The short answer is that LED bulbs use about a fifth of the power, last many times longer, and produce a whiter light. That is the summary. The useful part is knowing which of those differences you will actually notice, and where the marketing overstates things.
Here is the honest comparison, based on how these bulbs behave in a car rather than on a spec sheet.
Power draw
A typical halogen interior bulb draws around 5 watts. The LED replacement draws about 1 watt, sometimes less.
In practice this does not matter for your battery unless you leave a door open for hours. What it does affect is heat. A 5 watt bulb in a small plastic housing gets warm enough to discolour the lens over years of use. A 1 watt LED does not.
That heat difference is why the plastic around older halogen bulbs often looks yellowed. It is not dirt, it is heat damage.
Lifespan
Halogen interior bulbs are rated for roughly 1,000 to 2,000 hours. LED bulbs are typically rated for 25,000 hours or more.
For interior lighting, which is used in short bursts, both numbers are effectively "a very long time". A halogen bulb might last five years of normal use. An LED will outlast the car.
In practice the more common failure is not the bulb dying. It is the plastic housing clip breaking, or a contact corroding. Neither is affected by which bulb you fit.
Light colour
This is the difference you notice immediately.
Halogen bulbs produce a warm yellow light around 3,000K. LED bulbs are available from 3,000K warm white up to 8,000K and beyond, with 6,000K cool white being the most common replacement.
If you fit 6,000K, the cabin looks like a newer car. If you fit 3,000K warm white, it looks like a cleaner version of what you had.
There is no objectively better choice. It depends on whether you want the interior to look modern or to look warm.
Warm-up time
Halogen bulbs reach full brightness immediately, so there is no difference here for interior lights.
This matters for headlights, where some LED retrofits have a slight delay. For interior bulbs it is a non-issue.
Dimming behaviour
Some newer cars dim the interior lights on a fade rather than switching them on and off. Both bulb types handle this, though very cheap LEDs can flicker at low dim levels.
If your car has a fade function and you fit inexpensive LEDs, test the lowest dim setting before you finish the installation. If it flickers, a bulb with a better driver circuit fixes it.
Cost
A halogen interior bulb costs a dollar or two. A quality LED bulb costs three to five times that.
Over ten years the LED works out cheaper because you will not replace it. But the upfront cost is higher, and if you are selling the car next year the calculation changes.
The honest framing: you are not buying LEDs to save money. You are buying them because the light looks better and the housing will not discolour.
The CANBUS complication
This is the one real downside, and it only affects some cars.
European vehicles monitor bulb load. A halogen bulb draws about 5 watts. An LED draws 1. The car sees the lower load and assumes the bulb has failed, which triggers a dashboard warning.
There are two solutions. Fit CANBUS-compatible bulbs that include a resistor to mimic the original load, or add a load resistor to the circuit.
On most American and Japanese cars this does not apply. On Audi, BMW, Mercedes, Volkswagen, Volvo and their relatives, it usually does.
When not to swap
If your car already has factory LED interior lighting, there is nothing to change. Those are LED modules, not bulbs, and they are not serviceable.
If your interior uses LED modules in the trim for ambient lighting, those are also not replaceable. The bulb positions in a vehicle are separate from the ambient strips in most cars, but on some models the entire interior lighting is modular.
Check before you buy. If the part behind the lens is a circuit board rather than a glass bulb, you have a module and a bulb kit will not help.
What people actually notice after swapping
Brighter cabin. Whiter light. No more yellowing around the lenses.
Most people do not notice the power saving or the lifespan. They notice that the car feels newer inside, which is the reason most of them did it in the first place.
Frequently asked questions
Are LED interior bulbs brighter than halogen? A good LED produces more usable light from less power. The difference is noticeable in the cabin, particularly in footwell and trunk positions.
Do LEDs work in any car? They fit any socket that takes the equivalent halogen bulb size. The caveat is bulb monitoring, which some European cars use.
Will LEDs last the life of the car? Typically yes for interior use, since interior lights are used for short periods.
Do I need special bulbs for a European car? CANBUS-compatible bulbs if the vehicle monitors bulb load. That covers most German brands and several others.
Is the colour the same as factory? Only if you match the kelvin rating. 3,000K warm white matches halogen. 6,000K is cooler than factory on older cars.
Browse LED interior light kits for your vehicle.