I suspect you will not be happy with these. I could not find any LUMENS SPECS listed. What you need to be as close to the original 921 Incandescent automotive ratings is something in the WARM WHITE and have a LUMEN RATINGs of 450 or higher. I think the charts say a 40WATT Incandesent bulb is around 450 LUMENS
But you are on the right track by just buying ONE and trying it out first before you buy $150-$200 worth and finding out they will not be bright enough.
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Usually if a company doesn't list their LUMENS rating they are going to be dim for you.
The other concern you should be watching for is if the LED BOARD use a regulator chip on them so you can operate in a 10-20V DC range. These regulator chips may interfere with your radio frequency items like the TV, NOAH radio, radio scanners etc...
Each LED replacement board will save you around 80% of power for the same wattage/LUMENs being used. Will also save you on a BUNCH of heat being generated.
Read close to what all of us have to say...
Been there done that already
Roy Ken