Car.

    Final Drive 3.21:1
    1st Gear 3.57:1
    2nd Gear 2.04:1
    3rd Gear 1.34:1
    4th Gear 1.0:1
    5th Gear .82:1

    This seemed pretty tall to me. Calculating the speed in each gear at redline (6600 RPM) you get the following:

    1st Gear 44.5
    2nd Gear 77.9
    3rd Gear 118.6 Yikes!
    4th Gear 158.9
    5th Gear 193.8

    This is based on a wheel/tire cirumference of 81.64 inches which I measured.

    This is not the way the car drives. I have a simulation program that predicts acceleration, etc. given a car's stats and it came up with the numbers above. However, I had trouble believing them so I did the math myself in a spreadsheet. I came up with the same numbers.

    This is the formula I used:

    Rear tire circumference (in miles - .00128851) * 6600 RPM * 60 minutes in an hour / Rear Gear ratio / Trans Gear Ratio = Max Speed in gear.
    So I drove the car and wrote down the speed as indicated by the speedo when the tach was at 3300 RPM in each gear. I doubled this number to get what the speed would be at 6600 RPM. My car is still in the break in period so no 6600 rpm runs.

    Here is what I saw:

    1st Gear 37 @6600 RPM in 1st
    2nd Gear 66
    3rd Gear 100
    4th Gear 132
    5th Gear - Didn't do 5th.
    So what I see driving and the stats have a big discrepency!

    To actually get the speeds I observed, keeping the same trans ratios, the rear gear would need to be about 3.8:1!

    Does anyone know if there are new ratios for '04?

    Maybe my math (and the CarTest simulation program) are wrong?

    Maybe I am missing something?
    BTW, according to the simulation, using a 3.8 rear gear instead of 3.21 takes 2 tenths off the 1/4 mile time.

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  3. #2
    Mbai2's Avatar
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    Re: Confused about gear ratios (long)!

    There is definately something wrong with the calculated numbers. My first gear tops out around 40mph, 2nd~65, 3rd~95, 4th-speed limiter.


    Searching the net I found that the 5MT has a final drive ratio of 3.73. I'm not sure how correct this is but it brings the numbers a little bit closer to the real thing.

  4. #3
    JEM
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    Re: Confused about gear ratios (long)!

    Quote Originally Posted by BlueMalibu
    This is based on a wheel/tire cirumference of 81.64 inches which I measured.
    The manufacturer data for a couple different 225/50-17s per the Tire Rack website shows 804 revs/mile, which back-calculates to something like a 78in circumference.

    Taking third gear as an example, at 60MPH revs/mi=RPM, so:

    804*1.34*3.21=3460RPM

    (6600/3460)*60 = 114mph

    That's one tall third gear.

    You're also assuming your dash tach and speedometer are accurate.

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    BlueMalibu's Avatar
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    Re: Confused about gear ratios (long)!

    Quote Originally Posted by JEM
    The manufacturer data for a couple different 225/50-17s per the Tire Rack website shows 804 revs/mile, which back-calculates to something like a 78in circumference.

    Taking third gear as an example, at 60MPH revs/mi=RPM, so:

    804*1.34*3.21=3460RPM

    (6600/3460)*60 = 114mph

    That's one tall third gear.

    You're also assuming your dash tach and speedometer are accurate.
    Yea, that is why I actually measured the circumference myself - I wanted an accurate number. The speedo / tach could be off as you say.

    However, I just dug around a little and although Edmunds, Carpoint and others are showing the GM number of rthe rear gear ratio of 3.21 I did find some sites that list it as 3.73 for older models. I wonder if GM messed up the press data on the newer models. The GM data shows a reverse gear ratio of .82 and we KNOW that is WRONG.

    Hmm, I guess I will have to jack up the rear and check the rear gear ratio that way.

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    miscreant's Avatar
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    Re: Confused about gear ratios (long)!

    According to the actual build sheet on our 5 speed here on our lot, those listed ratios are correct.

    But I believe (IIRC) that you have to take 96%-97% of the tires circumfrence when off the vheicle (or off the ground) to account for loss of circumfrence from depression - ie. the tire does not stay round (flattens outward).
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    BlueMalibu's Avatar
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    Re: Confused about gear ratios (long)!

    Quote Originally Posted by miscreant
    According to the actual build sheet on our 5 speed here on our lot, those listed ratios are correct.

    But I believe (IIRC) that you have to take 96%-97% of the tires circumfrence when off the vheicle (or off the ground) to account for loss of circumfrence from depression - ie. the tire does not stay round (flattens outward).
    Does the built sheet list the final drive ratio? Or just the transmission ratios? If I lowered the circumference by your factor (which makes sense!) it only leads 1.5 mph difference in 1st gear and a 3.5 difference in fourth which isn't enough to explain the vast difference. I will jack up the car, hopefully 2nite, and twist the driveshaft by hand to estimate the ratio.

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    Re: Confused about gear ratios (long)!

    Quote Originally Posted by BlueMalibu
    Does the built sheet list the final drive ratio? Or just the transmission ratios? If I lowered the circumference by your factor (which makes sense!) it only leads 1.5 mph difference in 1st gear and a 3.5 difference in fourth which isn't enough to explain the vast difference. I will jack up the car, hopefully 2nite, and twist the driveshaft by hand to estimate the ratio.
    The build sheet says 3.21.
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  9. #8
    JEM
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    Re: Confused about gear ratios (long)!

    Quote Originally Posted by miscreant
    But I believe (IIRC) that you have to take 96%-97% of the tires circumfrence when off the vheicle (or off the ground) to account for loss of circumfrence from depression - ie. the tire does not stay round (flattens outward).
    The manufacturer's revs/mile number addresses that.

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    Re: Confused about gear ratios (long)!

    Quote Originally Posted by JEM
    The manufacturer's revs/mile number addresses that.
    Yep. That's why a 26" D tire revs/mile at about 804. I think he was actually physically measuring the tire.
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  11. #10
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    Re: Confused about gear ratios (long)!

    OK, no longer confused.

    I jacked the car up and twisted the drive shaft by hand. Took 3 and 3/4 turn of the driveshaft to get the rear wheels to spin around once. The car has a 3.73 gear or something very similar.

    I suspect all the 3.2 manual trans cars have this gear as the published ratio of 3.21:1 would not drive well, given the trans ratios. Amazing to me that all info on Edmunds, etc. is wrong. GM should correct the info because a 3.73 gear seems more favorable. I highly doubt that will happen though. And with the '05 6-speed coming out it will probably be a mute issue. Unless they get that wrong too.

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