How do you choose Neo Wedge halogen bulb LED replacement Bulb and Wedge  halogen bulb LED replacement Bulb for car's instrument clusters?

What are different of T3 T4.2 T4.7 Neo Wedge LED Bulb? Neo Wedge T3 vs T4.2 vs 4.7, T5 Wedge led bulb  vs T10 Wedge LED bulb.

Above images are Instrument Cluster Light Bulbs. Yellow light on dashboard, green light symbol on dashboard. T10 194 LED Bulb For Instrument Gauge Cluster Dash Lighting. T5 Wedge 18 27 74 85, T5 73 74 Instrument Panel Dash Light Bulb. T3 Neo Wedge LED BulbT4.2 Neo Wedge LED BulbT4.7 Neo Wedge LED Bulb

Here 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 for halogen bulb and led bulb 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 ( Europen standard ).

Take for example the "T4 neo wedge", 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.

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.

T4.2 Neo Wedge LED Bulb as T1-1/4 Wedge halogen bulb replacement.
Most halogen bulb uses imperial measurements, These imperial measurements are in increments of T1=1/8".  And most led bulb uses metric measurements that are in increments of T1=1mm.  So a T1-1/4 is a really stupid stupid way of saying this is a 4mm diameter bulb. I try to explain below.
1-1/4 = 1.25
1/8" = 0.125
1.25 x 0.125 = 0.15625"
0.15625" x 25.4 = about 4mm!

T10 Wedge LED Bulb as T3-1/4 halogen bulb replacement, Part no.194.
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

T5 Wedge LED Bulb as T1-3/4 halogen bulb replacement, Part no.74

T3 Neo Wedge LED Bulb as T1 halogen bulb replacement. Smaller in size, great for applications in most vehicle, common use. Ultra Bright LED, unique and more vivid color
Cross Reference: Cross Reference / Compatible socket bulb number: T3 - 39397-SA5-003, 35852-SEP-A02, 35853-SDA-A01
Application: Replace the light such as A/C Lights, Climate Control light, etc