FREE TOOL

Download Spec Manager free

Spec Manager is Freightliner's official tool for checking and comparing driveline specs — engine, transmission, tire size and rear axle ratio. Among other things it gives you the revolutions per mile for a tire size, which is what you need when you reprogram the ECM. It is free, it comes from the manufacturer, and here is the direct link.

Go to the official download

Opens freightliner.com — the manufacturer's site. You may need to create a free account.

What you can do with it

  • Get the revolutions per mile from your tire size — the number you program into the ECM when you change tire size, so the speedometer and the odometer read right.
  • Confirm the right rear axle ratio after a differential change.
  • Troubleshoot a low-power complaint by checking if the specs actually match.
  • See how engine, transmission, tire size and axle ratio affect fuel economy.
  • Compare driveline combinations before buying or changing a truck.
  • Check how many shifts a trip will take with a given setup.

How to get the revolutions per mile

  1. Open the link above and create a free account if the site asks for one.
  2. Enter your truck: engine, model, transmission, rear axle ratio — and above all the tire size you are running or about to install.
  3. Read the revolutions per mile the tool returns for that tire size.
  4. That is the number that goes into the ECM. If it does not match what is programmed, the speedometer and the odometer will read wrong.

If the number does not match and you need the ECM reprogrammed, write to us — we program by VIN with factory software.

Watch how it's done

Programmer Mode, part 4 — the same calculation, explained step by step.

@cartuningstarcts

Programmer Mode, part 4: adjusting tire size

Watch on TikTok

Watch on TikTok ↗

Changing the engine or the specs?

Spec Manager tells you which specs fit. Making the truck actually run with them is the other half. At CTS — a diesel electronics lab in Houston — we build our own engine wiring harnesses for Detroit Series 60 swaps in Cascadia, Peterbilt and Kenworth, plug & play with no cuts or splices, and we program ECMs by VIN.

See our harnesses

Spec Manager is free software from Daimler Truck North America. CTS – Car Tuning Star does not host, distribute or sell it, and is not affiliated with, endorsed by or an authorized dealer of Daimler Truck North America or Freightliner. The link above points to the manufacturer's official site. Brand and model names are used only to indicate compatibility.