I don't think that is correct. I've always heard that you need to have a written record to back up your claim for a deduction.

Here is what the Farmers Tax Guide publication says on page 24:

"Business use percentage. You can claim 75% of the use of a car or light truck as business use without any allocation records if you used the vehicle during most of the normal business day directly in connection with the business of farming. You choose this method of substantiating business use the first year the vehicle is placed in service. Once you make this choice, you may not change to another method later."

What I read in that paragraph is you can make that deduction is that vehicle is primarily for business use.  

I don't believe a family vehicle that is used for occasional use in business use will qualify.  That is how I read it.