Why do warmer air temperature's increase MPG?

DaveinOlyWA · 07-10-2005, 12:55 PM

with air conditioning you should still see a 5-10% increase

HTMLSpinnr · 07-10-2005, 02:00 PM

When outside temps were in the 80's and my original Integrity's were close to their wear limits, I was easily seeing 55mpg on average during my commute at 65-70mph and 36 miles each way with a best at 59.3mpg.

Since I've replaced my tires with new Integritys which only have a few thousand miles on them, and since temps are now avg 108-115F outside, and the air is running strong, my mpg is back down to 45-48mpg average. While we have a dry heat (6% humidity recently), we still need A/C to keep things reasonable.

The two factors (not sure which is stronger) combined seemed to have made a significant hit on my mpg. While rolling resistance and air density may be less, the need for full blast A/C (even when set at 78F) seems to counter any benefit.

As I type this, however, I"m in SoCal for the weekend, and I made a trip down to San Pedro (LA Harbor), My trip average there was well over 50mpg in the cooler temps. Drive back wasn't as good due to more hills, and the addition of CA gas to my tank (made it from Surprise to San Pedro - 405 miles +/- on one tank, not bad).

Other factors are that I need to re-check my tire pressure, and I'm probably due for an alignment since my tires are wearing unevenly (shimmy/vibration in the wheel during turns that coencides with wheel rotation) - no thanks to a couple of rolled curb to flat curb transitions I keep seeming to nail while parallel parking.

Once I get my pressures re-checked and my alignment eventually done, and once my tires have broken in a bit more, I'll be able to verify whether or not higher temps are the only true factor in my recent MPG drop.

HTMLSpinnr · 07-11-2005, 06:50 PM

Update:

Drive home from averaged 48mpg.

Checked pressure this morning - hot, they were at 35.5 average all the way around. I bumped them to 43/41 (hot) and my MPG on my way to work bumped back to the usual 52-53mpg @ 70mph, 102-108°F outside temp and A/C set at 78°F. Tire pressures really *do* make a big difference.

sdgeiger · 07-18-2005, 04:38 PM

The warmer spring helped my MPG (was getting around 50 MPG) in the very hilly suburbs of Philly. However, now that it has gotten very hot, I've been running the a/c a lot and have only been getting about 45-46 MPG.

Also, the week before Indpendence Day, I had a Thule Roof Box on my roof and drove 4 hours west of Philly with the A/C on, and I got only 36MPG.

DaveinOlyWA · 07-18-2005, 06:48 PM

air conditioning gives a bigger hit for me the slower i drive. at 35 mph i get about 60-65 without air 55 with.

at 60 mph, i get 54-56 without and 52-53 with.

twdusa · 07-18-2005, 07:11 PM

The warmer air temperature has helped my mileage! But if and when I run the A/C, I do take a good hit. The hit is less using 40mph in town for speed with A/C on.

07-10-2005, 02:00 PM	#22
HTMLSpinnr Senior Member Join Date: Dec 2003 Location: Surprise, AZ (Phoenix) Posts: 1,339 My Car: 2004 Prius Package: Pioneer #3 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 2	When outside temps were in the 80's and my original Integrity's were close to their wear limits, I was easily seeing 55mpg on average during my commute at 65-70mph and 36 miles each way with a best at 59.3mpg. Since I've replaced my tires with new Integritys which only have a few thousand miles on them, and since temps are now avg 108-115F outside, and the air is running strong, my mpg is back down to 45-48mpg average. While we have a dry heat (6% humidity recently), we still need A/C to keep things reasonable. The two factors (not sure which is stronger) combined seemed to have made a significant hit on my mpg. While rolling resistance and air density may be less, the need for full blast A/C (even when set at 78F) seems to counter any benefit. As I type this, however, I"m in SoCal for the weekend, and I made a trip down to San Pedro (LA Harbor), My trip average there was well over 50mpg in the cooler temps. Drive back wasn't as good due to more hills, and the addition of CA gas to my tank (made it from Surprise to San Pedro - 405 miles +/- on one tank, not bad). Other factors are that I need to re-check my tire pressure, and I'm probably due for an alignment since my tires are wearing unevenly (shimmy/vibration in the wheel during turns that coencides with wheel rotation) - no thanks to a couple of rolled curb to flat curb transitions I keep seeming to nail while parallel parking. Once I get my pressures re-checked and my alignment eventually done, and once my tires have broken in a bit more, I'll be able to verify whether or not higher temps are the only true factor in my recent MPG drop. __________________ Rick Johnson - Linux Geek, Private Pilot 2004 Prius - Tideland Green Pearl - Pioneer #3 (#9 pkg) - 108K miles - Factory EV Button, Homemade fog lamp grills, Goodyear Allegra tires 2002 Prius - Blue Moon Pearl - All options except NAV - 53.5k miles - Coastal Hitch Family Prius Score: 4 (2 mine, 1 parents, 1 aunt/uncle)

07-18-2005, 04:38 PM	#24
sdgeiger Join Date: Aug 2004 Location: Dresher, PA Posts: 89 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Re: Why do warmer air temperature's increase MPG? The warmer spring helped my MPG (was getting around 50 MPG) in the very hilly suburbs of Philly. However, now that it has gotten very hot, I've been running the a/c a lot and have only been getting about 45-46 MPG. Also, the week before Indpendence Day, I had a Thule Roof Box on my roof and drove 4 hours west of Philly with the A/C on, and I got only 36MPG.

07-18-2005, 07:11 PM	#26
twdusa Join Date: Mar 2005 Location: Eastern North Carolina Posts: 53 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Re: Why do warmer air temperature's increase MPG? The warmer air temperature has helped my mileage! But if and when I run the A/C, I do take a good hit. The hit is less using 40mph in town for speed with A/C on.

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07-10-2005, 12:55 PM	#21
DaveinOlyWA Opps !! I Did it Again!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 9,990 My Car: 2006 Prius Package: #4 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 6	with air conditioning you should still see a 5-10% increase

07-11-2005, 06:50 PM	#23
HTMLSpinnr Senior Member Join Date: Dec 2003 Location: Surprise, AZ (Phoenix) Posts: 1,339 My Car: 2004 Prius Package: Pioneer #3 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 2	Update: Drive home from averaged 48mpg. Checked pressure this morning - hot, they were at 35.5 average all the way around. I bumped them to 43/41 (hot) and my MPG on my way to work bumped back to the usual 52-53mpg @ 70mph, 102-108°F outside temp and A/C set at 78°F. Tire pressures really do make a big difference.

07-18-2005, 06:48 PM	#25
DaveinOlyWA Opps !! I Did it Again!! Join Date: Apr 2004 Location: South Puget Sound, WA Posts: 9,990 My Car: 2006 Prius Package: #4 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 6	air conditioning gives a bigger hit for me the slower i drive. at 35 mph i get about 60-65 without air 55 with. at 60 mph, i get 54-56 without and 52-53 with.