The best way to handle this in your situation is to enter the gasoline as an expense. You indicated that half of the money was paid by you for the gas. Use your actual gas expense you paid for the vehicle you do not own.
On your partner's return they should use half the miles for their expense deduction since they only paid for half the gas as well.
If you entered a vehicle under the rental activity or activities that you do not own, then you must delete that.
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