A base tune is only as good as the info that goes into it. The PCM uses your gear ratio and tire size to calculate vehicle speed. Vehicle speed controls your speedometer, your shift points, and torque converter lockup. Your injector size and cam specs decide whether the engine starts and idles or fights you every time you turn the key.

The good news is that most of these numbers take only a few minutes to find. Here's what to gather before you order.

1. Your PCM Service Number

This tells us exactly which computer you have and which operating system it's running. It's printed on the label on the PCM case, usually starting with "Serv. No." Take a clear photo of it before you ship. Labels get scratched in shipping, and the photo saves time.

Not sure if you have a P01 or a P59? Check the connector colors. Red and blue means P01, and green and blue means P59. Our P01 vs P59 guide covers the rest.

2. Your Rear Gear Ratio

A wrong gear ratio throws off your speedometer, and on an automatic it also throws off shift points and converter lockup.

If the rear end came from a GM truck or SUV, check the RPO sticker in the glovebox or on the door jamb. Common codes include GU4 (3.08), GU5 (3.23), GU6 (3.42), GT4 (3.73), and GT5 (4.10).

If it's from something else or has been changed, look for a tag on the differential cover bolts, or count it yourself: lift both rear tires off the ground, put the transmission in neutral, mark the driveshaft, and turn both rear tires together one full turn. The number of driveshaft turns is your ratio. 3¾ turns, for example, means about 3.73.

3. Your Tire Size

We need the overall tire diameter, not just the wheel size. You can find it on the tire sidewall. For a size like 275/60R15, the math works out to about 28 inches tall. If you're running a tire that lists its height directly, such as a 28x10.50R15, the first number is the diameter.

Give us the tire you're actually running, not the one that came on the vehicle.

4. Your Injector Size

If you're running the stock injectors that came with your engine, just tell us the engine and year. If you've swapped injectors, we need the size in lb/hr or cc/min, plus the brand and part number if you have it. Injector data is one of the biggest factors in whether a swap starts clean or floods and dies.

Not sure what size you need? Try our LS Injector Size Calculator.

5. Your Cam Specs

If the engine has a stock cam, just say so. If it has an aftermarket cam, send the specs from the cam card or the manufacturer's listing: duration at .050", lift, and lobe separation angle (LSA). The brand and cam name help too.

A bigger cam changes idle vacuum, airflow, and how the engine wants to be fueled at startup. That's why cammed engines get a different base tune. (More on that in Does a Cammed LS Need a Different Base Tune?)

A Few More Things to Have Ready

  • Transmission: 4L60E, 4L80E, manual, or something else
  • Throttle type: drive-by-cable or drive-by-wire
  • Engine size: 4.8, 5.3, 6.0, and so on
  • Fan setup: electric fans and how they're wired
  • Any other changes: MAF delete, O2 sensors removed, headers, or anything else that's not stock

Missing one or two items? Send what you have and we'll work through the rest with you.

Ready to Order?

Once you've got your numbers, ordering takes just a few minutes. Pick your package, enter your build details, and ship us your PCM.

  • Stock / Near-Stock Base Tune: $100
  • Cammed / Modified Base Tune: $150

Supports P01 drive-by-cable and P59 drive-by-wire PCMs. Typical turnaround is 2–3 business days after we receive it.

Order your mail-in LS swap base tune here →

Want a full pre-start checklist too? See our LS Swap Startup Checklist.