How Accurate Is Your Model A Ford Speedometer?

A Model A Ford speedometer was never a laboratory instrument, and ninety years of service only makes that more true. Many owners assume the dash readout is close enough—until a club run, a GPS pass, or a passenger's question reveals a steady offset at cruising speed.

Knowing how accurate your speedometer really is matters for touring comfort, honest bench racing, and configuring apps like A-Speed that display GPS-based road speed alongside estimated RPM.

Mechanical speedometers depend on a chain of moving parts between the wheel and the needle. Any change anywhere in that chain shows up as error on the dial.

• Tire size: The speedometer was calibrated for a specific tire diameter. Modern replacements, different aspect ratios, or simply worn tires that are smaller than when new will read fast or slow. Measuring rolling diameter helps you understand the offset.
• Worn speedometer gears: The pinion and driven gears inside the transmission or wheel path wear over decades. A tired gear set is one of the most common causes of persistent error.
• Cable issues: A binding, kinked, or partially disconnected cable can slip, stick, or flutter—especially noticeable at low speeds.
• Age and lubrication:Dry grease, stiff head units, and internal drag in the gauge itself can slow the needle's response or introduce friction that was not there when the car was new.
• Rear-end or tire changes: If the axle ratio or rolling diameter changed since the car left the factory, the original calibration may no longer apply at all.

Perfect agreement between a cable-driven gauge and GPS is rare—and not always necessary. What matters is knowing your offset so you can cruise confidently, judge RPM sensibly, and avoid fooling yourself about how fast the car is really moving.

A consistent error of two or three MPH at highway speeds is workable if you understand it. An unpredictable gauge that jumps or lags is a different problem—and worth fixing before long touring seasons.

The most reliable field check compares your dash reading to a trusted GPS speed source on a smooth, level road. You do not need special equipment beyond a safe stretch of pavement and a steady hand on the wheel.

1. Choose a calm road. Pick a flat section without heavy traffic, sharp grades, or tight curves. Steady cruise speeds are easier to compare than stop-and-go driving.
2. Warm up the car. Let tires and driveline reach normal operating temperature. Cold tires can affect rolling diameter slightly.
3. Hold a steady speed. At 30, 40, and 50 MPH (or your usual touring speeds), maintain an even pace for at least fifteen to twenty seconds.
4. Record both readings. Note the mechanical speedometer and the GPS speed at the same moment. A passenger with a phone mount makes this much easier.
5. Repeat in both directions. A slight road grade or wind can skew one pass. Averaging a run in each direction gives a fairer picture.
6. Calculate the offset. If your gauge reads 47 MPH when GPS shows 45, you are about two MPH fast at that speed. Check whether the error grows with speed—that points totire diameter or gear issues.

GPS speed measures how fast your position changes over the ground. It does not depend on tire wear, cable condition, or transmission gears. That makes it an excellent reference when you want to know true road speed on tour.

GPS can lag briefly during hard acceleration and may wander a little at very low speeds, but at Model A cruising speeds on open road it is generally steadier than a tired mechanical gauge. For a deeper look at GPS behavior in A-Speed, see the Help page.

A-Speed displays GPS-derived MPH or KPH in vintage-inspired gauge layouts designed for glanceability in a Model A cockpit. Many owners mount an iPhone beside the original dash and use A-Speed as a secondary speed reference—especially when the factory gauge is unknown, inconsistent, or purely decorative.

A-Speed does not replace a restored mechanical speedometer if you want the original look and feel. It does give you an honest, repeatable number for touring, RPM estimation, and comparing against what the cable-driven needle says. Pair it with an accurate rear-end ratio and measured tire diameter in Settings, and your RPM estimates become more meaningful too—especially with overdrive configured correctly.

If error is large, grows quickly with speed, or the needle sticks and jumps, internal gear wear or cable problems are likely. Vintage speedometer rebuild specialists can often recalibrate for your currenttire size and axle combination.

Until then, knowing your offset—and carrying a trustworthy GPS readout on tour—is the practical enthusiast approach. Fix the gauge when you are ready; drive with your eyes open in the meantime.