I'm trying to replace a Trane Weathertron thermostat with a Honeywell RTH3100C and the wires don't match. The instructions call for 9 wires on the Honeywell and I've only got 8.

The Trane labels are: R, T, O, G, Y, X2, B, and W

The Honeywell are: E, Aux, Y, G, O, L, R, B, and C

The installation manual cross references everything but the T, but there is not a reference for the L. The Trane had a place for an F which is referenced to L but there is no wire. The wallplate has a label that says C is optional when using batteries.

Several answers have said to tape off the T, but what goes to L?

So far the reference looks like:

Honeywell Trane
E X2
Aux W
Y Y
G G
O O
L??
R R
B B
C
T

What goes where?

HVAC 1000 handled this a few months back and I can't do better than his fine work. Try this https://www.askmehelpdesk.com/heating-air-conditioning/trane-weathertron-honeywell-rth7000-wiring-instructions-167641.html