gas and oil consumption at high speeds

[yucatec]

Dear Panthers,
I just returned to Virginia from Texas. My trip average over 3696 miles was 66.2 mph. West Texas two lanes are posted at 75 and I-10 is posted at 80. I routinely traveled 5mph over the speed limit and occasionally hit 95 when passing (with oncoming traffic coming quicker than I am accustomed to, I got around quick). The car was loaded with four dogs and gear - probably 650 pounds.

Scott just completed considerable work to make the 230k engine (130 since rebuild) more reliable and (slightly) more economical and powerful. It seemed more powerful.

On the trip to Texas I burned three quart of oil which alarmed me but I hadn't changed the oil before I left and did so in Texas. It burned 1 quart on my return.

Portal to portal it burned 230.5 gallons of gas which translates into slightly better than 16 mpg. On a slower trip out west last may I averaged 21 mpg

I don't know whether the increased consumption is due to the high speeds or is the first signs that I need a new engine. I fear the latter.

Donald McCaig

[Blaze86Vic]

Well, the MPG is surely a cause of the speed. Aerodynamic drag doubles from 55mph to 65mph.

The oil, almost any engine will burn oil when pushed. At the race track you are pretty much guaranteed to have to add a quart over the course of the weekend, unless you run thicker oil for the track like 15W-50 or 20W-50 because under heavy loads the oil will get hotter than normal, requiring a thicker oil to maintain spec viscosity. Under long trips at cruising speed you end up with the same situation. The entire engine becomes saturated with heat and things like oil get hotter than normal. It's a good idea to let the engine idle for a few minutes when you are finished with your trip, before you shut it off. This gives some time for things to cool off with some good oil flow.

[87_crown_vic]

on way to SF, 1200 miles, i was doing 70mph+ on way. before i left SF i added a quart. i use 10w40 Valvoline and dont have issues. on way home, the car hit the 100k mark and was sitting at 100,036 when i pulled in the driveway. car now has 103k. my Grand Marquis is getting oil blow by from the PCV valve. so it blows oil out and leaks from lower intake or rear main some also. it goes thru a quart between changes and is 1/2 qt low when it goes in for fresh stuff.

[85crownHPP]

high speeds with a load is a great way to loose mileage... like Jonnie said mpg loss over 55mph is exponential...
Re-calculate with a liesure trip before you jump to conclusions... Id still like to check out your wagon

[87_crown_vic]

i was well loaded with our clothes, coolers, stereo in the trunk, tent, etc. my trunk was full of stuff including my stereo and backseat was damn near too full to even recline seat to sleep in car on way to or from SF. my car was loaded and handled it decently at about 21mpg to SF and 23 back home doing 70mph+ with my LOPO

[gadget73]

The weight and the speed probably explain the fuel mileage. Mine really starts to drop as I get over 65-70. I can pull 19 at 65 mph, if I run 75-80 its about 13.

The oil though, 3 quarts to Texas sounds a bit high. Its not leaking from anywhere suddenly is it? Pushing a motor with miles hard will tend to make them suck down some oil but 3x as much going as coming sounds a little weird, unless you were running it harder on the way there.

[Enigma]

On my road trips from Saint Louis to Indianapolis 3 times a year, I average 65-75 mph (with an occasional 75-85 if I get a good semi to follow), and get 20-22 mpg. And I only add 1 to 1-1/2 quarts of oil between changes, and that's mostly from a leaky front crank seal that needs to be replaced. Engine has about 90k on it.

[Tiggie]

What's up with my motor? Man, the thing eats oil like crazy!" This is one ofthe more common questions that we get asked. It seems as though none of our Cougars have ever NOT used oil. The norm seems to be around 1-2 quarts between oil changes. So why does this happen, you may ask?
The problem is slightly undersized piston rings. Especially with the 1986-88 5.0L engines, the flat-top aluminum pistons were purposely made a tad smaller than a comparable forged piston, so that when the engine reached normal operating temperature the pistons would expand correctly. Since cast iron (block) and aluminum (pistons) expand at different rates, this makes sense...on paper. In reality, until normal operating temperature is reached, oil has the potential to get sucked into the combustion chamber if the piston rings are worn enough.

My mom's '87 T-Bird 5.0 uses 1 quart every 1000 miles and it has since the car was brand new. My dad took the car back, but Ford didn't consider it excessive. They consider 1 quart/800 miles excessive. I've used about 1.5 quarts in 2000 miles on my '88 with 180K on the clock.

And as Gadget said, check for external leaks. Mine was leaking at the lower rear intake seal.

For reference, I average 21 MPG on 80% highway @ 65ish MPH. I've checked the tripminder at quicker speeds and it usually reads 15-18, depending on terrain.