Most of what we write explains how to choose well within a category. This one is about categories where the right choice is to keep your money.
Fuel-saving OBD devices
Small dongles that plug into the diagnostic port and promise five to thirty per cent better economy.
The OBD2 port is, for practical purposes, a read-only diagnostic interface. A passive device plugged into it cannot alter fuelling, ignition timing or boost. There is no mechanism by which it could do what the listing claims. Teardowns of these devices routinely find a resistor and an LED.
The measurable effect is a possible parasitic battery drain from leaving it plugged in.
Magnetic and “vortex” fuel treatments
Clamp-on magnets, swirl inserts for the intake, and similar. The claimed physics — aligning fuel molecules, ionising air — does not correspond to how combustion works. Engine management adapts to conditions continuously, so anything that genuinely changed airflow would show up as a fuel trim correction, not free economy.
Ozone generators used while driving
These do remove odours, but ozone is a respiratory irritant and health agencies do not recommend ozone-generating devices in occupied spaces. A car is a very small occupied space.
There is a legitimate professional use — an empty, sealed vehicle, aired thoroughly afterwards. A plug-in unit running while you drive is not that.
Ultra-cheap dash cams
Not the category, the price tier. Below a certain point the compromises are all in the places that matter: a tiny sensor that produces unusable night footage, a low bitrate that smears number plates, and an internal lithium battery that fails after one summer on a windscreen.
A dash cam whose footage cannot identify anything is worse than none, because you believe you have evidence.
Bargain-bin USB-C cables sold as “fast charging”
A cable rated above 60W needs an internal e-marker chip. Cables without one silently cap charging, and nothing on screen explains why. The failure is invisible, which is exactly what makes it expensive: people replace the charger, then the phone battery, chasing a cable.
Tyre pressure valve caps that “monitor” pressure
The screw-on kind that change colour or contain a tiny sensor. They add mass at the valve, they are exposed to weather and theft, and the seal you are now relying on is the cap. A slow leak caused by the monitoring device is a genuinely common outcome.
Phone mounts with very long arms
Not a scam, just physics. A long arm is a long lever with a low natural frequency, which is precisely the range of vibration a car produces. The phone shakes, navigation becomes unreadable, and no amount of clamping force fixes it. Shorter arm, closer mounting point.
Radar detectors
Setting aside effectiveness, they are illegal to use, and in some cases to possess, in a number of European countries, including some where the penalties are severe. Check the law where you drive before considering one at all.
Where we would put the money instead
- A cabin air filter, changed on schedule. Cheap, and it fixes more complaints than any air purifier.
- Sound deadening on the front doors, before new speakers.
- A correctly rated USB-C cable and a charger that matches your actual devices.
- A plug kit and a decent compressor, which get you home when sealant will not.
- A battery tester, because most no-start calls were predictable.
None of those are exciting. All of them do what they claim.
