You know now that I think about it why would you even want to carry spare fuel inside a car? The light should come on before you run out, or better yet just don't run it much below a 1/4 tank. I never need more then 17-19 gallons to fill my 23.5 gallon tank. It doesn't pay to run it lower.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200