Prius mileage from gasoline bought in or near Kentucky

by Don Chesnut ©2005

Observations about gasoline mileage

miles per gallon classes

<40	<45	<50	<55	<60	>=60
worst	to				best

• My early questions about gasoline mileage to State regulations and inspections
• Response from State
• Subsequent observations about gasoline mileage

Mr. Lanny Arnold
Assistant Director
Division of Regulation and Inspection
Kentucky Department of Agriculture

Dear Mr. Arnold,

I have a concern that I'd like to bring to your attention; I hope you can help. I bought a Toyota Prius this spring and as you may know, it is a hybrid electric/gas car. It not only registers current miles/gallon in real time, it also calculates the cumulative average miles/gallon (from the last fillup) about every couple of minutes. In-city driving averages 51.4 miles/gallon over the last 8 fillups in Lexington, Kentucky, which is pretty good, I think. I recently made a two-and-a-half week trip to South Carolina and I noticed a significant difference in miles/gallon (three fillups there); the cumulative average miles/gallon was between 60.0 and 63.5. This seems fairly significant. After returning to Kentucky, I filled up again and the mileage dropped back to 51.2. I made another trip to Florida and filled up in South Carolina going south and did the same going north on the return trip. Mileage on these tanks of gas was in the 60's range again. Upon the return to Kentucky, after a refill, it was back to 51.3.

The topography in Florida was much flatter than in Lexington, but the topography where we were staying in South Carolina was much the same as Lexington's. Although it rained most of the time in Florida, the weather while in South Carolina was the same as Lexington's. The elevation difference between Lexington and these other two states was several hundred feet (about 600). I would expect a difference in partial pressures of oxygen to be significant only over several thousands of feet. My driving style was the same in all three cities and the interstate driving style didn't change either.

I used only 87 octane gasoline and I bought gas only at Sam's Club or at Shell stations (both in SC and both in KY).

The tentative conclusion that I have reached is that there is a difference in quality of gasoline or a difference in additives between the two states (whick, I guess, would affect the octane rating). Perhaps Kentucky requires additives that SC doesn't, or perhaps the octane rating is consistently incorrect in SC or in KY. I wouldn't think that SC would be selling higher octane gasoline at the 87-octane pumps, so I am wondering if the KY "87-octane" gasoline is considerably less than 87. I know that gasoline quality can fluctuate somewhat, but these differences seem to be consistent over a period of time and significant (about 10 miles/gallon).

Is this something that the state should be concerned about? Thanks for your time.

Respectfully,

Don Chesnut

Donald R. Chesnut, PhD
Geologist Emeritus, Ky. Geological Survey
Adjunct Prof., Geosciences Dept. Univ. Ky.

Donald R. Chesnut, PhD
Geologist Emeritus, Ky. Geological Survey
Adjunct Prof., Geosciences Dept. Univ. Ky.

Dear Dr. Chestnut,

Fuel in Kentucky must meet ASTM standards and federal guidelines as indicated in the Motor Fuel Quality Law, KRS 363.900 - 363.908. The law is designed to ensure that fuel meets gasoline specifications. Inspectors from our Department screen octane to determine if the fuel is being sold as advertised. The fuel may be further tested at a certified petroleum lab to determine if the fuel meets specifications for vapor pressure, distillation, contaminates, oxygen content, and alcohol/ether, as well as the antiknock index.

I can't give a definite answer as to "why" the fuel economy would be so different in South Carolina verses Kentucky. There are so many parameters that can affect gas mileage as you indicated in your e-mail.

If we could be of further assistance, please let me know.

Leslie Hageman

Kentucky Department of Agriculture
Division of Regulation and Inspection
107 Corporate Drive
Frankfort KY 40601

PHONE: 502/573-0282
FAX: 502/573-0303
mailto:leslie.hageman@ky.gov