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OK,, here is an interesting one---I have oversized wheels and tires on my 04 350z went from 17' stock wheels to 18" 245/40/18 front and 265/40/18 rear. here is my question? obviously my speedometer and odometer are going to be off. it is had to tell if i am reading fast or slow in the car and all the information given on the tire web sites for how speed and distance readings are impacted by Plus size wheel/tire setups are for a particular single size wheel/tire i guess assuming that you are putting 4 of the same size on the car. nothing is given if you have multiple sizes on one car. so, I am trying to find out where the input comes from for the speedometer and how the speed is calculated. I have the online maintenance manual and it talks about each wheel sensor sending a signal to the ABS or TCS box and that box sending a 8 pulse signal to the speedometer. however, it does not say how the box is manipulating the input from each wheel. does it take revolution counts from each wheel, average them and send a signal to the speedometer--or does it just use the signal from one wheel--if one wheel, which one?
 

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[/quote]


rob
Thanks for the response. I do not think these calculators will do the trick. Notice that they give you the difference for one tire to another. So, if I look for the difference for the new front wheels (which actually have a diameter slightly less than stock), the calculator tells me that my actual speed will be slightly less than indicated on the speedometer. then if i check to see the difference for the real wheels (which have a diameter slightly larger than stock) the calc says my actual speed with be slightly more than indicated. So, at this point which is it? I have both on my car. I know the answer is somewhere between the two but not sure where. Somehow the car is taking the reading from each wheel sensor and coming up with a speed. I just do not know how they are doing that. I cannot use the figures from the tire calc web sites because of the differences above.
 

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Doesn't the speedo signal come form the transmission? It wouldn't make sense if came from the wheels as the wheels rotate at different speeds even if they are the same size i.e. cornering.
 

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[/quote]


that's what I thought also, but according to the maintanance/repair manual the signal comes from the 4 wheel sensors thru the ABS controller and "sends an 8 pulse signal to the speedometer. sure wish we could find someone on this site that knew for sure. Late this afternoon I finally got a customer service guy at Nissan North America to listen to me, and did confirm with one of his engineers that the 4 wheels feed signals to through a processor to the speedometer as well as to the controller for the odometer. He also agreed to forward my questions viz fax to his engineers and get an a detailed answer Monday. /so, I faxed the info to him and we will see on Monday. Whatever I find out I will post here so everyone will know how this works.
 

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[/quote]


that's what I thought also, but according to the maintanance/repair manual the signal comes from the 4 wheel sensors thru the ABS controller and "sends an 8 pulse signal to the speedometer. sure wish we could find someone on this site that knew for sure. Late this afternoon I finally got a customer service guy at Nissan North America to listen to me, and did confirm with one of his engineers that the 4 wheels feed signals to through a processor to the speedometer as well as to the controller for the odometer. He also agreed to forward my questions viz fax to his engineers and get an a detailed answer Monday. /so, I faxed the info to him and we will see on Monday. Whatever I find out I will post here so everyone will know how this works.
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OK the person that said the transmission was the source of the signal for the speedometer is correct. although each wheell has a speed sensor, those signals are not passed to the speedometer at all--only to the ABS,. there is a revolution sensor at the trans which counts revoltions of a particular "Idler gear" in the transmission and send a signal on to the speedomenter
 

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Sorry to drag up such an old thread, but I'm wondering about the true source of the vehicle speed that ultimately gets fed to the gauge cluster (speedometer).

From the service manual (2007 - DI-4 & DI-5) it seems pretty clear that the source of this signal is the sensors in the wheels. Also, you can see in the second image below that it states the engine speed (RPM) comes from the crankshaft position sensor.

Does anyone have a reference in the service manual or any other technical document that shows otherwise and can verify what the NISSAN tech told 'swilso12'?

I'm working on a project car and will be using the ECU and other guts from the 350z to make dealing with NATS issues easier. I wanted to use the 350z gauge cluster, but it would be more difficult and expensive if I had to setup the wheel speed sensors in all 4 corners.

Thanks
Chris



 

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It is my understanding that the engine speed is measured using the crank position sensor and that the ABS sensors are used to calculate the vehicle speed. The ABS counts the revolutions and that is converted to vehicle speed using a nominal tire circumference.
 
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