If I change the rear tires only to 285/25/19, will the accuracy of the speedometer be affected? They are currently stock.

 

I switched to a 285/35/19 on the rear and only see a 1mph difference in what my speedometer says vs. my GPS or Ultraguage digital mph reading.

You posted a 285/25/19
Not sure about this size though

 

I don't think you can get a 285 25 19 My book shows they don't make it. your stock tire is 26" tall as long as you stay close to 26.tall ( 25.9 to 26.3) there is no change in your speedometer. have you done a search ?
any way here are a few tire sizes for you.
stock 255/35-19,, 10,0 wide 3.5 wall height 26.0 tall
-------255/30-20,, 10.0 wide 3.0 wall height 26.0 tall
-------265/35-19,, 10.4 w 3.7 ------------26.3 ---
-------285/30-19 ,,11.2-------3.4--------------25.7---
-------285/35-19,, 11.2-------3.9--------------26.9---
-------295/30-19,, 11.6-------3.5--------------26.0

------265/30-20,, 10.4--------3.1--------------26.3-
------255/30-20,, 10.0--------3.0--------------26.0--

jim

 

It is my understanding (and I'm about to test this) that the speedo is PRIMARILY driven by the speed sensors in the front wheels.

I say this because, when I went with the oversize tires on the rear, my speedo originally disagreed with my GPS by 1 mph. Now, it seems to agre with it.

But my TACH, at 80 mph, shows about 300-350 less rpm - and THIS cannot be denied. I don't think oversize REAR tires make any noticeable difference in the Speedo.

But again, I'm about ready to jack up the rear and turn the wheels and see what I get... (As the R&D manager at work often says: Why are we discussing it? TEST IT!)

 

thanks u r right testing it is the real way. we could also road test.
maybe, we can get: A few cars to give us at maybe three speeds,, and of course it has to be in high gear. jim
tire size,,, Speedometer reading, and tachometer reading

 

Thanks for the quick input. I was hoping the speed sensor was primarily front wheel driven.

I especially appreciate the tire size correction. My source had a typo and I did not catch it. I will probably go with the 285/35/19 which is what I suspect my source meant to write. Plus, I think they will look great!

 

After installing 285/35/19 rear tires. My copilot and i tested speedometer accuracy with my ipad air with WAZE navigation app. The speedometer and WAZE reported exact same speeds at 30, 40, 50, and 60 mph. This testing was repeated twice for each speed.

 

Just use this simple calculator : Tire size calculator

 

Yeah right, Math Teachers!!!!

 

When these tires wear down, I'm going with the 285's! And I double checked my source: IT WAS WRONG!

 

Everyone should have a smart phone and Torque. I can display the GPS (actual) speed right next to the OBD (car calculated) speed. No guessing or even thought required.

Just been playing with an 8" Windows tab and my smart phone. Both can connect with Bluetooth and music sounds the same that way or using the AUX input.

Clarion M303 is neat just need to do something about all of the unused wires in back.

 

 

The speed sensors for the XFire are located in the Transmission Solenoid Assembly (TSA) which is mounted & connected to the Transmission Control Module (TCM). There are 2 sensors in the transmission (NAG1 Auto-Shift). Since these serve as the primary inputs to the overal speed control system, the drive-train plays a major part in the overall electronic speed control of the vehicle, which is a "no brainer". (Note: The cruise control system gets inputs from additional multiple sources to control the vehicle speed.) So by changing the rear wheel/tire geometry, one does imact the accuracy of the speedometer readout. But the change is so minor over the range of tire sizes available, most folks wouldn't see a difference (see attached).

What I wasn't able to identify is whether the electronics are sophisticated enough in our XFires for the ECM to automatically adjust the Speedo readout for a detectable speed difference seen by the tranny speed sensors when a different rear wheel/tire setup has been installed. In theory, if any difference was seen by the ECM compared to factory setup, the ECM could make an adjustment so the Speedo maintained its factory accuracy.

 

Thought about that (biggest is the wired remote cable). Problem is that now that it is in it really does not want to come out again. Am going to wait for the SM CD to arrive so I can see how to remove the glove box. I am assuming that I can reach the back of the radio with it out.

Anyone purchasing I would advise against the remote mike. The internal mike should be fine and I have not found a good place to put the external one.

Besides the Clarion I needed the Metra 40-EU10 antenna adapter, Scosche VW01B Power/Speaker connector, and a pair of Porsche/VW/Mercedes radio removal keys, plus nine red crimp butt splices. (Probably better to trim to fit and solder but was lazy. In retrospect probably part of the congestion behind the radio).

I am very slow, ringing everything out and checking twice and took about three hours all told. Took almost as long to figure out how to set the clock (manual is available on line from Clarion).

Did print out all of the diagrams and cautions for reference. Does really being the electronics into this century.

Have shortened the tightening screw in picture & no longer blocks display.

 

Ok, but when I put the bigger tires on the back, my speedometer went from being 1mph off the GPS to agreeing with the GPS. BUT, at 80mph, I lost some ~350 RPM.

This implied to me that the sensors in the front wheels drive the speedometer, and others here agreed.

Oh, and my car is a six speed.

 

This thread was about tire size and speedo accuracy..........edit....okay Pizza see you have us back on track.

 

"The speedometer receives wheel speed data from the CAB via the CAN data bus"

So it is coming from the antilock braking module and probably a meld of all four wheel sensors plus steering and lateral acceleration factors. We would need to know the ABS programming to know which is chosen in the event of a mismatch or what fudge factors are thrown in and when.

Computer may use other sensors but that is where the speedo gets its signal.

 

Where did you find that?

 

2005 Crossfire Service Manual (one that is online, I have not received the 2007 CD yet) page 8J-70 (1024) lower half under Instrument Panel Gauges.

Just above under FSS it has "4 Wheel Speed Sensors (WSS) signals for calculating vehicle speed and distance from the CAB".

Question is how the ABS develops the speed value.

 

I'm still thinking the SPEEDO is based on the front wheels, but we KNOW the ABS has to know what the rears are doing as well.

This is where it's hard to say, so much of this stuff is in the electronics and you can't simply LOOK, like in the old days where you could follow a wire and know - now it's all an input or inputs to a microP which then talks to another microP and yadda, yadda, yadda.

I bet Rudy knows...