Gas mileage variance..

[Limited-Time]

On my resent trip to and from Cleveland I monitored the gas mileage. I wanted to compare the actual mileage, (by doing the math) with the reported mileage. (On Board Computer) While the computer was pretty close it was actually a little low on the freeway numbers and a little high on the surface roads. That made sense because it calculates a running average. While over all I thought the car did outstanding (Full sized 4 door caddy, late 90's with v8) getting over 26 MPG at 75 plus MPH. However I did see a marked drop in mileage on one tank of gas, we lost over 2 MPG. Just after the fill up I noticed the average gas mileage (per the OBC) begin to drop. It continued to drop until our last refill at which time it began to climb, it continued to climb until it reaches the original average. We ran high test the entire trip, so that rules out octane variances. All I can think is that maybe the one tank contained 10% ethanol??

 

[angus63]

Re: Gas mileage variance..

Fuel varies quite a bit from load to load. Saying fuel is fuel is like saying Old Milwaukie Lite and Heineken are both beers!

 

[tashasdaddy]

Re: Gas mileage variance..

also elevation enters in to the formula.

 

[Bob_VT]

Re: Gas mileage variance..

Ton's of variables....

Most manufacturers instant mpg readings are a touch high.......

Using the old stand by method of checking your odometer and gallons added is better assuming you are getting a correct odometer reading. If you changed tires or there was an optional tire size ..... you can not be sure that the odometer is accurate. The "accepted" variance is 3 1/2% ........

I have not found too much impact on the e10 gas since that is the only gas they sell here.

 

[JB]

Re: Gas mileage variance..

After driving Mercedes MLs for 9 years and monitoring the real-time trip computer I find several factors affect mileage more than one might think.

Changes in elevation (doesn't have to be a "hill"), changes in wind direction, what sort of traffic you are in (following a semi about 10 car lengths back can add 2-3 mpg at 70).

The real-time readouts were a lot more revealing than averages calculated since last fill-up or reset.

On the ML320 Interstate mileage varied from 18 to 25, depending on the above factors.

 

[Pony]

Re: Gas mileage variance..

I usually see about a 5mpg difference between reformulated and non reformulated gas. One of the things that drives prices up is the numerous different gas reformulations........I wouldn't doubt that on a long trip you would at some point get something that id reformulated enough to decrease your mpg.

 

[tommays]

Re: Gas mileage variance..

I dont drive a LOT anymore BUT in the fall of 1980 and 1981 i took 3 week cross country Motorcycle trips in the 8500 mile range

At the time it was Not and EFI motor and the power changes from different elevations and Weather conditions like a cool evening were very easy to feel


When you go over something like HIGH independence pass(12000') you were really wishing you had a turbo and EFI to lean out the really rich motor

 

[Andy in NY]

Re: Gas mileage variance..

We travel from upstate NY on the canadian border to Springfield, MA about 5 times a year, and I notice the same trend EVERYTIME. Coming home we always get 3-4 mpg LESS than going down. NY up until the last month or 2 did NOT have ethanol in the fuel, Mass DID.

 

[Limited-Time]

Re: Gas mileage variance..

I understand the variables to a degree. The trip was an up and back, same route both directions. Weather was almost identical on both travel days. Refuels were all in the 15 gal. range, extremely close considering no stops were "planned" prior to the trip. Gas mileage varied only slightly on all other legs (tanks) of the trip. All gas was purchased from major name chains, Exxon, Citgo, Shell, and Pilot. I did not however note the actual octane ratings, only know I always purchased the Premium Grade.

 

[angus63]

Re: Gas mileage variance..

If your car doesn't call for premium, and very few do, your just farting dollars out of the exhaust for nogood reason. If your car calls for 87 or better, use 87. Unless you shaved the heads, a high lift cam and a chip to play with timing. Then just go racin' and ignore the mileage!!

 

[Limited-Time]

Re: Gas mileage variance..

Cadillac calls for Premium Fuel Only right on the fuel gauge.

 

[angus63]

Re: Gas mileage variance..

Cadillac calls for Premium Fuel Only right on the fuel gauge.

And Grey Poupon too?? Actually, check the manual for the minimum recommended octane rating and use that. Premium can mean 89 to 94 octane depending where you are. It'll save a few bucks and not hurt nothing!

 

[zoe'sdad]

Re: Gas mileage variance..

Terrain and traffic affect my mileage the most. Driving to Disney last week with the cruise set at 70 and the AC on we got the following results. Ponchatoula to Tallahasee, 17.8 mpg. Tallahasee to Orlando to Lake City, 18.1 mpg. Lake City to Pascagoula, 17.5 mpg. Pasagoula to Ponchatoula, 21.5 mpg. The drop was noticible once we started to hit the hills past Pensecola. Also we were stuck in slow moving traffic for about an hour due to a wreck on the 75 and we followed the 27 to the resort.

 

[SgtMaj]

Re: Gas mileage variance..

I would be surprised if any of your fillups didn't contain 10% ethanol.

For a 2 mpg drop in fuel economy, if the problem was isolated to the fuel and not a result of other variables (which isn't really possible, but let's talk theoretically for a minute), out of 26mpg... that's a 7.7% decrease in fuel economy. In order to cause that big a drop in fuel economy at the fuel level, you would have to replace approximately 10% of the fuel with a completely non-combustable material, such as water. Why 10% instead of 7.7%? Because some of the fuel doesn't burn in the cylinders anyway.

So even if you could rule out headwinds, traffic, curves in the road, uphill slopes, etc. as the cause of the drop, it couldn't be because you bought 10% ethanol fuel... it's more likely that you bought fuel with either water in it... or more likely... fuel that still had winterizing chemicals in it... or even very old gas.