Originally Posted By: chief
If got a 86 Toyota pick up I use to go back and forth to the farm with. Its a 60 mile round trip on country back roads. I always get 29-31 MPG. I buy BP 87 at the same place and very seldom get any spark knock. I needed gas the other day and a tanker was dropping gas at the BP so I went on down the road a ways and stopped at an independant station thats sells a high volume of gas and diesel. No sooner than I left it started spark knocking like crazy. On that tank of gas I got 24 MPG. Next fill up with BP its back to 29 MPG. Question is this. Would my 03 Nissan frontier or 07 Nissan altima which is fuel injected and computer controlled dropped in mileage like the Toyo did or would the computer compensated it any to try to maintain mileage? The Toyo has a carb and no computer.
The knocking seems to make it fairly safe to assume that you got a bad batch of gas.
Fuel efficiency sometimes seems to vary from brand to brand because of poorly calibrated gas pump flow meters. If a gas pump's meter reads on the high side your fuel efficiency will appear to be less than it actually is. For example, if I pump 15 gallons of gas according to the meter on the gas pump, but actually get 14.25 gallons, which is a 5% error, then my calculated mpg will appear to have decreased. Sometimes it works the other way and you end up with more gas than the meter reads, but I would bet that most of the time the error works in the station owner's favor.
This week one of our local news stations reported that 1,000 out of 1,700 gas pumps at Sunmart gas stations in Texas were only dispensing 9/10ths of a gallon for every gallon that registered on the gas pump. Due to the shear volume of Sunmart gas pump meters registering a 10% error on the high side, the case was referred to the Texas Attorney General for investigation.
I once thought about marking the 2.5 gallon plastic gas container that I use for my lawn mower in .5 gallon increments. I was then planning to visit several local gas stations to see if there were any obvious differences from one station to the next. I have a strong hunch that the pumps at a local Shell station that I like to use dispense more gas than their pumps indicate because I always seem to average ~2 mpg more when I buy from that particular station. Anyway, I never got around to performing this test but may do so after reading about the stuff that Sunmart seems to have been pulling.
Here is a link to the news article:
http://www.click2houston.com/newsarchive/16957571/detail.html