RPMs <--> speed question
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Joined: Jan 2006
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From: Schaumburg (Chicago) IL
RPMs <--> speed question
Lately I have been feeling a change in my car. Everyone tells me that it drives the same but I am having serious doubts. Here is what I feel:
For instance, previously my car used to be at 1800 RPMs in the 6th gear going at 80, but now it is at 1400 RPMs at the same speed. And this is true at all the speeds - for the same speed now, it goes at lower RPMs.
Now I have 2 questions:
1. Is this possible that it actually changed or I am having unnecessary doubts.
2. If its possible and it did happen, does this mean the car got better or worse?
3. What could've caused it?
For instance, previously my car used to be at 1800 RPMs in the 6th gear going at 80, but now it is at 1400 RPMs at the same speed. And this is true at all the speeds - for the same speed now, it goes at lower RPMs.
Now I have 2 questions:
1. Is this possible that it actually changed or I am having unnecessary doubts.
2. If its possible and it did happen, does this mean the car got better or worse?
3. What could've caused it?
Melting Slicks
Joined: Nov 2005
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From: clackamas oregon
unless someone stole your rearend and replaced it with another. this is not possible. are you driving your car 
this is not possible. if you said it now runs at a higher rpm. maybe a clutch slipping. but lower??? no way 
maybe your tach is going wacky, or change the spedometer back to miles per hour, not kilometers per hour. that mught help. do you notice people flying by you looking at you oddly????? 
this is not possible. if you said it now runs at a higher rpm. maybe a clutch slipping. but lower??? no way maybe your tach is going wacky, or change the spedometer back to miles per hour, not kilometers per hour. that mught help. do you notice people flying by you looking at you oddly?????
Last edited by rgtkst; Feb 3, 2006 at 01:33 AM.
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Changing brake rotors could also cause this. Thing is you'd have to go to a smaller diameter roter or a rotor the same diameter but ligher to get this kind of change. If you went to a lighter rotor with larger diameter you would have to push the engine slightly harder to get the same power to the wheels. Even though the rotors might be lighter the added diameter increses the moment of inertia (inertial resistance to rotation) and in turn require the engine to push harder at the same speed (and actually slightly decrease RWHP). This is the same explanation for a tire or wheel change possibly causing the difference.
Intermediate
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From: Wichita KS
Originally Posted by Stephen Hopkins
Changing brake rotors could also cause this. Thing is you'd have to go to a smaller diameter roter or a rotor the same diameter but ligher to get this kind of change. If you went to a lighter rotor with larger diameter you would have to push the engine slightly harder to get the same power to the wheels. Even though the rotors might be lighter the added diameter increses the moment of inertia (inertial resistance to rotation) and in turn require the engine to push harder at the same speed (and actually slightly decrease RWHP). This is the same explanation for a tire or wheel change possibly causing the difference.
The relationship between RPM and speed is a purely mechanical connection. The equation is:
Speed = RPM/(Gear Ratio*Final Drive)*(2 * tire rolling radius * pi)*(60/5280).
Inertia has absolutely nothing to do with. The only way it can change is to alter one of the above variables. A 22% change, however, is huge. Can't be the tire. I'm guessing it's an instrumentation error.
Melting Slicks
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From: clackamas oregon
Originally Posted by Stephen Hopkins
Changing brake rotors could also cause this. Thing is you'd have to go to a smaller diameter roter or a rotor the same diameter but ligher to get this kind of change. If you went to a lighter rotor with larger diameter you would have to push the engine slightly harder to get the same power to the wheels. Even though the rotors might be lighter the added diameter increses the moment of inertia (inertial resistance to rotation) and in turn require the engine to push harder at the same speed (and actually slightly decrease RWHP). This is the same explanation for a tire or wheel change possibly causing the difference.
Last edited by rgtkst; Feb 3, 2006 at 02:24 PM.
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From: .... NJ
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Originally Posted by Stephen Hopkins
Changing brake rotors could also cause this. Thing is you'd have to go to a smaller diameter roter or a rotor the same diameter but ligher to get this kind of change. If you went to a lighter rotor with larger diameter you would have to push the engine slightly harder to get the same power to the wheels. Even though the rotors might be lighter the added diameter increses the moment of inertia (inertial resistance to rotation) and in turn require the engine to push harder at the same speed (and actually slightly decrease RWHP). This is the same explanation for a tire or wheel change possibly causing the difference.
Stephen Hopkins you are no Stephen Hawking
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From: Supporting the Corvette Community at Abel Chevrolet in Rio Vista, CA 707-374-6317 Ext.123
St. Jude Donor '08
Originally Posted by Stephen Hopkins
Changing brake rotors could also cause this. Thing is you'd have to go to a smaller diameter roter or a rotor the same diameter but ligher to get this kind of change. If you went to a lighter rotor with larger diameter you would have to push the engine slightly harder to get the same power to the wheels. Even though the rotors might be lighter the added diameter increses the moment of inertia (inertial resistance to rotation) and in turn require the engine to push harder at the same speed (and actually slightly decrease RWHP). This is the same explanation for a tire or wheel change possibly causing the difference.
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From: ================== Houston, Texas www.englandgreen.com ================== Necessary Evil™ __________
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Originally Posted by RichieRichZ06
I want a little of what you are are smokin cause obviously it is better than mine
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From: Supporting the Corvette Community at Abel Chevrolet in Rio Vista, CA 707-374-6317 Ext.123
St. Jude Donor '08
Originally Posted by Paras
My car does 1800 rpms at 80 in 6th gear... somethings not right with you car...
Mine does about the same.
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Originally Posted by Stephen Hopkins
Changing brake rotors could also cause this. Thing is you'd have to go to a smaller diameter roter or a rotor the same diameter but ligher to get this kind of change. If you went to a lighter rotor with larger diameter you would have to push the engine slightly harder to get the same power to the wheels. Even though the rotors might be lighter the added diameter increses the moment of inertia (inertial resistance to rotation) and in turn require the engine to push harder at the same speed (and actually slightly decrease RWHP). This is the same explanation for a tire or wheel change possibly causing the difference.
