Is Your Car's Speedometer Actually Accurate?

Your car's speedometer might be lying to you. Not by much—a few mph. But enough to matter if you're trying to avoid a speeding ticket or wondering why the car next to you is keeping pace when your speedometer says you're going faster.

How Speedometers Work

Older vehicles use mechanical speedometers with a cable connecting to the transmission. Modern cars have electronic speedometers that use tire speed to calculate how fast you're going.

Sensors measure how fast your tires rotate, convert that to mph, display the number. Simple in theory. Accurate in practice? Not always.

What Throws Them Off

Tires are the biggest factor. Since modern speedometers calculate speed based on tire rotation, tire condition matters.

Bigger tires than stock? Your speedometer thinks you're going slower than you actually are because it's calibrated for stock tire size. Smaller tires have the opposite effect.

Low tire pressure changes the effective diameter of your tire. Worn tread does too. All of this throws off the calculation.

Then there's the intentional inaccuracy. Many automakers make vehicles display speeds faster than you're actually going, especially performance cars, to silently persuade drivers not to go too fast.

Your speedometer says 75 mph. You're actually going 72 mph. Close enough that you don't notice, but enough to keep you slightly more cautious.

It's a liability thing. Better to have speedometers read slightly high than slightly low and risk customers getting speeding tickets.

GPS Apps Are More Accurate

Several apps on the Apple App Store or Google Play have live speedometers. Some are traditional GPS navigation apps. Some are dedicated speedometer apps.

Google Maps has a speedometer feature that came from Waze after Google purchased the app in 2013. Google Support has a disclaimer though: "Speedometers shown in the Google Maps app are for informational use only. Make sure to use your vehicle's speedometer to confirm your actual driving speed."

Translation: Don't blame us if you get a speeding ticket because you relied on our app.

GPS-based speed readings are generally more accurate than your car's speedometer because they measure actual distance traveled over time, not tire rotation speed.

Your car's speedometer can be thrown off by tire size, tire pressure, tread wear, and manufacturer calibration. GPS measures actual movement regardless of what your tires are doing.

Keenan Thompson from Detroit Free Press wrote that after driving his personal car and several press vehicles, he's noticed his own car's speedometer matches navigation apps while some other cars are off by a few mph.

That tracks. Some manufacturers calibrate speedometers more accurately than others. Some intentionally read high. Some are just off because of how they calculate speed from tire rotation.

Does It Actually Matter?

For everyday driving, a few mph difference doesn't matter much. Your speedometer says 65, you're actually going 63. Not a big deal.

It matters in two situations:

• Speed limits. If your speedometer reads high, you're fine. If it reads low and you’re already going a few over the limit, that's how you get tickets.

• Fuel economy. Speed affects fuel efficiency. If your speedometer is off, you might not realize how fast you're actually going and wonder why your gas mileage sucks.

How to Check

Pull up Google Maps or Waze next time you're on the highway. Compare the app's speedometer to your car's. If they match, great. If they're off, now you know.

If your speedometer seems way off—like 10+ mph difference from GPS readings—get it checked. It could be a sensor issue, major tire size mismatch, or other mechanical problem.

For most cars, being off by 2-3 mph is normal. Annoying, but normal.

The Bottom Line

Car speedometers can be inaccurate by a few mph, caused by tire size, tire pressure, tread wear, or intentional manufacturer calibration to read slightly high.

GPS apps are generally more accurate because they measure actual distance traveled, not tire rotation speed affected by all those variables.

Use Google Maps or Waze to check if your speedometer is accurate. Just don't rely solely on apps while driving. They're for reference, not official speed measurement.

And regardless of what any speedometer says—car's or app's—respect the speed limit. Whether you're going 2 mph slower or faster than you think doesn't matter if you're doing 80 in a 55 zone.

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