Speedometer vs VSS Pulses / Mile

OptimizePrime

Goblin Guru
My smooth brain has never heard of VSS Pulses / Mile until just recently. If you're familiar with it's purpose and calc, this won't be that interesting. If you have modified your wheel & tire size and / or changed the transmission final drive you're going to want to update this value in HPTuners so your speedometer reads correctly, RPD (if you have one) reads correctly, and what you're seeing in your logs is accurate.

In my case, I went from an F35 3.82 fd to an F23 4.17 fd along with some bougie wheels/tires and wondered why my speedometer wouldn't go past 20mph. This calculation is insanely easy.

Step 1 is to get your revs / mile. Head over to www.tiresize.com and type in your brand & size. In my case I was using 285/35/19 r888r's


What you're looking for is the revs / mile figure. In my case it was 773
21438


Step 2 is to take this number and multiply it by the number of teeth on your reluctor (speedometer ring on your diff). For F35's it's 85 and for F23's it's 29.

773 * 29 = 22,417 vss pulses / mile for F23
773 * 85 = 65,705 vss pulses / mile for F35

Insert this number into your vss pulses / mile input and write it to your vehicle. Your speedo is now accurate.
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