Upgraded to Bigger Tires? Here's Why Your Speedometer Is Lying

Face up to it:

You lifted your truck and slapped on some big tires because it looks cool.

But now you gotta pay the piper - your speedo is off and that can really mess things up.

That shiny new set of 35s might look awesome, but without proper recalibration, your speedometer could be off by several miles per hour, affecting everything from your actual speed to your nav system to your tranny’s shift points.

Let’s dive into why recalibration matters and how to get your truck’s systems back in sync with your new meaty treads.

Why You Need to Recalibrate

Your vehicle’s computer calculates speed based on how many rotations your wheels make over a given distance. Larger tires cover more ground with each rotation, meaning your actual speed is faster than what your speedometer shows.

Too complicated?

Big circle go further than small circle in one turn.

This creates several issues:

• Your speedo reads slower than your actual speed
• Your odo will accumulate miles more slowly than you’re actually driving
• Tranny shift points will occur at incorrect speeds, causing sluggish performance
• Fuel economy readings will be inaccurate(but you’re driving a truck, so YOLO)
• Electronic stability control and other safety systems may not function optimally

As one truck owner described it: “The biggest difference I noticed after installing bigger tires was when I’d roll on the throttle to accelerate a little, the truck would sometimes stay in the same gear instead of downshifting, because the speed-to-throttle ratio at certain speeds wouldn’t trigger the downshift.”

Recalibration Options

How do you do this?

You’ve got choices:

1. Plug-and-Play Calibration Tools

These dedicated devices connect to your OBD-II port and allow you to program your vehicle’s computer with the new tire size.

The short version is:

You’re going to spend a lot of money to use this one thing that only works on one vehicle probably one time.

The longer version:

Pros:

• Simple to use with minimal technical knowledge required
• One-time setup that permanently fixes the issue
• Often includes additional features like diagnostic code reading
• No permanent installation required

Cons:

• Can be expensive ($80-200)
• May not be available for all vehicle makes/models
• Some units are “married” to a single vehicle and can’t be used on multiple trucks

Popular options include the Rough Country Speedometer Calibration Device, Hypertech Speedometer Calibrator, and FlashCal.

2. Tuners and Programmers

If you’re already considering performance tuning for your truck, many tuners include speedometer calibration as a feature.

Tuners are fine, but be aware that you may do something with a tuner that VOIDS YOUR WARRANTY.

Pros:

• Offers multiple benefits beyond speedometer calibration
• Can adjust other performance parameters simultaneously
• Often provides more customization options

Cons:

• More expensive than dedicated calibration tools
• More complex to use
• May require additional subscription fees for custom tunes
• May void your warranty

Popular options include SCT, Bully Dog, and Edge programmers.

3. Inline Calibration Modules

These devices physically install between your vehicle’s speed sensor and computer.

Pros:

• Permanent installation
• No need to keep a device on hand for adjustments
• Often less expensive than full tuners

Cons:

• Requires more installation work
• Stays in your vehicle permanently
• Fewer additional features

4. Software Applications

For some vehicles, specialized software like AlfaOBD can access your vehicle’s computer systems to adjust tire size settings.

The biggest downside to these things is that you need to figure out how to run and use the software. If you’re just messing with big tires, custom software isn’t worth it. Buy something or pay someone to do it.

Pros:

• Often less expensive than dedicated hardware
• Can do other stuff
• May work on vehicles with no other options

Cons:

• Requires technical knowledge
• May require additional hardware adapters
• Can trigger warning lights that need to be reset

5. Dealer Programming

Some dealerships can reprogram your vehicle’s computer to account for the new tire size.

Depending on what you’ve got, your dealer might only be able to reprogram to factory tire sizes. That means no 44" Swampers!

If you can somehow work it in to some other service, you might be able to get this for free, but don’t count on it.

Pros:

• Pro installation
• Don’t need to buy tools
• Won’t void warranty

Cons:

• Often limited to factory tire size options
• Can be expensive ($100-300)
• Many dealers refuse to do this modification because they are dumb

Installation Gotchas

Don’t do any of this without Before attempting to recalibrate your speedometer, be aware of these common pitfalls:

1. Accurate Measurement Is Critical

Don’t just go by the tire size printed on the sidewall. The actual diameter can vary by manufacturer and even by how much air pressure you’re running.

It has literally been like this for years, making many of us wonder:

“Why is this 35 33 inches tall?”

Who knows.

Anyway, for the most accurate calibration, measure your actual tire diameter:

• Place your truck on level ground
• Ensure tires are inflated to your normal driving pressure
• Measure from the ground to the center of the axle and multiply by two, or
• Measure the overall diameter from the ground to the top of the tire

2. Warning Lights and Error Codes

Many calibration procedures will temporarily trigger warning lights on your dashboard. Don’t panic! This is normal and usually resolves after completing the full calibration process, which may include:

• ABS system initialization
• Clearing fault codes
• Performing “sleep cycles” where you turn the vehicle off completely
• Taking a test drive to allow systems to recalibrate

One Ram owner shared: “I changed the tire size and all of a sudden no speedometer, check engine light, and ABS light. I freaked out! But following the full procedure fixed everything.”

3. Compatibility Issues

Not all calibration tools work with all vehicles. Before you buy, try to match:

• Make and model
• Model year
• Engine type (some tools don’t work with diesel engines 🫠)
• Existing modifications

4. Warranty Concerns

Modifying your vehicle’s computer settings could mess up your warranty. Some strategies to minimize this risk:

• For the love of Pete, call whoever holds your warranty and find out
• Use tools that can revert to stock settings before dealer visits
• Choose dealer programming if available
• Document that your calibration only corrects for the tire size change

5. GPS Verification

After recalibration, verify your speedometer accuracy using a GPS speedometer app on your phone. Your calibration should get your speedometer within 1-2 mph of the GPS reading.

DIY or Professional Help?

For many 4x4ers, the DIY approach using a calibration tool offers the best balance of cost and convenience, especially if you plan to make further tire changes in the future.

But it might make sense to get a pro to do it, especially when there’s more computer stuff to deal with.

As one Toyota owner discovered when trying to find a free method to recalibrate: “At this point I would say that running through that procedure does not work. The speedometer is still off. That’s a bummer, I was really kind of hoping this would work.”