High Temps vs MPG

[radmanGTO]

All ... we here in Knoxville TN, as in much of the mid-East, have been suffering from excessive 97-102 degree heat for 30 successive (we tied that record today) days with high 90+ temps and no rain. In regards to that issue, I have seen a dramatic decrease in my mpg when the temps have been above 95 for weeks at a time.

I run the same 32 miles each way (64 miles) to work each day on mostly interstate roads. As my '06 Solara SLE V6 has over 45K miles, I am quite watchful in regard to speed, gas useage and where I buy my 89 grade gas. I run at a constant 70 mph and don't drive like I did with my previous LS6 RamAir TA ... :>))

So, when the temps are +95, my mpg is 27-28 mpg without fail and has been that way for these many weeks. At temps in the low 90's and lower, it is dead on at 30-31 mpg.

Has anyone else seen that kind of major change with signiificant temps and might their be some mechanical issue that I can correct?
Regards,
Radman

[Gadgetjq]

Radman,

In theory, your mileage numbers should be slightly 'better' with warmer temperatures. Two reasons. The air temperature sensor is one factor that determines how rich or lean a mixture is sent to the cylinders. Wamer temperatures mean less dense air so the amount of fuel being injected should be (in theory anyway) less.

The other reason mileage should (again in theory) be higher is because you're driving through less dense air so less air resistance than colder (more dense) air.

Now, here's the wildcard. If your area has a different fuel blend for summer and winter those blends can have a huge effect on mileage. If more oxygenates are added in summer (as they are here in California) mileage tends to go down by 1-2 mpg compared to winter blends.

Cheers

[philosophicaljt]

^^really? since colder air is denser air and thus there is more oxygen in colder denser air, the car should run better and get better gas mileage in colder temp's.

[Gadgetjq]

^^really? since colder air is denser air and thus there is more oxygen in colder denser air, the car should run better and get better gas mileage in colder temp's.

To some extent you're correct, an engine will produce more 'power' with cooler denser air mainly because more air (and fuel) will be packed into the cylinder. The original post isn't about power though, it's about mileage.

Oxygen comprises just under 21% of the air we (and our engines) breathe no matter what the temperature. Nitrogen is around 78% and the remainder are all the other gasses like carbon dioxide, xenon, argon, neon, carbon monoxide and others.

Bottom line, no matter what the temperature or air density the '02 sensor will have the final say and will try to keep the mixture somewhere between 13:1 and 14.1:1 depending on coolant temperature and whether the car is cruising or accelerating.

Cheers

[Chinky420]

all other things being equal (velocity, distance, coefficient of friction, etc) the higher temp will make the engine run less efficiently. given a constant velocity, the engine will have to spin faster, producing more power to maintain the same speed.

that being said...consider also the fact that the accessories will be working harder (air conditioning, radiator/fans, etc), meaning that more power is going to be diverted out of the powertrain to power those accessories.

all that combined, i'm sure it explains the 2-4 mpg loss in the greater heat.

[radmanGTO]

Thanks for all of your replys ... I was also considering that as the temp goes up, gas more easily vaporizes and the liquid expands. As the liquid expands, there is not as much explosiveness per injector pulse as with a cooler liquid. I thought that 95 degress may have been some magic transformation point.

Also, this expansion issue is one that I believe the Feds are filing against the oil co's on behalf of all of us. Meaning, that filling up at 4PM in the hottest part of the day will be worse than filling up at 7AM ... ie, the cooler liquid is denser versus the same less dense gallon being pumped at 4PM ... after which it condenses to a smaller volume. Certainly these expansions and contractions would not be gallons in our tank, but taken each day millions of times, that can be significant $$ profit for the oil co's for gallons not really pumped.

Regards,
Radman

[radmanGTO]

As an UPDATE ....

Whatever the issues were then, the temps are still high for the fall season here in Knoxville .... 80's .... and very dry ....

The mpg is back up to 30/31 mpg @ 70mph .. 32 miles each way on I40 ...

Radman

[Gadgetjq]

As an UPDATE ....

Whatever the issues were then, <snip>The mpg is back up to 30/31 mpg @ 70mph .. 32 miles each way on I40 ...

Ah ha, a clue! Check with your gas station and ask if their supplier just switched to a 'winter' fuel blend. Here in California fuel mileage always goes up in winter because the seasonal blend includes less oxygenate and other 'stuff' designed to reduce emissions than summer blends... I.E. more 'gasoline'.

Your original mileage drop might simply have been a switch to summer blend.

Cheers

[radmanGTO]

Your correct ... but, we haven't had a change in formulation yet in this area... although, now that you mentioned it, I can't wait for some better milage ...

Regards,
Radman