After final drive install - does your tach/speedo read incorrectly?
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From: Chicago
Re: final drive problem (black2001ITR)
The accuracy of your tachometer and speedometer readings will be unaffected.
However, the relationship between the two will be affected. For example, with the stock ITR, first gear will redline at 8400 RPM at 40 mph. Swap in the 4.93 ATS FD, and first gear will redline at 8400 RPM at 36 mph. Both gauges are still accurate, but the relationship between their readings changes.
However, the relationship between the two will be affected. For example, with the stock ITR, first gear will redline at 8400 RPM at 40 mph. Swap in the 4.93 ATS FD, and first gear will redline at 8400 RPM at 36 mph. Both gauges are still accurate, but the relationship between their readings changes.
Trial User
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From: Jasma
Re: final drive problem (nsxtasy)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxtasy »</TD></TR><TR><TD CLASS="quote">The accuracy of your tachometer and speedometer readings will be unaffected.
However, the relationship between the two will be affected. For example, with the stock ITR, first gear will redline at 8400 RPM at 40 mph. Swap in the 4.93 ATS FD, and first gear will redline at 8400 RPM at 36 mph. Both gauges are still accurate, but the relationship between their readings changes.</TD></TR></TABLE>
"Accurate" is a relative term
However, the relationship between the two will be affected. For example, with the stock ITR, first gear will redline at 8400 RPM at 40 mph. Swap in the 4.93 ATS FD, and first gear will redline at 8400 RPM at 36 mph. Both gauges are still accurate, but the relationship between their readings changes.</TD></TR></TABLE>
"Accurate" is a relative term
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From: Chicago
Re: final drive problem (Dan GSR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dan GSR »</TD></TR><TR><TD CLASS="quote">yeah i belive , he's a little confused as to what the lower final drive acutally does</TD></TR></TABLE>
Think of it this way:
The tachometer measures engine revolutions per minute, which are detected from the crankshaft (at the end of the engine).
The speedometer measures road speed based on, and detected from, revolutions of the drive axle (rear axle on RWD cars, front axle on FWD cars).
The gearing is part of the transmission, which is in between the engine and the rear axle. Whether you are in first gear, or second gear, or third gear, etc, the tachometer and the speedometer both read accurately. Changing the FD (final drive gear) is changing part of the transmission, but doesn't affect how revolutions are measured at the engine before the transmission, or at the axle after the transmission.
Does that help?
Think of it this way:
The tachometer measures engine revolutions per minute, which are detected from the crankshaft (at the end of the engine).
The speedometer measures road speed based on, and detected from, revolutions of the drive axle (rear axle on RWD cars, front axle on FWD cars).
The gearing is part of the transmission, which is in between the engine and the rear axle. Whether you are in first gear, or second gear, or third gear, etc, the tachometer and the speedometer both read accurately. Changing the FD (final drive gear) is changing part of the transmission, but doesn't affect how revolutions are measured at the engine before the transmission, or at the axle after the transmission.
Does that help?
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From: Fairbanks
Re: (black2001ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by black2001ITR »</TD></TR><TR><TD CLASS="quote">the thing is, the speed vs tacho has not changed... that is why i am asking if it should...</TD></TR></TABLE>
Hmm, you should definitely feel it. You can easily putt around town in 3rd gear from a roll with ATS FD.
Hmm, you should definitely feel it. You can easily putt around town in 3rd gear from a roll with ATS FD.
FSAE
Joined: Jan 2002
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From: Drinking Beer, UT
Re: (Cosworth)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Cosworth »</TD></TR><TR><TD CLASS="quote">
Hmm, you should definitely feel it. You can easily putt around town in 3rd gear from a roll with ATS FD. </TD></TR></TABLE>
Ha, yea a friend of mine thought he was in reverse and was in 4th, the car actually moved about 10 ft. This with my ATS of course.
If you cannot tell that you got a new final drive installed, then you didnt get it installed.
Hmm, you should definitely feel it. You can easily putt around town in 3rd gear from a roll with ATS FD. </TD></TR></TABLE>
Ha, yea a friend of mine thought he was in reverse and was in 4th, the car actually moved about 10 ft. This with my ATS of course.
If you cannot tell that you got a new final drive installed, then you didnt get it installed.
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From: New York
Re: (92TypeR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 92TypeR »</TD></TR><TR><TD CLASS="quote">
Ha, yea a friend of mine thought he was in reverse and was in 4th, the car actually moved about 10 ft. This with my ATS of course.
</TD></TR></TABLE>
awesome
now thats low gearing
Ha, yea a friend of mine thought he was in reverse and was in 4th, the car actually moved about 10 ft. This with my ATS of course.
</TD></TR></TABLE>
awesome
now thats low gearing
Darth ModerVader
Joined: Jan 2001
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From: Mustafar
Re: (black2001ITR)
Did you note the exact speed/RPM figures before the installation? Assuming you're talking about the swap from USDM to JDM ITR final drives, the difference is "only" ~3 MPH in first gear and ~5 MPH in second (although the difference obviously becomes larger in the higher gears). In fifth gear @ 70 MPH (USDM is turning 3752 RPM and JDM is at 4080 - a difference of 328 RPM).
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From: Chicago
Re: final drive problem (Dan GSR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dan GSR »</TD></TR><TR><TD CLASS="quote">oh, i know what it does, hell i've installed a bunch</TD></TR></TABLE>
I knew that.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dan GSR »</TD></TR><TR><TD CLASS="quote">just think the original post maker, is not sure why his rpms to speed is different from before</TD></TR></TABLE>
Yes, I know; my additional explanation was for his benefit, not yours.
Padawan is right; the numbers may be extremely close. In fact, you can figure out very easily how much they would change, just by dividing the gear ratios. For example, here are the ratios of the various FD (final drive) gears:
USM stock: 4.400
JDM stock: 4.785
ATS: 4.928
So, for example, let's say you change from the USM stock FD to the JDM. Your speed at redline in first goes from 40.3 mph to 40.3 * (4.400 / 4.785) = 37.1 mph (which is where Padawan gets that 3 mph figure from). 3 mph is not a huge difference, and you might not even notice it on your speedometer. And, as implied by El Pollo Diablo, the gauges are not always absolutely accurate, which could obscure the change in the FD ratio as well.
I knew that.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dan GSR »</TD></TR><TR><TD CLASS="quote">just think the original post maker, is not sure why his rpms to speed is different from before</TD></TR></TABLE>
Yes, I know; my additional explanation was for his benefit, not yours.
Padawan is right; the numbers may be extremely close. In fact, you can figure out very easily how much they would change, just by dividing the gear ratios. For example, here are the ratios of the various FD (final drive) gears:
USM stock: 4.400
JDM stock: 4.785
ATS: 4.928
So, for example, let's say you change from the USM stock FD to the JDM. Your speed at redline in first goes from 40.3 mph to 40.3 * (4.400 / 4.785) = 37.1 mph (which is where Padawan gets that 3 mph figure from). 3 mph is not a huge difference, and you might not even notice it on your speedometer. And, as implied by El Pollo Diablo, the gauges are not always absolutely accurate, which could obscure the change in the FD ratio as well.
Thread Starter
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Joined: Aug 2002
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From: Sydney, NSW, Australia
Re: final drive problem (nsxtasy)
i was the installed and i know the ratio is different... i counted the teeth on the crown wheel and drive shaft, here is some interesting news for you...
aussie ITR has 4.7, my ats is 4.9, last night i tested my cars speedo against another 2 cars just to make sure the speedo of that car was not out... anyway, my speedo is out about what it is meant to be...
maybe my speedo was out all along????
my car does pick up quicker as well..
aussie ITR has 4.7, my ats is 4.9, last night i tested my cars speedo against another 2 cars just to make sure the speedo of that car was not out... anyway, my speedo is out about what it is meant to be...
maybe my speedo was out all along????
my car does pick up quicker as well..
FSAE
Joined: Jan 2002
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From: Drinking Beer, UT
Re: final drive problem (black2001ITR)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by black2001ITR »</TD></TR><TR><TD CLASS="quote">
maybe my speedo was out all along????
</TD></TR></TABLE>
Do you still have the same sized wheel/tire combo?
maybe my speedo was out all along????
</TD></TR></TABLE>
Do you still have the same sized wheel/tire combo?
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From: Fairbanks
Re: (black2001ITR)
A sure way to find out is by spending ~$30 and do a dyno run.
Max out in 2nd & 3rd gears and see where your top sped is. Assuming your tire/wheel diameter is same as stock, you should readily be able to calculate the theoretical top speed in each gear and compare to the dyno run.
G'luck.
Max out in 2nd & 3rd gears and see where your top sped is. Assuming your tire/wheel diameter is same as stock, you should readily be able to calculate the theoretical top speed in each gear and compare to the dyno run.
G'luck.
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