Amp draw before and after LED.
 

1978 to 1991 9th gen chevy/GMC suburban and pickups with the same lighting scheme.

OE running lights draw 3.75 amps.
OE brake lights draw an additional 3.5 amps.
OE halogen low beams soak up 10 amps.
At a stop light after dark sitting there with the brakes pressed the electrical system demands 17 amps.
That's a lot if you are trying to be out driving around with no alternator.

With a mix of mostly LEDs and some incandescent instrumentation and running lights it uses 2 amps (mostly due to OE dash lights I never changed over to LED and the front blinker/running light that I can not switch to LED or my blinkers wont blink)
LED brake lights added an additional 0.5 amps
LED head lights use 2.5 amps.
At a stop light after dark sitting there with brakes pressed the electrical system now demands only 5 amps.
12 amps makes a big difference.

That is significant! Headlights especially.
Arguably taking your foot off the brake and engaging the parking brake would save a little bit extra. :-)

Great information oil pan!

Nice info, thanks for sharing. Agreed, 12A is pretty huge reduction when alternatorless.

lessens the chance of rearenders by keeping them on though.

I could further reduce the running light draw.
The front blinker and running light, a 2 filament #1157 bulb. If I really wanted to I could remove and rewire the front blinker/running light so that the running light function is accomplished with an LED and put something like an #1156 bulb for the actual blinking of the blinker.
I just need something that draws a little more than an amp on one side of the blinker circuit to cycle the blinker module.

Doing this would only save about 0.5 to 1 amp when the running lights are on, hence the reason I have not messed with it.

Also the dash lights. Those are a pain and would only save up to about 1 amp.
Most of them are burned out in my 454 gas suburban so I need to take care of them at some point.

I did similar in my Accord. LED headlights, brake lights (except the center one) and corner lights. Big reduction in amps draw. Thumbs up.

I have been pondering on this with my Civic Hybrid. It is alternatorless, using a dc-dc converter to keep the 12v battery alive. So at night, that is amperage draw pulling off of the hybrid battery, and that is current that is not being utilized to drive the vehicle, requiring more regen. If you think an alternator has draw, you oughta feel what happens to an already underpowered civic when it kicks in full regen. It does make for a nice engine braking effect when decelerating though...
Have you looked at swapping to an LED friendly blinker module? What model Chevy are you working on? I know my 03 Silverado had an electronic module on the back of the fuse block, but GM does have an LED friendly variant available.

As much as I like the economy gain from the few LED's I've put in my car, the dome light swap to LED actually lost me some economy.
As I replaced the 8W uncandescent bulb with a 10W led grid, and did the same with the boot light.

Once at night I needed something from the boot and could not find it in the weak orangy shine of the boot light. I had to use my phone photo light to retrieve what I sought.

I had enough. I ordered some 10 Watt 4x6 SMC LED grids and put them in.
Now you could do brain surgery in the boot. And the dome light is bright enough to not only light up the interior, but also anything surrounding the car. At night I unlock my car from a distance, which fires up the dome light, so it helps me avoid the dog turds in the street ;)

In all honesty I enjoy the LEDs that reduce my economy most :)

I also am a big fan of the LED's in the dome lamps. In the Silverado I used to drive, I swapped out the 194 bulbs in the overhead console map lights for LED units. They were brighter, and sure helped you see things in the cab at night. I don't know if it was the light color, or how the light acted with the "reflector" of the map light, but it also caused much less glare when driving. I considered that a win all around.