I was getting roughly 19-20 mpg with roughly 1/3 highway miles and on the next tank i popped in a bottle of techron concentrate to make sure it was clean and got rotten mileage, 14 or so - same gas, chevron premium - different station and all city driving.

Is that normal?

 

I have the same issue. Put in a bottle of Chevron fuel system cleaner and started to see lower gas mileage...

 

It messes with your gas gauge and you can't get a full tank which is why your seeing a reduced mpg. Give it a few tanks when its out of your system and you'll be able to see it. I went from a tank every 4 days to a tank a week after some techron fuel injector cleaner. I drive 300 miles a week give or take btw.

 

I've always found the tank with the bottle would give me horrible mileage, then on the next tank I would start noticing a difference in performance/mpg.

 

Hmm, weird just never seen that effect in other cars but good to know.
thanks all!

 

Yeah anytime I put in any additive to my gas it gets lower MPGs and it is the same on my Dad's maxima

 

eh, Never noticed anything And you shouldn't.

 

I use a similar product from Lucas and it causes the same problem in my G and Honda Pilot.

It normally takes 2-3 tanks before things are back to normal

 

FYI
Gasoline has additives mixed in when its produced.
Some of these additives are added to clean gunk and debris.

Source : Myself I work in a gas plant