I'm using a fuel heater on my Geo Metro. An image can be seen here
www.livesteaming.com/metro.htm
I have to admit I was nervous about using but after running it disconnected from the fuel supply for a week I hooked it up.
The copper fuel line got too hot to touch without fuel. With fuel it is just warm.
I cannot tell you if this mod has made a difference because I do not have the time to test every mod individually. I'm happy though with the current approx. 62 mpg.
TH
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