O.T. Gas mileage using cruise control

[AK]

I made a 600 mile trip using mostly highways.

I had my cruise control set for 55 mph.

Previously in 2015, I got 28.4 mpg.

But this last trip I got 29.8 mpg.

Is that possible since it is 7 years later?

Thanks.

[Dean Hoffman]

Wind? Did you use the same mixture of gasoline? My dad would claim that engines and other parts would loosen up over time and allow a little better fuel mileage.

[invalid unparseable]

That and other factors like time of year and temperature. I get about one mpg less using AC in summer and of course summer and winter gas is different.

[Ed Pawlowski]

Isn't that dangerous? A few times a year I make either a 2200 mile or

2800 mile trip. At 55 I'd be a hazard on the highway and possibly cause accidents. Much of the trip is I75 and I95.

I set the cruise at 80 to keep up with traffic. At some points, the speed limit does drop to 55 but cars are still moving 75 to 80. The lower speed would also add about 10 hours to the round trip. I average about 27.5 mpg.

[T]

My mileage went up 1-1/2 MPG when I switched from E15 to pure gasoline. So unless you can control the exact type of gasoline, you really can't say.

Plus the temperature and the wind and the tires and the weight of the occupants

[Jim Joyce]

My thought, exactly. Moving with the flow of traffic is how I was taught. Going faster than the flow, or slower than the flow, can be quite dangerous. Personally, I think going slower than the flow is the more risky of the two.

We've been doing a 4000-mile road trip twice a year, MS to MA via the scenic route, and our experience matches yours. Posted speed limits are a guide, but the actual traffic flow is how we roll.

[micky]

And don't carbon deposits increase the compression ratio as they increase?

[Ed Pawlowski]

Many cars will give an instant readout of mpg. When I lived in Ct and worked in MA I took the highway for about 10 miles. I wanted to see the difference at different speeds so picked a flat area and each day did a different speed where it was posted at 65. Never a lot of traffic. Did

70, then 65, then 60, but at 55 I was a danger on the road as cars came up fast behind me. Cancel experiment.

[Cindy Hamilton]

Congratulations. You've developed a viable perpetual motion machine.

Keep taking those trips, and you'll soon be up to 1000 mpg.

[Wade Garrett]

The question you should be asking: Is that a statistically significant difference vs. just random variation?

The mileage difference is small-- only 1.4 MPG which is less than 5%.

[AK]

It was not dangerous.

Quite a lot of cars and trucks were also doing 55 mph.

I did not mention it, but I used mid grade gas. Premium is recommended.

Andy

[AK]

You are right.

Andy

[Scott Lurndal]

A 4% difference is easily within the margin of error, particularly if you had a tailwind some of the time or if the terrain was different between the two trips.

[Ed Pawlowski]

You must have been on a road with a speed limit of 40 mph if everyone wad doing 55. Certainly not on I95.

[Clare Snyder]

I don't know about you, but MY cars don't get a chance to build up carbon - - - -

[Bob F]

I had a '73 Suburban that was so carboned up that it would diesel and just sit there coughing and hacking after I turned it off.

[Marilyn Manson]

Cheap "fix": Leave it in gear when you shut it off.

(I know, too late now.)

[Clare Snyder]

Most common problem causing deiseling on a mid-70's GM was some jerk diffling with the idle speed control making the throttle not close fully when shut off. The isle solenoid is supposed to control the idle, not the throttle screw - ex mechanic speeking here - - - If carbon was the problem it would ping and rattle under light throttle conditions

[Mark Lloyd]

[snip]

Since it's called "deiseling", I would expect the compression is enough to ignite it. What I'm wondering about is the fuel pump.

[Snag]

It's the same way a model airplane glow plug engine runs . There's a hot spot in the cylinder that ignites the fuel . Fuel pump got nothing to do with it .