Speedo correction

[Fully Vetted]

I'm getting ready to replace my factory 17" A molds with Series One's which are 18's with 335/35 PS2's. Any idea how far off my speedo is going to be and the most efficient way to correct it?

I did a search and the following thread is a couple of years old so I thought I would check and see if it's still relevant.


http://www.zr1.net/forum/showthread.php?t=11556&highlight=speedometer+correction

[1991ZR1]

Are you sure the tires aren't 335/30/18?

When I changed to 335/30/18, I checked the diameters of the original tires and the 335/30/18 on the Tire Rack website. The diameters were very close. That was a long time ago so you should probably check again with the specific tires that you are getting.

[Fully Vetted]

You are correct. They are 335/30/18. Not sure where I got 335/35/18. I don't even think they make a tire in that size. Anyway, the overall diameter shows to be 25.9 so I'll compare that to what I have on there currently.

If there is a difference or when I go to 410's is the preferred method of speedo correction with these correction boxes or is it with a Haibeck speedo gear swap?

[Paul Workman]

Are you sure the tires aren't 335/30/18?

When I changed to 335/30/18, I checked the diameters of the original tires and the 335/30/18 on the Tire Rack website. The diameters were very close. That was a long time ago so you should probably check again with the specific tires that you are getting.


Yep!

Handy tool for tire diameter comparison (C-Link). (http://robrobinette.com/tire_diameter.htm)

Variance % (speedo error) = 100D1/D2

where D1, D2 = respective tire diameters.

In the case of the 315/35/17 vs. 335/30/18, the difference is approx 1%; not enough to get me to change anything. But, CCmano's digital speedo correction would interest me, should I ever need to calibrate the speedo.

P.

[GOLDCYLON]

You are correct. It is 335/30/18. Not sure where I got 335/35/18. I don't even think they make a tire in that size. Anyway, the overall diameter shows to be 25.9 so I'll compare that to what I have on there currently.

If there is a difference or when I go to 410's is the preferred method of speedo correction with these correction boxes or is it with a Haibeck speedo gear swap?

WVZR-1 appears to be all over this upgrade. He would tell you for it right do it with the correct speedo gear swap. The correction boxes will work in a pinch. However WVZR-1 is a wealth of knowledge on this subject. GC

[Jagdpanzer]

Check this link
http://www.rimsntires.com/specs.jsp

[secondchance]

I'm getting ready to replace my factory 17" A molds with Series One's which are 18's with 335/35 PS2's. Any idea how far off my speedo is going to be and the most efficient way to correct it?

I did a search and the following thread is a couple of years old so I thought I would check and see if it's still relevant.


http://www.zr1.net/forum/showthread.php?t=11556&highlight=speedometer+correction

Fully Vetted,

Funny you should ask. I just had Michelin 275 35 18s (front) and 335 30 18s (rear) mounted last Saturday. Prior to this I was running Michelin Pilot Sport 335 30 18s in the rear.
Speedo reading at 60 mph was reading 61 (actual) on my Garmin - error is negligible in my opinion.
BTW you will love these PS2s.:cheers:

[VetteVet]

Speedo error can be corrected with the PROM programming. The only issue is that the FX3 curves will be shifted by the amount of the VSS signal error. This will illustrate it:

VSS signal(4000 pulses per mile) is output directly to FX3 module and ECM in parallel. ECM measures frequency of incoming signal and converts to MPH using the road speed constant variable. ECM outputs speed info digitally on the 8192 baud serial line to the CCM. CCM takes the speed info and displays it on the LCD. ECM also generates a signal (2000 pulses per mile) to the Cruise Control module. This is not a 2:1 pulse divider circuit in the ECM. The ECM actually generates the 2k pulse/mile signal based on the measured MPH.

Therefore, adjusting the road speed constant variable will correct any speed errors due to tire/wheel or rear ratio changes. Speedo will read correctly and cruise will operate properly. The FX3 damping adjustments will be affected by the amount of the VSS error.

When I swap in my 4.10 gears, I intend to adjust via the ECM and my FX3 damping adjustment points(in MPH) will shift approximately 19% in a stiffer, more aggressive direction, which is a good thing as far as I am concerned.

Jep