I don't know why you'd call this cheating. I think it's a very practical approach. She's driving anyway, so it's not like there's any impact. Plus the 3 mile hop to work means you should arrive without being sweaty or needing to change clothes. If you have a bike with an upright riding posture, you might even ride in like a Dane, at a lower peed, but dressed for work and ready to go.
When Spring comes, you might opt to modify the routine occasionally by going in with her in the AM, but making a nice ride of going home the scenic way (if one exists).
In any case, it's your ride, and your life, so there's no such thing as cheating.
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