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How do you calculate RPMs?


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Is there a way to calculate RpM for any given speed and any given gear? I have just bought a 2006 manual 997 and I find I am coming too close to over-revving on downshifts, i.e. shifting down a gear too early. I am used to a much higher engine note and the 997's low-pitched growl is a lower pitched noise that the howl i am used to from other cars i have owned, and constantly leads me to think that I am doing 3,000 RpM when in fact it's closer to 5,000 RpM If I knew that, for example, at 5,000 RpM in 4th I cannot change down to 3rd, it would help me learn my new car better.

There's probably a formula that takes into account RPM, gear ratio, final drive ratio, and wheel diameter.

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Is there a way to calculate RpM for any given speed and any given gear? I have just bought a 2006 manual 997 and I find I am coming too close to over-revving on downshifts, i.e. shifting down a gear too early. I am used to a much higher engine note and the 997's low-pitched growl is a lower pitched noise that the howl i am used to from other cars i have owned, and constantly leads me to think that I am doing 3,000 RpM when in fact it's closer to 5,000 RpM If I knew that, for example, at 5,000 RpM in 4th I cannot change down to 3rd, it would help me learn my new car better.

There's probably a formula that takes into account RPM, gear ratio, final drive ratio, and wheel diameter.

Learn to drive by that big ol' tach right smack in the middle; it just takes a little practice. I believe the digital speedo just beneath it is VERY accurate, and it's very easy to read, but when I'm accelerating up past 6 Grand in fourth, I'm too busy hanging on for dear life and watching out for wandering behemoth SUV's (this is on my daily commute) and the CHP to worry too much about actual mph.

Cheers

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I guess what I'm looking for is a way to know, for any given speed, what the RpMs are in each gear. I never watch the speedo but I do drive on the tach, but if I am in 4th at 5,000 RpM what would the RpMs in 3rd, etc? Problem is that I don"t know until I get there!

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Just a word of caution, Gavins, the re-limiter on our cars does not protect us from a missed down shift. Only on an over rev when up shifting. Be careful. You are asking the right question. Good job.

I agree with 355bhp, I drive by tach too, not so much MPH.

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Richard that is EXACTLY the informtion I have been looking for. If I read the chart correctly, it is telling me that the maximum RPMs for a downshift from 5th is 6,000, from 4th is 5,750, from 3rd is 5,000. I am just wondering whether there is any difference in the numbers you show between the car you show, and my car, which is a 2006 C2 USA spec with standard 18" wheels. Really helpful - thanks again.

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The ratios diagram is for the MY2005 G97/01 transmission, and the SIT also shows a ratio tables and speeds which is based on 18" wheels. I believe the rolling circumfrence of 19" wheels would be very close to the 18" because of the lower profile tyres. You can read the complete SIT, which is a mine of information, by becoming a Contributing Member here. It is worth every cent, just for the SITs and TSBs alone. Sorry for the blatent plug, but :renntech:

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post-41447-1240072911_thumb.jpg

Richard, I have done the chart for my car. According to the manual my gear ratios are 3.91, 2.32, 1.61, 1.28, 1.08 and 0.88 with a 3.44 final drive. I am running 265/40 ZR 18 at the rear, which gives me a rear tyre circumference of 210.24 cm, versus 207.22 cm for the 295/30 ZR 19 rear tyres and 209.10 cm for the 305/30 ZR 19 rear tyres. The maximum downshift RPMs come out a bit different. I am assuming of course that my excel skills haven't produced GIGO (Garbage In Garbage Out) but it's good to have done the analysis. Thanks for the inspiration and I will certainly consider joining. I am one month into 997 ownership and loving every moment.

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