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Speedometer Calibration

 
Old 02-10-2019, 03:31 PM
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woodshop57
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Default Speedometer Calibration

Mid-summer in 2018, I purchased a set of OEM C6 wheels and installed them (along with new tires, of course) on my 1996 LT4 CE. But I've noticed that since changing from all 17" wheels and tires that had been on the car to the now 18" fronts and 19" rears, that my speedometer is off....and quite a bit actually. When my speedometer indicates 75 MPH I am actually doing 82 mph. Because of the disparity between displayed and actual, I now have to drive with a GPS app on my phone so I can more readily know just how fast I am driving.

Is there a mechanical thing that I need to change to bring the actual and displayed speeds closer together or is it something that has to be done through some kind of ECM tuning or something? Any help or guidance anyone can provide will be greatly appreciated!
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Old 02-10-2019, 03:59 PM
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Originally Posted by woodshop57 View Post
Mid-summer in 2018, I purchased a set of OEM C6 wheels and installed them (along with new tires, of course) on my 1996 LT4 CE. But I've noticed that since changing from all 17" wheels and tires that had been on the car to the now 18" fronts and 19" rears, that my speedometer is off....and quite a bit actually. When my speedometer indicates 75 MPH I am actually doing 82 mph. Because of the disparity between displayed and actual, I now have to drive with a GPS app on my phone so I can more readily know just how fast I am driving.

Is there a mechanical thing that I need to change to bring the actual and displayed speeds closer together or is it something that has to be done through some kind of ECM tuning or something? Any help or guidance anyone can provide will be greatly appreciated!
An LT4 and a ZF can be sometimes corrected 'mechanically'. I'm quite sure a late ZF was built with 42 tooth driven gear. What tire size do you have on the rear now. It can be approximated using tire diameters easily. It's difficult to attempt using an approximation of 10%+ at the mentioned 75MPH. Most GPS/MPH apps I'd think accomplish .00 MPH. If your TACH is thought to be accurate (parks @0 with KEY-ON/ENGINE OFF) it would be good to know the RPM at the GPS 82MPH in 4th gear.

Rear is I assume a 3.45 stock?

It might be interesting to check RPM at 45, 55 & 65 MPH (GPS) in 4th gear. I'd note also 'cluster' MPH @ those GPS MPH also.

*** Using the VSS in the car maximum correction would be a 40 tooth so I'd buy a 25513047 GM gear and 15552872 o-ring, just swap it and see what you've accomplished. A gear and an o-ring that's a very easy swap.Using some calculators that exist and maybe a 285/35-19 tire it suggests a 38.xxx tooth count. Your VSS will do 40 but nothing smaller. What size are your tires?

Last edited by WVZR-1; 02-10-2019 at 05:38 PM.
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Old 02-10-2019, 09:32 PM
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Originally Posted by ihatebarkingdogs View Post
If the car now has a 42 tooth gear. And 75 INDICATED is exactly 82 MPH ACTUAL, l. I did the math for the following gears:

38 tooth is a .096% correction, 82 MPH actual will indicate 82.9 MPH
39 tooth is a .072% correction. 82 MPH actual will indicate 80.8 MPH.
40 tooth is a ..05% correction. 82 MPH actual will indicate 78.75 MPH.

40 teeth isn't going to get it. The 39 is probably better in that the error will become less as the tires wear. But it requires a different VSS sensor in the transmission. WVZR will have to tell you if it exists or not for the ZF. I don't know if it is or not.

Personally, I use the odometer to calculate error. I reset the car's trip odometer and my GPS odometer and compare the two readings after a good distance of mostly straight-line highway or interstate travel. Divide one reading into the other and you get the error. Add teeth to lower the reading, Remove teeth to raise the reading.

If there isn't a different offset VSS available, you're going to have to resort to an electronic ratio adjuster.
We don't know that his 75 is 75.0 or 75.999 and that his 82 is 82.0 or 82.999. The 40 tooth trial is likely a $12 - 14 expense and then it is what it is. I've done corrections using actual dimensions of the tire on the car not advertised by the tire guys.. Pretty easy! An ERA for the OP I'd think is a 'stretch' and not required!

Measured miles and odometer are certainly the 'actual confirmation'.

Last edited by WVZR-1; 02-10-2019 at 09:34 PM.
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Old 02-11-2019, 01:11 AM
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Maybe you can find an old Hypertech Power Programmer III out there somewhere. Those will reflash the computer for tire/wheel changes.
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Old 02-11-2019, 08:36 AM
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It might get interesting when the OP returns. I had assumed (but asked) axle ratio and tire size. I assumed maybe a 19" popular C6 tire. Comparing a 285/40R17 and 285/35R19 the OP's 'error' is considerably greater than what a person might expect using customary 'tire guy' information.

Axle ratio and tire size is going to be very important when he returns. I also assumed since the 1st time poster mentioned '96 LT4 that it would of course be a ZF. Maybe nothing is correct. The 42 tooth expected to be stock driven if a ZF is from some SPO information I expect is correct.

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Old 02-11-2019, 10:18 AM
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If the OP has (or has access to) Jet DST it can also be corrected in the PCM. However, my question is what size tires are actually on the back of his car?! That's a huge speedo error, and if it's just from tire diameter then I'm going to suggest that he needs to replace the tires with ones that are the proper diameter for a C4 (25.7"-26.1").
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Old 02-11-2019, 11:27 AM
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I just did one of these on my 96 LT1 A4 after I re-geared the rear end. Worked good once i got the wiring figured out.

https://www.ecklerscorvette.com/corv...1984-2004.html
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Old 02-11-2019, 03:32 PM
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The tires on the front of my LT4 are now: 255/40ZR18 and on the rear they are 285/35ZR19. Prior to this change to 18/19" combo, I had 17" all the way around with 255/45/ZR17 on the front and 285/40/ZR17 on the rear. So the 18"s have a little short sidewall than the 17" and the same holds true on the 19".

I tried to order the part suggested by WVZR-1 from GM Parts on-line and they don't carry it any longer and don't have any suggestions as to a source so I don't know for sure what I am going to do. No one in my near-area of north central Illinois will or can tune the car either.

I don't drive the car in the snow and salt so i can not get RPM information when driving certain speeds in fourth gear. I doubt it is helpful but when I am driving 82 MPH in 6th gear (3:45 ratio rear end), my RPMs are at 1950 rpm.

Last edited by woodshop57; 02-11-2019 at 03:57 PM.
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Old 02-11-2019, 04:19 PM
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Originally Posted by woodshop57 View Post
The tires on the front of my LT4 are now: 255/40ZR18 and on the rear they are 285/35ZR19. Prior to this change to 18/19" combo, I had 17" all the way around with 255/45/ZR17 on the front and 285/40/ZR17 on the rear. So the 18"s have a little short sidewall than the 17" and the same holds true on the 19".

I tried to order the part suggested by WVZR-1 from GM Parts on-line and they don't carry it any longer and don't have any suggestions as to a source so I don't know for sure what I am going to do. No one in my near-area of north central Illinois will or can tune the car either.

I don't drive the car in the snow and salt so i can not get RPM information when driving certain speeds in fourth gear. I doubt it is helpful but when I am driving 82 MPH in 6th gear (3:45 ratio rear end), my RPMs are at 1950 rpm.
The gear was available on eBay when I mentioned it by part #. I believe your first move needs to be a 'confirmation' of the rear axle ratio. Using 'tire guy' information you shouldn't have the 7+ MPH differential.. You've a good bit going on there. I believe an accurate tach @ 1950 with a ZF would/should be nearly 91 MPH in 6th gear.

Greg @ Blower Works on the forum here would likely have the gear also.

A 3.73 gear @ 1950 RPM (if accurate) would get very close to your mention MPH.

***** If you found a 3.73 ratio and the previous owner did a correction using a 45 tooth driven (considered a 1% accurate w/2.85/315-17) then it may be possible to use a driven change to correct it. I'd prefer a 'mechanical' vs ERA correction. I'd say it's time to count the pinion rotations vs 1 rotation of rear wheel/tire.

Last edited by WVZR-1; 02-11-2019 at 04:45 PM.
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Old 02-11-2019, 05:10 PM
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That's what I was going to suggest - after replacing the D36 with a D44 and messing with the gearing, I did the math and used the Power Programmer to make the necessary adjustments - it's within 1MPH of GPS, which is closer than any other vehicle I've ever owned.

Originally Posted by gpierce View Post
Maybe you can find an old Hypertech Power Programmer III out there somewhere. Those will reflash the computer for tire/wheel changes.
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Old 02-11-2019, 07:33 PM
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Originally Posted by woodshop57 View Post
The tires on the front of my LT4 are now: 255/40ZR18 and on the rear they are 285/35ZR19. Prior to this change to 18/19" combo, I had 17" all the way around with 255/45/ZR17 on the front and 285/40/ZR17 on the rear. So the 18"s have a little short sidewall than the 17" and the same holds true on the 19".
That tire is 4.7% taller than the typical 25.6" tire, or 3% taller than the later non-Z51 285/40/17 that came on base C4s. It's really too tall, and you should have gone with a 285/30/19 instead. But, even a 5% difference should have only given you half the error you are getting (i.e., 78.5mph actual against an indicated 75mph). So something else is going on here. My 96 LT4/ZF6 car is geared stock, and it actually reads about 1-2mph faster than is really going on the highway with 275/40/17 (25.6") rear tires.
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