Oil Level/Tire Pressure MPG Myth

anj48 · 08-08-2006, 07:58 PM

Being a reader of this and other Prius oriented web sites - when I got my Prius a little over a year ago I dutifully pumped up the tires to 42/40 and at the first oil change added 3.5 Qt. full synthetic (a little below the fill line). I’ve maintained this for the last year in belief that it optimized my mpg but now I don’t think so. What happened?

I was to my dealer for the steering shaft recall and had them do the 25k oil change/tire rotation at the same time. Predictably they used dino oil filled to a little over the mark on the dipstick and set the tire pressure back to spec. Since I fried my compressor I didn’t get the pressure back up before I took a long trip – about 300 mi – the next day. Much to my surprise I didn’t notice a real mileage hit – I’ve been getting 51-52 mpg (MFD) for the last couple of months using the car mostly for commuting – at my fill up after the trip the MFD showed 50.8 mpg. So getting back to my routine commute (approx 95 mi daily) I just left the tire pressure and oil alone to see what would happen. Now after two more fill ups (52.0 and 52.3 mpg via MFD), nearly 1000 miles of commuting over the same roads I always drive (mix city, secondary, freeway – lots of hills and turns), same driving style, similar weather and A/C use, always get gas at the same place – very consistent. I see no difference in mpg, actually if anything it’s been a hair better.

While I know this falls a short of a research protocol I still think it is a solid observation. Any effect of tire pressure above spec or slightly low oil level on mpg, if it exists at all, must be so miniscule as to not be worth attending to. I am ready to take this whole concept and throw it into my bin of assertions that have more to do with emotions and expectations than reality. From now on I will keep my tires at spec, use regular oil and fill to the line just like any other car.

(Note: same posted on Prius Online)

Rangerdavid · 08-08-2006, 08:07 PM

i have had my Prius a little over 3 weeks now. it has 1400 miles on it, original oil, original tire pressure, and i have a life time 48.3 MPG and on the trip i just returned from, over 400 miles round trip, I averaged 49.7 MPG, all interstate miles. I understand the EPA mileage for highway is 51, so I'm not that far off with dealer pressure, oil, etc. and I'm still in the break in period.

anyway, i'm still as happy today as I was when we first put out deposit down. What a wonderful car!!

john1701a · 08-08-2006, 08:16 PM

Even if there was no MPG gain at all from the higher tire-pressure or the lower oil-level, there is still a reason to maintain that anyway.

Tire wear has been proven to slow as a result of never allowing the pressure below the minimum (35/33). Keeping it at 44/42 helps out even the most forgetful of owners.

As for having the oil level lower, you are simply using less oil. That's a good thing no matter how you look at it.

jamarimutt · 08-08-2006, 10:24 PM

Overinflated tires produce a harsher ride and I wonder if they cause rattles and may cause suspension problems later on. Using less oil than recommended means that the same oil is circulating over and over, so that you may need to change it earlier.

My experience over 44,000 miles is that dyno oil and 35/33 work just fine.

hobbit · 08-08-2006, 10:28 PM

Tires run at less than their design pressure will wear unevenly.
.
An overfull crankcase will cause more thrashing and foaming, as well
as a viscosity-friction hit. And more oil may show up in that sump
that seems to form in the bottom of everyone's intake manifold.
.
This stuff ain't a myth. Go snoop around cleanmpg.com for some
vindication.
.
_H*

john1701a · 08-08-2006, 11:03 PM

<div class='quotetop'>QUOTE(jamarimutt @ Aug 8 2006, 09:24 PM) [snapback]299960[/snapback]</div>

Quote:

Using less oil than recommended means that the same oil is circulating over and over, so that you may need to change it earlier.[/b]

Not in a Prius. 5,000 miles is actually premature. The engine simply doesn't get worked anywhere near as hard as a traditional vehicle, which have the same change interval. Take a look during the next service. It comes out surprisingly thin & clean compared to what you'd expect.

jayman · 08-08-2006, 11:16 PM

<div class='quotetop'>QUOTE(jamarimutt @ Aug 8 2006, 09:24 PM) [snapback]299960[/snapback]</div>

Quote:

Using less oil than recommended means that the same oil is circulating over and over, so that you may need to change it earlier.
[/b]

I've posted Used Oil Analysis run a bit over 1,000 miles past the recommended oil change interval. At least with my Prius, there is no issue at all.

Consider that the EU market Prius has an oil change interval of 12 months or 16,000km, twice as long as our North American Prius cars. It's all in the quality of the oil. And - essentially - all the motor oil does is endlessly circulate around the motor

molgrips · 08-09-2006, 07:09 AM

I put my tyres up to 42/40 for a few hundred miles and didn't notice a big difference in MPG.. ok so it wasn't for very long but that's because I DID notice a big difference in everything else. I can't decide wether the alarmingly skittish handling through bumpy corners was worse than the road noise or the vibrations rattling the trim all over the car...!

As for oil - I doubt a couple of cups extra would hurt. I've only done 5k miles in mine so haven't had the chance to check it out. I'm assuming the dealer changed it at 4.5k when they passed it on to me.

I do notice some tyre wear right on the edges of the tyres though, about an inch in maybe. People are saying that 35/33 is too low in terms of tyre wear, regardless of mpg.

tnthub · 08-09-2006, 09:11 AM

Short of extensive testing it is nearly impossible to quantify the results of a couple of pounds of tire pressure or a half quart difference in oil....

A person making the exact same 200 mile trip two days in a row could easily achieve a 2-3 mpg difference simply by changes in the bp or humidity... Too many variables happen during the course of a tank of gas (especially in a Prius)m to accurately quantify subtle changes short of simply following through over and over again to see a trend or change.

If someone can quantify results then that is great. All I know is from my racing experience where if I fail to extimate the performance of my vehicle within .01 or .02 I will probably lose. Such factors as weather, tire pressure, amount of fuel, and hundreds of other variables come into play and until the variables can be accurately quantified any changes, such as tire pressure, cannot be accurately evalulated.

On the street, if a person drives the same route for a month, makes a change, and drives the same route for a month, the variables often will equalize so the person has a good idea (although not conclusive proof), of how a change, such as tire pressure, affects economy.

tomdeimos · 08-09-2006, 09:17 AM

I've seen no problems with oil up to the full mark but I never overfill beyond that.
For tires I see 2-3 mpg difference from Toyota spec to tire sidewall spec.

All of you are making up stuff without measuring it. It is very simple to to measure but you can't do it by tankfuls. There are too many variables. You must measure 1 change at a time and do it so nothing else varies.

I check every change I make at constant speed on level road with no wind, and at matching conditions.
I use the car mpg readout and reset it to measure over a short test route. This way changes of 1 mpg are meaningful and repeatable every time.

Over a tankful I see variations way beyond what I am trying to detect.

The car is very predictable and on any regular route if you get 42 mpg when you normally get 50 you can be sure something changed.

08-08-2006, 07:58 PM	#1
anj48 Join Date: Apr 2006 Posts: 24 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Being a reader of this and other Prius oriented web sites - when I got my Prius a little over a year ago I dutifully pumped up the tires to 42/40 and at the first oil change added 3.5 Qt. full synthetic (a little below the fill line). I’ve maintained this for the last year in belief that it optimized my mpg but now I don’t think so. What happened? I was to my dealer for the steering shaft recall and had them do the 25k oil change/tire rotation at the same time. Predictably they used dino oil filled to a little over the mark on the dipstick and set the tire pressure back to spec. Since I fried my compressor I didn’t get the pressure back up before I took a long trip – about 300 mi – the next day. Much to my surprise I didn’t notice a real mileage hit – I’ve been getting 51-52 mpg (MFD) for the last couple of months using the car mostly for commuting – at my fill up after the trip the MFD showed 50.8 mpg. So getting back to my routine commute (approx 95 mi daily) I just left the tire pressure and oil alone to see what would happen. Now after two more fill ups (52.0 and 52.3 mpg via MFD), nearly 1000 miles of commuting over the same roads I always drive (mix city, secondary, freeway – lots of hills and turns), same driving style, similar weather and A/C use, always get gas at the same place – very consistent. I see no difference in mpg, actually if anything it’s been a hair better. While I know this falls a short of a research protocol I still think it is a solid observation. Any effect of tire pressure above spec or slightly low oil level on mpg, if it exists at all, must be so miniscule as to not be worth attending to. I am ready to take this whole concept and throw it into my bin of assertions that have more to do with emotions and expectations than reality. From now on I will keep my tires at spec, use regular oil and fill to the line just like any other car. (Note: same posted on Prius Online) __________________ 2005 pkg #6 since 7/14/05

08-08-2006, 08:07 PM	#2
Rangerdavid Senior Member Join Date: Jun 2006 Location: Boone, North Carolina Posts: 1,066 My Car: 2006 Prius Package: #5 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 11	i have had my Prius a little over 3 weeks now. it has 1400 miles on it, original oil, original tire pressure, and i have a life time 48.3 MPG and on the trip i just returned from, over 400 miles round trip, I averaged 49.7 MPG, all interstate miles. I understand the EPA mileage for highway is 51, so I'm not that far off with dealer pressure, oil, etc. and I'm still in the break in period. anyway, i'm still as happy today as I was when we first put out deposit down. What a wonderful car!! __________________ ******************************* Rangerdavid Boone, North Carolina 2006 Silver Pine Mica, Package #5 All weather mats, mud flaps, PC front window heat shield ScanGauge II BT Stiffening Plate Engine Block Heater (installed by FireEngineer) Rear Bumper Protector Wheelskins Two-Tone Leather Steering Wheel Cover Belkin cup holder unit for PDA/mp3 Player Winter front grill wind blocker

08-09-2006, 09:11 AM	#9
tnthub Senior Member Join Date: Jun 2006 Posts: 463 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Short of extensive testing it is nearly impossible to quantify the results of a couple of pounds of tire pressure or a half quart difference in oil.... A person making the exact same 200 mile trip two days in a row could easily achieve a 2-3 mpg difference simply by changes in the bp or humidity... Too many variables happen during the course of a tank of gas (especially in a Prius)m to accurately quantify subtle changes short of simply following through over and over again to see a trend or change. If someone can quantify results then that is great. All I know is from my racing experience where if I fail to extimate the performance of my vehicle within .01 or .02 I will probably lose. Such factors as weather, tire pressure, amount of fuel, and hundreds of other variables come into play and until the variables can be accurately quantified any changes, such as tire pressure, cannot be accurately evalulated. On the street, if a person drives the same route for a month, makes a change, and drives the same route for a month, the variables often will equalize so the person has a good idea (although not conclusive proof), of how a change, such as tire pressure, affects economy. __________________ 2002 Champion New England Dragway 2001 GM High Tech Shootout Winner 1999 Champion Oxford Dragway http://www.tntdragracing.com

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08-08-2006, 08:16 PM	#3
john1701a Senior Member Join Date: Jan 2004 Location: Minnesota Posts: 2,715 My Car: 2004 Prius Package: Pioneer #3 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 3	Even if there was no MPG gain at all from the higher tire-pressure or the lower oil-level, there is still a reason to maintain that anyway. Tire wear has been proven to slow as a result of never allowing the pressure below the minimum (35/33). Keeping it at 44/42 helps out even the most forgetful of owners. As for having the oil level lower, you are simply using less oil. That's a good thing no matter how you look at it.

08-08-2006, 10:24 PM	#4
jamarimutt Join Date: May 2004 Posts: 972 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Overinflated tires produce a harsher ride and I wonder if they cause rattles and may cause suspension problems later on. Using less oil than recommended means that the same oil is circulating over and over, so that you may need to change it earlier. My experience over 44,000 miles is that dyno oil and 35/33 work just fine.

08-08-2006, 10:28 PM	#5
hobbit Senior Member Join Date: Mar 2005 Location: Bahstahn Posts: 2,976 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Tires run at less than their design pressure will wear unevenly. . An overfull crankcase will cause more thrashing and foaming, as well as a viscosity-friction hit. And more oil may show up in that sump that seems to form in the bottom of everyone's intake manifold. . This stuff ain't a myth. Go snoop around cleanmpg.com for some vindication. . _H*

08-09-2006, 07:09 AM	#8
molgrips Join Date: Jul 2006 Posts: 188 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I put my tyres up to 42/40 for a few hundred miles and didn't notice a big difference in MPG.. ok so it wasn't for very long but that's because I DID notice a big difference in everything else. I can't decide wether the alarmingly skittish handling through bumpy corners was worse than the road noise or the vibrations rattling the trim all over the car...! As for oil - I doubt a couple of cups extra would hurt. I've only done 5k miles in mine so haven't had the chance to check it out. I'm assuming the dealer changed it at 4.5k when they passed it on to me. I do notice some tyre wear right on the edges of the tyres though, about an inch in maybe. People are saying that 35/33 is too low in terms of tyre wear, regardless of mpg.

08-09-2006, 09:17 AM	#10
tomdeimos Join Date: Feb 2004 Location: Lexington, MA Posts: 995 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	I've seen no problems with oil up to the full mark but I never overfill beyond that. For tires I see 2-3 mpg difference from Toyota spec to tire sidewall spec. All of you are making up stuff without measuring it. It is very simple to to measure but you can't do it by tankfuls. There are too many variables. You must measure 1 change at a time and do it so nothing else varies. I check every change I make at constant speed on level road with no wind, and at matching conditions. I use the car mpg readout and reset it to measure over a short test route. This way changes of 1 mpg are meaningful and repeatable every time. Over a tankful I see variations way beyond what I am trying to detect. The car is very predictable and on any regular route if you get 42 mpg when you normally get 50 you can be sure something changed.