Help recalibrate 98 Camaro Speedometer

[georgefr]

Hey Guys,

This is my first posting, and thanks in advance for your help on this.

I just had my old 4.10 geared 10 bolt rear end swapped for a 3.91 geared 12 bolt. My tuner made an educated guess on speedometer recalibration. His guess resulted in a speedo that reads an indicated 60 mph at an actual 64 mph. I'd like to get it closer. I thought the speedo was accurate before the swap, but in retrospect, who knows?

In the Speedometer window of VCM Editor I see my tuner changed the original Tire/Gear from 5.95 to 6.24, the General VSS Pulses per Mile from 57648 to 54968, Final Drive Ratio from 965.2734 to 920.3906, while keeping Rev-Trans at 17.000. The original numbers were apparently installed on my computer by a speedo recalibration by a Hypertech Power Programmer when I changed my gears from stock 3.42 to 4.10 about 6 years ago. My tuner apparently adjusted those numbers by the ratio of 4.10/3.91 which equals a 4.859% difference.

I have seen postings in this forum that describe how to calculate VSS Pulses, and I calculate 52366. I'd guess the Tire/Gear is the ratio between tire diameter (25.6 inches for me) and rear end gears, which I calculate to be 6.547. I was unable to find a posting on how to calculate the Final Drive Ratio.

Here are my questions......
1. Was my tuner correct in modifying these three entries? If not, which entries should be modified?
2. Are my tuner's entries correct or are mine better?
3. If you like my calculations better, and the Final Drive Ratio must be changed, what are the steps to calculate this entry?

Thanks so much for your help!!

[Russ K]

http://www.thetuningdoctor.com/HPTun...gearchange.swf

Russ Kemp

[georgefr]

Russ,

Thank you so much for this link. I watched the video and I have a few more questions.

1. The video shows how to make changes to the Tire/Gear number. As I suspected this is the ratio of tire diameter to gear ratio. If the default value for tire size is 24.9 inches and my car was initially calibrated for 4.10 gears and stock tires by a Hypertech Power Programmer , then why was my value 5.95 and not 24.9/4.10=6.07?

2. Regardless of the original 5.95 and the present 6.24 value that my tuner provided, I guess my calculated value of 25.6 (thetirerack.com's value for my tires) / 3.91 = 6.55 would be correct, or should I use 24.9/3.91=6.37, or is it my call?

3. I notice in the video that no changes are made to the VSS Pulses per mile. The value of the cell is 47018. Should I change my VSS Pulses to this value or should I use one of the three other values in my original posting?

4. I also notice in the video that no changes are made to Final Drive Ratio. The value of the cell is 805.1953. Should I change my Final Drive Ratio to this value or should I use one of the other two values in my original posting?

Thanks!
George

[Russ K]

If you have stock dia tires, then use 24.9/4.10=6.074.

Russ Kemp

[georgefr]

My tires are 25.6 inch diameter and recall, I now have a 3.91 gear. I'm leaning toward 6.55 for Tire/Gear. Sound right?

What numbers would you suggest for VSS Pulses and for Final Gear ratio given my tire size and gear ratio?

[georgefr]

Well, guys, I've got it figured out. My speedo is now within 1% accuracy, an accuracy greater than the placement of the highway mile markers I tested the speedo against. I'm satisfied with that. Thanks to Russ K for the clues.

For the purpose of helping out any others researching this problem, here is the long story.....

My tuner had it right. Tire/Gear, VSS Pulses, and Final Drive Ratio can simply be adjusted from stock values by the percentage of gear ratio change, assuming you still have stock tires. My problem was the Hypertech Power Programmer installed values were incorrect. I've probably had this much speedo error all along and didn't know it.

Stock values for a 98 Camaro with M6 should be 7.29, 47018, and 805.1953 respectively for these values. I got these from a stock tune in the Repository on this website. These numbers are calculated based on the stock tire size of 275/40R17. If you look up the manufacturer's stated diameter of this tire you will find 25.6 inches. The value GM uses is 24.9 inches which exactly compensates for the squish this tire experiences when the weight of the car loads it. If you look up the revs/mile on this tire the manufacture provides 811. Do the math and you will find this 811 does indeed correspond to the effective diameter of 24.9 inches, not the unweighted diameter of 25.6 inches. Lesson 1, measure your tires as installed on the car from axle center to pavement, then double that to determine their effective diameter to be used in any calculations.

Lesson 2, if you have changed gears from stock (3.42) to 4.10, for example, that ratio is 4.10/3.42=1.19883. Decrease Tire/Gear by this ratio, increase VSS Pulses by this ratio, and increase Final Drive Ratio by this ratio. My tuner had that exactly right.

Lesson 3, when you enter the new value of Tire/Gear and hit the Commit button, the VSS Pulses and Final Drive Ratio are automatically calculated (at least that seemed to be how my beta version worked). It seemed to allow me to input small changes from these values, but it would not tolerate larger changes and gave error messages. Since the values the program suggested were pretty close to those I had calculated, I simply used the suggested values. Once I got on the highway and timed the car through the mile markers I determined I was now only 2.5% off. I readjusted the Tire/Gear value by this amount, VSS Pulse and Final Drive Ratio automatically adjusted themselves, and a retest though the mile markers shows it's probably as accurate a speedometer as I need.

If you change tire size then adjust these three values by the ratio of the old vs the new tire diameters. Be sure to use the loaded tire measurement, not the unloaded diameter provided by the manufacturer. You could also use the ratio of the revs/mile, which is the same ratio as loaded diameters.

If you happen to be running the 245/50R16 tire size, the manufacturer gives its revs/mile as 813. For most folks that is close enough to 811 that it isn't worth recalibrating the speedo for this small change. If the speedo was 100% accurate at 60 mph on the 275, the error on the smaller 245 tire would be less than 0.2 mph at an indicated 60 mph.

Thanks for your help,
George

[carlitos2012]

hi
i know is bin 7 year but can you help me calibrate my car?
i have a 1998 ls1 automatic trans am, now i have fully build trans and 4.10 gears and 305 35 18 tires.
i have hp tuners but don't know how to calculate, can you help. thanks