Does the JDM final drive throw off speedo?
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From: Southern California, U.S.A.
Re: Does the JDM final drive throw off speedo? (Rdriver)
I was actually wondering about this myself. When you change the final drive without changing the speedo drive or driven gears, how is the speedo not affected?
A detailed explanation would be nice-thanks
A detailed explanation would be nice-thanks
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From: WI
Re: Does the JDM final drive throw off speedo? (b19coupe)
because speed is measured by how manytimes the differential turns.
if you somehow made the diff larger diam or smaller diameter then it would mess something up.
if you somehow made the diff larger diam or smaller diameter then it would mess something up.
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From: Southern California, U.S.A.
Re: Does the JDM final drive throw off speedo? (sgT)
Next time I have a tranny apart on the bench, I am definitely going to look at the workings of the diff/speedo gears.
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Re: Does the JDM final drive throw off speedo? (b19coupe)
wheels are tunring at 65mph... whatever revolutions per minute that is.
1) axles are turning at same revolutions per minute.
2) differential is turning at same revolutions per minute.
3) vehicle speed sensor is detecting differential speed directly off of the VSS gear which is on one end of the differential.
4) final drive gear is on the other end of the differential connecting the transmission gearing to the clutch/flywheel/crank.
they are two different gears (final drive and VSS) although they are both directly rotating at the same speed as the differential.
1) axles are turning at same revolutions per minute.
2) differential is turning at same revolutions per minute.
3) vehicle speed sensor is detecting differential speed directly off of the VSS gear which is on one end of the differential.
4) final drive gear is on the other end of the differential connecting the transmission gearing to the clutch/flywheel/crank.
they are two different gears (final drive and VSS) although they are both directly rotating at the same speed as the differential.
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Re: Does the JDM final drive throw off speedo? (Rdriver)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Rdriver »</TD></TR><TR><TD CLASS="quote">
Any way, my speedo is 10 off for some reason.</TD></TR></TABLE>
what size tires are you using? if you alter sizing, that throws the speedo off.
Any way, my speedo is 10 off for some reason.</TD></TR></TABLE>
what size tires are you using? if you alter sizing, that throws the speedo off.
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From: Charlotte, NC, US
Re: Does the JDM final drive throw off speedo?
I thought I might add on here, I have a post going at G2IC here it is.
So I just purchased a GSR cluster from a member here and it occurred to me that there may be different calibrations for the two clusters. So I did a little research and this is what I found, the GSR Speedometer component is different compared to an LS one. Yet they have the same power steering /vehicle speed sensor.
Speedometer
GSR: 78120-SK7-A21
LS: 78120-SK7-41
VSS
GSR/LS: 56500-PR3-010
I also looked at the differences between the Tachometers, they are different, obviously, cosmetically yes but mechanically? Who knows.
GSR: 78125-SK7-A91
LS: 78125-SK7-A12
So until I receive my GSR cluster I won't know for sure, so in the mean time I need some assistance. If you have a 90-93 GS/LS/RS with a 92-93 GSR cluster what is your RPM reading at 50 MPH? Also if anyone has an open cluster with the speedometer part out, look on the back of it, there should be a number like 1258rpm/km or something like that. This should be the calibration number used to match the gearing of the car to the speedometer, I am curious if these are the same or not.
Well I know it is plug and play its the differences in RPM that I am worried about. By my calculations I get a 72 RPM difference between the GSR and the LS because of final drives. Also it is interesting to note that the GSR/LS have the exact same differential part, which also means they have the same gear to drive the VSS.
Differential:
GSR/LS/RS/GS: 41100-P21-000
So that means the only difference is the final drive and the speedometer. My hunch is that the speedometer is calibrated for the 4.4 gearing and by putting it in the LS with 4.266 gearing you will be at 72 higher RPMs and thus be speeding.
The difference is small yes, but its a 1.6 MPH difference, and a 72 RPM difference by my calculations. But I still have to test that.
Speedometer
GSR: 78120-SK7-A21
LS: 78120-SK7-41
VSS
GSR/LS: 56500-PR3-010
I also looked at the differences between the Tachometers, they are different, obviously, cosmetically yes but mechanically? Who knows.
GSR: 78125-SK7-A91
LS: 78125-SK7-A12
So until I receive my GSR cluster I won't know for sure, so in the mean time I need some assistance. If you have a 90-93 GS/LS/RS with a 92-93 GSR cluster what is your RPM reading at 50 MPH? Also if anyone has an open cluster with the speedometer part out, look on the back of it, there should be a number like 1258rpm/km or something like that. This should be the calibration number used to match the gearing of the car to the speedometer, I am curious if these are the same or not.
Well I know it is plug and play its the differences in RPM that I am worried about. By my calculations I get a 72 RPM difference between the GSR and the LS because of final drives. Also it is interesting to note that the GSR/LS have the exact same differential part, which also means they have the same gear to drive the VSS.
Differential:
GSR/LS/RS/GS: 41100-P21-000
So that means the only difference is the final drive and the speedometer. My hunch is that the speedometer is calibrated for the 4.4 gearing and by putting it in the LS with 4.266 gearing you will be at 72 higher RPMs and thus be speeding.
The difference is small yes, but its a 1.6 MPH difference, and a 72 RPM difference by my calculations. But I still have to test that.
Re: Does the JDM final drive throw off speedo?
VSS monitors the rotations of the wheels cause the VSS gear is attached to the diff housing. This means altering gear ratios or final drives does not alter VSS signal. The only thing that alters the VSS signal in relation to actual speed, is the diameter of the tire. If you run a larger tire, the car is traveling further than intruments are indicating (speedo reads low). If you run a smaller tire, the car thinks it's going faster than it actually is (speedo reads higher).
My 205/40/16 tires are 4% smaller than stock ITR tires. Therefore My speedo reads 4% higher (at 100mph on the speedo, I'm actually only doing 96).
Here's a pic of the diff with the VSS gear on it. As you can see, you change your FD without effecting the VSS gear at all:
My 205/40/16 tires are 4% smaller than stock ITR tires. Therefore My speedo reads 4% higher (at 100mph on the speedo, I'm actually only doing 96).
Here's a pic of the diff with the VSS gear on it. As you can see, you change your FD without effecting the VSS gear at all:
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From: Charlotte, NC, US
Re: Does the JDM final drive throw off speedo?
VSS monitors the rotations of the wheels cause the VSS gear is attached to the diff housing. This means altering gear ratios or final drives does not alter VSS signal. The only thing that alters the VSS signal in relation to actual speed, is the diameter of the tire. If you run a larger tire, the car is traveling further than intruments are indicating (speedo reads low). If you run a smaller tire, the car thinks it's going faster than it actually is (speedo reads higher).
My 205/40/16 tires are 4% smaller than stock ITR tires. Therefore My speedo reads 4% higher (at 100mph on the speedo, I'm actually only doing 96).
Here's a pic of the diff with the VSS gear on it. As you can see, you change your FD without effecting the VSS gear at all:
My 205/40/16 tires are 4% smaller than stock ITR tires. Therefore My speedo reads 4% higher (at 100mph on the speedo, I'm actually only doing 96).
Here's a pic of the diff with the VSS gear on it. As you can see, you change your FD without effecting the VSS gear at all:
uhhhh, yeah it does. changing the final drive would change the speed reading, the only thing that is different between a G2 LS and GSR is the final drive. Regardless of your gear if you change the final drive, then you are driving the VSS at the same ratio. So if I change the Final drive to 4.4 then the VSS is being driven at the same ratio, and vis versa. It DOES affect the reading, not by much but it does.
Re: Does the JDM final drive throw off speedo?
No it doesnt. VSS measures the speed of the rotating differential.
By changing the final drive, you are altering the relation of engine rpm vs diff rotation. Not diff rotation vs tire rotation. TO MEASURE SPEED, YOU NEED TO KNOW TIRE DIAMETER & DIFF RPM. It has nothing to do with engine rpm, mainshaft rpm, or countershaft rpm...
It doesn't matter what motor, gearbox etc. The only thing that matters is that you use the correct size tires for the VSS & diff...
EXAMPLE/PROOF: If you took the ring-gear off the diff and pushed the car down a hill, would the speedometer read the vehicles speed?
/thread
By changing the final drive, you are altering the relation of engine rpm vs diff rotation. Not diff rotation vs tire rotation. TO MEASURE SPEED, YOU NEED TO KNOW TIRE DIAMETER & DIFF RPM. It has nothing to do with engine rpm, mainshaft rpm, or countershaft rpm...
It doesn't matter what motor, gearbox etc. The only thing that matters is that you use the correct size tires for the VSS & diff...
EXAMPLE/PROOF: If you took the ring-gear off the diff and pushed the car down a hill, would the speedometer read the vehicles speed?
/thread
Last edited by 94eg!; Aug 4, 2009 at 07:28 PM.
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Joined: Feb 2004
Posts: 50
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From: Charlotte, NC, US
Re: Does the JDM final drive throw off speedo?
No it doesnt. VSS measures the speed of the rotating differential.
By changing the final drive, you are altering the relation of engine rpm vs diff rotation. Not diff rotation vs tire rotation. TO MEASURE SPEED, YOU NEED TO KNOW TIRE DIAMETER & DIFF RPM. It has nothing to do with engine rpm, mainshaft rpm, or countershaft rpm...
It doesn't matter what motor, gearbox etc. The only thing that matters is that you use the correct size tires for the VSS & diff...
EXAMPLE/PROOF: If you took the ring-gear off the diff and pushed the car down a hill, would the speedometer read the vehicles speed?
/thread
By changing the final drive, you are altering the relation of engine rpm vs diff rotation. Not diff rotation vs tire rotation. TO MEASURE SPEED, YOU NEED TO KNOW TIRE DIAMETER & DIFF RPM. It has nothing to do with engine rpm, mainshaft rpm, or countershaft rpm...
It doesn't matter what motor, gearbox etc. The only thing that matters is that you use the correct size tires for the VSS & diff...
EXAMPLE/PROOF: If you took the ring-gear off the diff and pushed the car down a hill, would the speedometer read the vehicles speed?
/thread
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Joined: Jun 2009
Posts: 162
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From: Vancouver, BC
Re: Does the JDM final drive throw off speedo?
No it doesnt. VSS measures the speed of the rotating differential.
By changing the final drive, you are altering the relation of engine rpm vs diff rotation. Not diff rotation vs tire rotation. TO MEASURE SPEED, YOU NEED TO KNOW TIRE DIAMETER & DIFF RPM. It has nothing to do with engine rpm, mainshaft rpm, or countershaft rpm...
It doesn't matter what motor, gearbox etc. The only thing that matters is that you use the correct size tires for the VSS & diff...
EXAMPLE/PROOF: If you took the ring-gear off the diff and pushed the car down a hill, would the speedometer read the vehicles speed?
/thread
By changing the final drive, you are altering the relation of engine rpm vs diff rotation. Not diff rotation vs tire rotation. TO MEASURE SPEED, YOU NEED TO KNOW TIRE DIAMETER & DIFF RPM. It has nothing to do with engine rpm, mainshaft rpm, or countershaft rpm...
It doesn't matter what motor, gearbox etc. The only thing that matters is that you use the correct size tires for the VSS & diff...
EXAMPLE/PROOF: If you took the ring-gear off the diff and pushed the car down a hill, would the speedometer read the vehicles speed?
/thread
well put
Re: Does the JDM final drive throw off speedo?
If the VSS ring gear on the new diff is different, you will need a different VSS. Doesn't have anything to do with the final drive or gear ratios...
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