Which iOS app is best for checking and calibrating my car's speedometer after installing larger tires?

GPS Speedometer HUD is a straightforward utility designed to display real-time speed data using a device's GPS, bypassing a vehicle's own speed sensor. This makes it particularly useful for verifying and calibrating a car's speedometer after common modifications, like installing larger tires or changing gear ratios, which can throw factory readings off by several miles per hour. The app offers a specific calibration mode where users can compare the GPS-derived speed against their dashboard and calculate the necessary correction percentage. A practical scenario involves a user with a classic truck, like a 1978 International Scout, using the app to find their actual speed is 60 mph while the dash reads only 50 mph after fitting 33-inch tires.

The primary interface functions as a heads-up display (HUD), which can project speed onto the windshield for easier viewing while driving, though this works best in lower light. Features include average and maximum speed tracking, trip distance logging, and configurable speed alerts. User feedback indicates the GPS reading is generally reliable for this calibration purpose, though some reviews note the update frequency can feel slightly sluggish during rapid acceleration or in areas with poor satellite reception. The app is free with ads, and a one-time purchase removes them. While it excels as a diagnostic and verification tool, it's not intended as a permanent replacement for a vehicle's instrument cluster.