P J Moran

I just burned a PROM in which I bumped the torque converter lockup point to 56mph. During my test drive, I determined that according to my speedometer, it's locking up at 62mph. Based on traffic, I would say my speedo's pretty close, but I'm not sure.

Either my speedo's off or the VSS is off. Which is more likely correct? Since the VSS in my car (cable driven speedo) simply watches the thingy on the back spin around, it's not necessarily accurate, either (it depends on proper cable "speed"). However, it could be correct and my needle just needs to be pulled and rotated about 6mph worth.

I'm babbling, now, but you see what I mean. I'm trying to decide if I need to put "corrected" numbers on the PROM or move the speedo needle, or both. Perhaps even the VSS is adjustable! Comments?

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'87 IROC 5.7l TPI - original owner!

Andy89RS

Find a stretch of road with highway mile markers, and bring a friend with a stopwatch. Drive along at indicated 60 mph when there's little enough traffic so you don't need to change speed, and compare indicated speed with calculated speed. Or you could just find a shop that does speedo calibration.

JP84Z430HP

As far as what to put into the PROM, I'd use whatever "feels" right.

I'd be willing to say that the VSS is right, but not necessarily. One way to find out is with a scan tool. Compare the VSS on the scan tool with the speedo. If they are different the speedo is definately off, but that's no assurance that the VSS is correct. There is a formula somewhere for taking tire size, rear gear ratio, trans ratio, etc....to give you the correct MPH.

Maybe you could go flying past a cop and see what he clocks you at when he gives you the ticket! LOL

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Working on:
'84 Z28 LG4 305 with 200,000 original miles!
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350
using primarily GMPP parts.
Block is STILL in shop! (Waiting for my to buy the rods!)

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Starting to look like the Kicker poster child!

P J Moran

What is the best way to verify speedometer accuracy? Remember the old Vericom? I have used it before for this and think I'm currently calibrated that way. Is radar reliable?

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'87 IROC 5.7l TPI - original owner!

BowtieNut

The most accurate way is with a GPS. They are accurate to like 1/10 mph.

Grim Reaper

You can adjust the "Pulses to Mile" constant on most of the later ecms. It may not be defined in your eprom, but it is there (somewhere). The Pulses to Mile constant will not adjust your speedo, even if you have an electronica speedo. But you can use it to syncronize your speedo to the ecm.

Once they are syncronized, then any variance between the speedo and actual speed can be cured with speedo gears. If, for some reason, your VSS system does not reflect the different speedo gears (they do on electronic speedos), you can again adjust the "Pulses per Mile" constant on the eprom.

I made a similar change within my eprom as I noticed that the display I got from Diacom was off by 1% from what my speedo actually said. Adjusting the "Pulses per Mile" cured this.

P J Moran

Using my new scan tool, I watched the vehicle speed and found:

SPEEDO ECM DIFF
--20------9----11
--30------21----9
--40------32----8
--50------43----7
--60------54----6
--70------65----5
--80------76----4
--90------86----4
-100------97----3

Interesting, huh? I also measured the rollout of a rear tire, determined the number of wheel revs/mi, etc. and charted the theoretical speeds at various engine RPM's. Those calculations verified the ECM (VSS) values within 1 mph. It looks like the VSS is damn close, and my speedo's way off!

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'87 IROC 5.7l TPI - original owner!


[This message has been edited by P J Moran (edited February 23, 2001).]

Kevin91Z

iTrader: (1)

What tires are you running? Do you have the original 245/50-16's or did you go larger or smaller?

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West Coast GM Shootout 2001!
1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.25 @ 107.18 MPH
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a73camaro

I wouldn't buy into the GPS speed. How can your position on the earth be off by 300 feet, but speed is within a 1/10 of a MPH.

I'm with Andy89RS, mile markers are within inches and you can get down to tenths of a second with a stop watch.

Note: If you got it, set your cruise control when you hit 60 MPH. You should clear a mile in exacly 60 seconds if your speedo is right.

P J Moran

Stock (new) tires. Again, the calculated speeds based on rollout and RPM "matched" the VSS, so I suspect it's correct. The speedo error is huge at low speeds and gets "better" at higher speeds.

I could move the pointer, but that would result in a constant change. What I need is a progressive change. I don't know how a speedo actually works (though I have had several apart). It looks like a magnet or something spinning that sorta "drags" the pointer along. I suspect either the spring rate needs to be increased (stiffened) or something in the spinning part needs adjusting.



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'87 IROC 5.7l TPI - original owner!