Im almost 100% sure I have. Why would it leak if the fuel cap is loose? I mean, shouldn't the fuel system be sealed anyway and not depend on pressure generated by a gas cap to not leak?

I've also noticed a significant decrease in gas mileage, and not just from this tank but the last few. Its gotten alot worse with this one but I think I have a couple broken teeth on my gears for the odometer/trip meter.

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1963 Ford F100 - Cummins Swap?
1985 Mercedes 300SD - 210K
1992 Ford Festiva - 92K
2001 Ninja 250 - 8K