Which wheels drive the speedo

[04lss]

Hi All,

I apologize if this has been answered, I checked in the search and didnt see anything. I just ordered a set of RTR tech 7s and some falken tires from AM. I got 275 40 19s in the front which have a greater circumference than the OEM 235 50 18s, and in the rear, i have 285 35 19s which are smaller. I know my speedo is going to be slightly off, but what id like to know is which wheels impact the speedo, front or rear?

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[StangTime]

Speed data is obtained from the transmission.

 

[BmacIL]

Speed data is obtained from the transmission.

That is incorrect. It is from rear wheel speed sensors.

 

[StangTime]

That is incorrect. It is from rear wheel speed sensors.

So what's this in the service manual going on about? (OSS) output shaft speed sensor located at the rear of the transmission sends speed data onto the CAN bus.
The wheel speed sensors (four of them) are for the ABS system.

 

[BmacIL]

So what's this in the service manual going on about? (OSS) output shaft speed sensor located at the rear of the transmission sends speed data onto the CAN bus.
The wheel speed sensors (four of them) are for the ABS system.

The PCM may use that, particularly for the A6/A10 cars for shift schedules, but the indicated speed in the cluster is coming from the rear wheel speed sensors.

Want some practical evidence? Swap to a different rear axle ratio without changing anything else. If it relied on the OSS, the speed would be off. In reality, it matches GPS speed because it's from the rear wheel speed sensor.

 

[WildHorse]

Want some practical evidence? Swap to a different rear axle ratio without changing anything else. If it relied on the OSS, the speed would be off. In reality, it matches GPS speed because it's from the rear wheel speed sensor.

Then why does my handheld tuner ask for tire size and/or gear ratio changes? I can confirm that dropping the rear tire diameter resulted in the speedometer being off slightly. Also, my GPS speed rarely matches the speedometer. Disconnect the rear wheel sensors and see what happens ;)

 

[BmacIL]

Then why does my handheld tuner ask for tire size and/or gear ratio changes? I can confirm that dropping the rear tire diameter resulted in the speedometer being off slightly. Also, my GPS speed rarely matches the speedometer. Disconnect the rear wheel sensors and see what happens ;)

You just proved my point?

 

[WildHorse]

You just proved my point?

I dunno? Did I? Lol. but generally, I tend to believe everything you say and learn from your knowledge, despite my stupid questions haha.

 

[BmacIL]

I dunno? Did I? Lol. but generally, I tend to believe everything you say and learn from your knowledge, despite my stupid questions haha.

Changing the tire diameter changes the revs per mile actual but the wheels speed sensor just sees revs. If the OSS was used, changing the rear axle ratio without changing the programmable parameter for the rear axle would result in the speedo being off. The reality is that you can do a gear swap, change nothing and your speedometer indicated speed accuracy will remain for a given gear+rpm.

 

[Zathras]

But, going back to the first post in this thread, aren't we talking about the effect of a change in tire size? If the rear wheel speed sensor(s) provides data in revolutions/min, then when you put a tire/wheel combo on with a different diameter, won't the revs/min change? For example, if your OEM tires were 235/50/18, at 60 MPH, that would be about 740 wheel revolutions/minute. Which is multiplied by some factor to convert to the "60 MPH" we see on the dashboard.

If you put on a tire/wheel that is 5% smaller diameter, and drive at a true 60 MPH, the wheel speed sensors are going to read a higher rotational speed, something like 780 wheel revolutions per minute. Which, unless something is reprogrammed, would seem like it's going to be converted to an indicated speed 63 MPH when the computer calculates the speed based on the wheel RPM.

 

[WildHorse]

when you put a tire/wheel combo on with a different diameter, won't the revs/min change

They do. I had to adjust the tire size in my handheld. Now the speedo is bang on.

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