Tire size/Speedometer accuracy
 

Re: Tire size/Speedometer accuracy
 

Re: Tire size/Speedometer accuracy
 

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

Re: Tire size/Speedometer accuracy
 

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!)

Re: Tire size/Speedometer accuracy
 

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

Re: Tire size/Speedometer accuracy
 

Re: Tire size/Speedometer accuracy
 

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.:cool:

Re: Tire size/Speedometer accuracy
 

Just use this simple calculator : Tire size calculator

Re: Tire size/Speedometer accuracy
 

Yeah right, Math Teachers!!!! :p

Re: Tire size/Speedometer accuracy
 

Re: Tire size/Speedometer accuracy
 

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.

Re: Tire size/Speedometer accuracy
 

:p:p:p:p

http://www.stitchdiva.com/media/Tuto...ireCutters.jpg

Re: Tire size/Speedometer accuracy
 

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.

Re: Tire size/Speedometer 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.

Re: Tire size/Speedometer accuracy
 

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.

Re: Tire size/Speedometer accuracy
 

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

Re: Tire size/Speedometer accuracy
 

"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.

Re: Tire size/Speedometer accuracy
 

Where did you find that?

Re: Tire size/Speedometer accuracy
 

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.

Re: Tire size/Speedometer accuracy
 

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...

Re: Tire size/Speedometer accuracy
 

Assuming your feedback is valid on the loss of RPM (which I have no reason to doubt), then that verifies that the electronics do compensate for a change in rear wheel/tire setup. Bigger size would require lower RPM (drive shaft speed) to maintain equivalent gound speed. The fact that you've indicated a change in Speed readout indicates to me that the overall accuracy of the Speed is within an acceptable range, which would explain the small +/- change.

When you say the MPH changed, at what speed did your see that? Most Speedos aren't exactly linear throughout their operating range. Was always a problem back in "the good old days" of mechanical hardware. Most likely improved with today's electronics, but a smaller impact.

And per previously mentioned, there are quite a few sensors providing input to overall speed control of the XFire, which has to be a factor in what the Speedo reads out. I do recall seeing a different notation on the 6-speed manual. Gotta go back & look at it now!

Interesting thread!

Re: Tire size/Speedometer accuracy
 

And if the sensors on the front wheels do drive the Speedo, your experience would still verify that the electronic sensors in the Tranny are "doing their thing" when a bigger geometry is installed at the rear. If the rear wheels are turning a slightly lower RPM to maintain same ground speed, the front wheels would be turning the same RPM as before. Speedo would give the same readout. If the electronics did not compensate for the increased geometry at the rear, then the fronts would be turning at a higher RPM too resulting in the Speedo being off (slightly higher).

Re: Tire size/Speedometer accuracy
 

If you really care, you could raise the whole car (jackstands or lift please) and with key on spin one wheel at a time and see if anything registers on the speedo. Then start engine & let rears turn at idle in third. That would be a good set of indicators.

Might set a code also so be ready to clear.

ps "lost 350 rpm at 80" - maybe the tires grew... Nah going from 26 to 28" diameter only drops 200 rpm (2800 to 2600 in 6th). More likely the programming has a fudge factor and the speedo is non linear. Again, need to compare to GPS.

Re: Tire size/Speedometer accuracy
 

Interesting reading, but I did not find my answer.
The Corpus Christi TX-area Deputy insisted he clocked me at 82 mph today. My cruise control was set to 78. My tire sizes are exactly as called out in the owners manual, so my question is - is the speedo on the 05 Crossfire prone to reading 5 mph off? Need to know to keep my daughter ticket free.
First time in 45 year of driving I have ever told a cop I didn't believe his radar. I've been pulled over more in the last 4 weeks than I have in the last 15 years. Cops sure like pulling it over. Cop said, "It looks fast.", so I showed him when I drove off that it was. He followed me for another 10 miles.

Re: Tire size/Speedometer accuracy
 

See if you can get a GPS unit that gives the MPH and have someone note the readings on the speedo and the GPS.
I find my speedo and GPS pretty much agree with each other from what I see.

Re: Tire size/Speedometer accuracy
 

Using my GPS, I have found that my last two Crossfires read 1-2 mph lower than reality when compared to my Garmin. I thought that was unusually accurate. My Ranger shows 80 when my GPS says 77.

Re: Tire size/Speedometer accuracy
 

Mine is spot on - 60 on speedo = 60 on GPS, 75 = 75

Re: Tire size/Speedometer accuracy
 

;) Find a long straight road and set cruise control to 60mph. Time one mile, it should read one mile per minute. If you time it for 5 minutes, it will give you a much closer reading of actual speed (the more miles/minutes the more accurate). Make sure you set CC right at 60 and don't do it on a hilly or twisty road. Good luck! ;)

.

Re: Tire size/Speedometer accuracy
 

Re: Tire size/Speedometer accuracy
 

Thank you once again 180 and Pizzaguy! Downloaded a speedometer app. LIke PizzaGuy's case, it was real close. 75 on the car showed 74 on the app. 78 on the car was 78 on the app. I have to concur that the speedometer is pretty darned accurate.

Re: Tire size/Speedometer accuracy
 

Traffic court is not a place to expect justice. Best to find a lawyer who specializes in this sort of thing (some even guarantee "no points"). Figure $200-$250.

Re: Tire size/Speedometer accuracy