If a 3.9 FD is installed, how is the speedo recailbrated?

 

hm your speed is not based by RPM.. so it will not make a difference...

The only time it'll be off is if the actual circum of the wheel is changed.

 

you sure about that? i thought the speed is measured at the axle stubs? if thats the case then neither gearing or tire size wont make a differance right?
if im wrong sorry bout that . but that was my understanding

 

there is no recalibration needed with different final drives -- i've done it a couple times and made sure

the only difference (other than acceleration) is you're at different rpm's at the same speed

 

On the 300zx, speed was read off a speedometer pinion in the transmission. If you changed final drive, you also had to change the speedometer pinion. On the 350z, speed is read off the stub axles after the final drive. No matter what you change your final drive to, it will read speed correctly. Although, if you change you tire circumference, speed will read incorrectly.

 

im having trouble figureing how you say the tire circumferance will change the speedometer.... it doesnt mater how big or small the is . if the axle turns 1 time then the tire turns one time. if the tire makes one revolution then its imposible for the axle its conected to spin slower or faster. right?

 

The facts are that the tire diameter will change the speedo reading. The final drive will not.

Respect
JET

 

explain how? the speed is measured from the axles. the axles turn the same speed of the tires. so how does it change the reeding?

you say its fact. but give me nothing to support the supposed facts you state..

im not trying to be an *** but explain where i am wrong.

the only fact i see is it is a fact that its impossible for the tire to make less than one revolution or more than one revolution than what the axle bolted to it is making

 

All do respect they both change speedo.

If you increase tire diameter you change your final drive ratio. because there is a greater distance the tire surface travels with every 360 turn you change the total distance the wheel moves with each turn. So say you have a tire diameter of 35.6 you increase your wheel size with out plus sizing your tires. you will increase your tire diameter. Say the tire diameter is now 37.5.
Distance / time = speed. so 35.6in/ .25sec = 569.6 in/sec and the new size is 37.5/.25 = 600in/sec. Or if you want it in ft per second... 2848ft/min and 3000ft/min. Or miles per hour. 32.36 MPH Vs 34.09 MPH. So tire size makes a diffrence. (60 mph base 5280 feet per mile X 60 min = 316800 ft/hr.)

Since final gearing is the ratio of output shaft RMP and Drive Shaft RPM you change the rate of rotation of your wheel so you will effect the time portion of the above equation. So instead of 35.6in/.25sec you may have 35.6in/.20sec (.05 sec per rotation which is WAY MORE THAN 3.917 will give you, but times exagerated to show the point) So that increase of .05 your now at 50.5681 MPH so the final drive made a diffrence.

Basicly it this tire size effects the distance traveled, gearing effects the time that distance is traveled. With those in mind you see why increasing the gear ratio makes your car quicker.

Thats the exact portion of it. The realistic portion is your numbers will not be as drastic of a change but the same applies to your car. Simple fact of physics you change your tire size or gear ratio you change your spedo output. it will either be slower or faster depending on your mods.

 

I should stay out of this thread, but there is a lot of misinformation being posted. ( AHHHHH, isn't the internet GREAT! )

Diff gear ratios have NO effect on the speedo in a 350Z. Period. Speed is measured at the stub axels, which are the output shafts of the diff., past the gear. Tire diameter changes WILL change speedo readings as the distance traveled per revolution will change. Tire diameter changes will also change the effective gear ratio. I'm currently running about a 4.013 ratio(yes, I made this # up) with my 3.917 350EVO diff & 255/40 RT 615's.

Way back, in my youth, I owned taxi's, and I am well aware of the benifits of altering speedo gears!

 

question, are the spedo sensors located before or after the input shaft (or service manual calls propeller shaft) conects to the pinion gear? The Ring attaches to the diff, and that spins the stub axels right? that would make the sensors after the pinion gear set? Any thing changing your gear ratio will whange your vehicle speed and acceleration. Your speedo will keep up with the input shaft speed because presume a) the tires are stock diameter and b) since its on the stub axels the computer dosnt give a F#ck about your diffy gear. It just count shaft RPM and calculates speed from stock tire size inputed in the computer? sound correct so far?

So yes, now that I understand that the stub axels are the input for the speed sensors. I will conseed the diffy gear effecting speedo from previous posted statments.

Math was correct but my mental picture of spedo input was wrong. At least you got your math lesson for the day. Now you can tell how quickly it takes your tire to make one full revolution at any given speed.

 

ACTUALLY on the 350z the speed is picked up off the front wheels, so changing your rear end or final drive has no affect your wheel size does affect the speeds because the larger the OVERALL wheel and tire size the more surface it will cover for every rotation, again you can have a 20" wheel with the right tire and your OVERALL diamter will be the same as stock. All this info i am sure is correct so do some research before you try to prove me wrong, Thanks!!

www.1010tires.com has a good tire size calculator check it out go to the tech section

-George

 

Im pretty sure I read just a bit ago that there are speed sensors on the back too. ref EC-528 in service manual

Component Description ABS0015G
The ECM receives two vehicle speed sensor signals via CAN communication line. One is sent from unified
meter and A/C amp., and the other is from TCM (Transmission control module). The ECM uses these signals
for ASCD control. Refer to EC-635, "AUTOMATIC SPEED CONTROL DEVICE (ASCD)" for ASCD functions.

and BRC-95 refrence points 2,3,12,13 are the wheel sensor locations that relay speed to the unified meter and TCM respectively.

and BRC-96
TCS Function AFS000ZG

The wheel spin of the drive wheels is detected by the VDC/TCS/ABS control unit from the wheel speed
signals from the four wheels, so if wheel spin occurs, the rear wheel right and left brake fluid pressure
control and engine fuel cut are conducted while the throttle value is restricted to reduce the engine torque
and decrease the amount of wheel spin. In addition, the degree the throttle is opened is controlled to
achieve the optimum engine torque

Straight from the service manual 4 signals.

EDIT

SPEEDOMETER
VDC/TCS/ABS control unit (with VDC system) or ABS actuator and electric unit (control unit) [without VDC
system] provides a vehicle speed signal to the unified meter and A/C amp. with CAN communication line. Unified
meter and A/C amp. provides a vehicle speed signal to combination meter for speedometer with communication
line between unified meter and A/C amp. and combination meter.

So the VDC/TCS/ABS control unit takes all 4 signals relays it to the unified meter and A/C amp then provides the speed to combination meter for speedometer. So some where after all the comparison the speed signals are combined and sent down the line and ending up at the guage.

again straight from the servic manual

 

That is correct it detects speed from all four wheels for use of traction control, vdc, etc. but the signal used to determine the vehicle motion for the speedo is picked up from the front wheels.

 

After realizing that the speed sensors were in a diffrent location, I decided to go to the source. The service manual. if the service manual is wrong than were all screwed. I tried to include as complete of refrence info as I could.

 

I think I added the speedometer section to my edit jsut as you posted. the VDC controll blah blah takes all 4 signals and feeds them out. Or at least that is what my understanding of the service manual. If I'm missing something please put out the refrence to the section. This damn book is only like 2000 pages.

I'm not trying to be sarcastic I just realy am currious now about whats going on with the speed sensor in relation to the speedo.

 

Will someone put the rear of their car up on jackstands and spin the $#!% out of the tires to see if the speedometer moves. My car is in the body shop, so I can't.

 

it doesnt . have you ever done a burn out? needle doesnt move

 

Although there are sensors at each wheels, which measures slip for VDC, Traction Control, and ABS, the sensor for the speedometer are in the front wheels. Do a burnout while not moving forward and you will see that the speedometer will not rise, indicating that the speed sensors for them are at the front wheels.

Therefore, changing the final drive gear will have no effect on the speedometer, because it only makes the drive shaft spin faster (see "A" in the picture). However, changing to bigger tires will affect the speedometer, because it reduces the revolution per distance of all A, B, and C -- going 20mph will be displayed lower.

----------
Hypothetically, if the sensors for the speedometer are located at position "A," then changing the final drive gear will affect the displayed speed, because the shaft will be spinning faster, while the car is traveling at the same rate -- imagine the engine RPM in 2nd gear vs 1st at say 20 mph, where 2nd gear is the stock final drive, and 1st gear is the aftermarket final drive.

Now, if the speedometer sensors are positioned at location "B," then changing the final drive will have no effect on the speedometer speed, because position "B" will not spin faster or slower with a different final drive gear -- only the the drive shaft, "A," will.

However, if a bigger-diameter tire is put on the wheel, position A, B, and C will spin slower while the car is traveling at the same rate. Thus, this will change the displayed speed (see "Rear" and "Front").

Tonylltell, this is what most on this post are saying.

 

very well explained.. my whole thing was people say for fact it will change things but not explain how. im guessing the speed sensor measures how long it takes for the shaft to make one revolution? and if the wheel is smaller or larger then it would make sense.
i knew the gearing wouldnt change things as speed is measured after gearing. my flaw was thinking a revolution of the wheel if it matches the revolution of the axle then the speed reading wouldnt change.

understood now

thanks