Weather conditions effect on MPG

lessgascar · 11-09-2007, 07:43 PM

We're in Houston and purchased an '07 in March this year. In this part of the country, heat and humidity can play a big role in the performance of any auto. Summers here are particularly brutal; frequent highs in the mid-90s, humidity 75% and up, and more importantly, dew points in the mid-70s and up.

This summer was one of the wettest on record, which kept the dew points and humidity high but the temps low. This meant the A/C could be kept on 71 with the lowest fan setting. Our mileage ranged from the mid-40s to low 50s, but most frequently in the 40s. It's finally gotten cooler in the last 3-4 weeks, which has lowered the dew points to the 50s and below. Now we're getting consistent 50+ MPG.

I'm looking for members who live in the Sun Belt, where these weather conditions predominate, to relay any shift in performance they've seen since the seasons have changed. My son's science project involves tracking MPG vs. humidity/dew point to see if there is a correlation, and any feedback would help.

F8L · 11-09-2007, 11:36 PM

A drop in humidity and temp will mean more HP but the denser air is more difficult to cut through. So for racing a low Density Altitude is awesome but I've never really thought about how that would effect fuel economy. Generally speaking density altitude is the "effective" altitude of a geographic location. In Sacramento I have seen the DA raise to 3,000ft. during a hot humid summer and go as low as -2800ft. in the late fall when temps and humidity (dew point) are low and the barometric pressure is high. This roughly translates into a feeling of driving at a much higher altitude than you are currently at.

It seems like it is a mixed bag. Look at this description from my DA calaculator site.

"Air density is affected by the air pressure, temperature and humidity. The density of the air is reduced by decreased air pressure, increased temperatures and increased moisture. A reduction in air density reduces the engine horsepower, reduces aerodynamic lift and reduces aerodynamic drag."

As part of his study he may want to use the DA calculation formula to keep track of this variable. It is HUGE in the drag racing field. Many racers carry expensive weather stations with them on race days to track conditions and try to make elapsed time predictions based on DA.

JimboK · 11-10-2007, 10:53 AM

Not exactly what you asked for, but you might consider the Prius MPG Simulator for calculating "what-ifs" for virtually any kind of weather-related (among others) variable. In particular, it will adjust for variations in temperature (with or without climate control), wind speed and direction, temperature, humidity, and barometric pressure. It's inexpensive and downloadable, with Windows and Palm OS versions available.

hobba · 11-12-2007, 08:25 AM

I have an 05 with Pkg 6 and about 18000 miles. Recently here in SW FL, the weather has been cool and no humidity. I've noticed that my battery shows green most of the time as opposed to blue, and that my mileage is some of the best I've ever experienced. My last tank was 54.9 MPG. I run the tires at 40/38 as I have always done. The thing I don't understand is that these weather conditions have occured before without this result. Something has changed and I'm not sure what, but I'm pleased with the results. I do know the battery likes cooler conditions and I have used my AC less. But once again, this is no different than past seasonal changes.

11-09-2007, 11:36 PM	#2
F8L Collecting Data on Nature Join Date: Aug 2006 Location: Sacramento, CA. Posts: 3,997 My Car: 2005 Prius Package: #4 Nominated 1 Time in 1 Post TOTM Awards: 0 Friends: 15	A drop in humidity and temp will mean more HP but the denser air is more difficult to cut through. So for racing a low Density Altitude is awesome but I've never really thought about how that would effect fuel economy. Generally speaking density altitude is the "effective" altitude of a geographic location. In Sacramento I have seen the DA raise to 3,000ft. during a hot humid summer and go as low as -2800ft. in the late fall when temps and humidity (dew point) are low and the barometric pressure is high. This roughly translates into a feeling of driving at a much higher altitude than you are currently at. It seems like it is a mixed bag. Look at this description from my DA calaculator site. "Air density is affected by the air pressure, temperature and humidity. The density of the air is reduced by decreased air pressure, increased temperatures and increased moisture. A reduction in air density reduces the engine horsepower, reduces aerodynamic lift and reduces aerodynamic drag." As part of his study he may want to use the DA calculation formula to keep track of this variable. It is HUGE in the drag racing field. Many racers carry expensive weather stations with them on race days to track conditions and try to make elapsed time predictions based on DA. __________________ 2005 Prius California Rangeland ConservationCoalition Ranchers, Environmentalists, And Agencies Working Together For The Benefit Of All.

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11-09-2007, 07:43 PM	#1
lessgascar Join Date: Nov 2007 Posts: 2 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	We're in Houston and purchased an '07 in March this year. In this part of the country, heat and humidity can play a big role in the performance of any auto. Summers here are particularly brutal; frequent highs in the mid-90s, humidity 75% and up, and more importantly, dew points in the mid-70s and up. This summer was one of the wettest on record, which kept the dew points and humidity high but the temps low. This meant the A/C could be kept on 71 with the lowest fan setting. Our mileage ranged from the mid-40s to low 50s, but most frequently in the 40s. It's finally gotten cooler in the last 3-4 weeks, which has lowered the dew points to the 50s and below. Now we're getting consistent 50+ MPG. I'm looking for members who live in the Sun Belt, where these weather conditions predominate, to relay any shift in performance they've seen since the seasons have changed. My son's science project involves tracking MPG vs. humidity/dew point to see if there is a correlation, and any feedback would help.

11-10-2007, 10:53 AM	#3
JimboK One owner, low mileage Join Date: May 2006 Location: Chesterfield, VA Posts: 2,060 My Car: 2005 Prius Package: #2 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 3	Not exactly what you asked for, but you might consider the Prius MPG Simulator for calculating "what-ifs" for virtually any kind of weather-related (among others) variable. In particular, it will adjust for variations in temperature (with or without climate control), wind speed and direction, temperature, humidity, and barometric pressure. It's inexpensive and downloadable, with Windows and Palm OS versions available.

11-12-2007, 08:25 AM	#4
hobba Member Join Date: Jun 2006 Location: Estero, FL Posts: 50 My Car: 2005 Prius Package: #6 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I have an 05 with Pkg 6 and about 18000 miles. Recently here in SW FL, the weather has been cool and no humidity. I've noticed that my battery shows green most of the time as opposed to blue, and that my mileage is some of the best I've ever experienced. My last tank was 54.9 MPG. I run the tires at 40/38 as I have always done. The thing I don't understand is that these weather conditions have occured before without this result. Something has changed and I'm not sure what, but I'm pleased with the results. I do know the battery likes cooler conditions and I have used my AC less. But once again, this is no different than past seasonal changes.