I would not recommend this type of method for saving gas.

1. Don't assume the driver is good.
2. It is very annoying and rude to have a tailgater when you are trying to drive any type of vehicle.
3. Don't assume the driver knows you are there.
4. It's a great way to get a huge rock chip on your wind screen
5. It's only increasing your chances of a big accident which will cost you much more money than the couple dollars in gas you saved.

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'84 Westfalia, 1.9L
'73 FJ40, 2F