Okay bit of an update to my chart, specifically for cross-referencing the neo-wedge bulbs if you want to stick with halogens instead of LEDs. I have learned more about miniature bulbs in the last few days than I ever cared to know. I started to get really confused with how bulbs are described online and found it was because of metric vs yankee measurements.
Take for example the "Type B" Neowedge 10mm-base bulb that is used in quite a bit in our truck. When searching for LEDs you'll want to search for a "T4 neowedge". The T stands for tubular shaped and the 4 in this case is the diameter of the bulb in millimeters, so a 4mm bulb. You may see these come up as T4.2 which is fine. Don't get a T4.7 as you'll see in the picture below, these have a 12mm base which is too wide for our instruments.
Okay fine. Now websites like Autolumination call this a Type B/T4 newedge but that doesn't help if you want to find an OEM
halogen replacement. Unless you go to Toyota and pay $10 per bulb, it's hard to find an aftermarket
halogen bulb that works. That is where the imperial T-bulb measurements come into play and where it gets confusing. In searching for halogen bulbs I found descriptions like T1.25 or T1-1/4 which I could not correlate to T3 or T4 you see for LEDs.
I then learned that these imperial measurements are in increments of 1/8"...WHAT?? So a T1-1/4 is a really stupid stupid way of saying this is a 4mm diameter bulb. I'll try to explain.
1-1/4 = 1.25
1/8" = 0.125
1.25 x 0.125 = 0.15625"
0.15625" x 25.4 = about 4mm!
Let's try another example with your more common "194" bulb that is often times called a "T10" bulb. According to Americans this is a T3-1/4 bulb:
3-1/4 = 3.25
1/8" = 0.125
3.25 x 0.125 = 0.40625"
0.40625" x 25.4 = about 10mm. That is why they call it a T10 bulb
These imperial T-measurements come in handy if you're trying to find the right size colored filter/condom for halogen bulbs to change the color.
Okay I swear I'm done