Are you referring to the CI-0000009 per the MTH bulb chart?
If this is indeed the bulb you're referring to, take a look at this recent OGR thread where member Arnoldoo mentions using the LED-8352WW to replace the incandescent bulbs in an MTH house.
There appears to be some issue with the shape of the plastic base. If you do go this route, please post progress to this thread to add to the collective knowledge.
And based on a sample of 2, I measured an MTH passenger car bulb. 1.5 Watts at 14V DC, 2-1/4 Watts @18V DC. The relationship between AC and DC power measurements is tedious but these numbers should head you in the right direction. For the design of the LED-8352WW as shown in the linked thread, I'd estimate a ~5x savings in power. It's a tedious technical discussion as to why you don't get more power savings so I'll leave it at that.
