How about a game? Let each driver get points both for how fast s/he drove and for their trip MPG. Lowering fuel consumption while trying not to lose too much time should make both of you happy, and I know (but don't tell her) that the laws of physics will be on your side: You can considerably reduce FC while taking only slightly longer to get where you're going.
I assume that remapping the wife's ECU for economy is out of the question?
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread