MPG, What matters most, Oil type or Viscosity?

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Jun 14, 2002
S. W. Wisconsin
After about 1,500 miles on our 2002 Grand Am I kept a record of MPG for the next 2,244 miles and she Ave.35.3 MPG.Most of these miles were on factory fill oil with maybe two fillups after free oil change by dealer. The Dealer Tech.put in 10W30 weight oil I dont know what brand or if it was SJ,SL? All I know is it came out of an oil drum. I checked the MPG again for 807 miles and Ave. 32.4 this was on the Dealers 10W30 oil. I blamed the almost 3 MPG drop on running the AC ALL the time in this HOT spell. Now after changing the oil & filter myself and putting in Chevron Supreme Dino SL 5W30 oil,the MPG went up. The first fillup after my oil change was today, 428 miles--11.9 Gal.= 35.9 MPG. This with the AC still on all the time. So what happened here? Was it the Chevron Dino 5W30 acting like a Synthetic? Or was it the 5W30 Vs.10W30 Viscosity oil? Or was it just cheap 10W30 oil the Dealer used? OR something else? Bill
Sounds like the vehicle is still very new and is still breaking in. 5w/30 SHOULD give you better gas mileage because it is thinner/less viscous oil. The engine is still new though and may take a while to wear in to be able to really compare gas mileage. (adding one passenger can make a difference in gas mileage too!) The suspension also wears in and may need an initial adjustment after maybe 5000 miles. The vehicle book probably shows what oil to use and I bet it shows the 10w/30 to use above freezing temps, it will protect the engine better but may get slightly less gas mileage. I have used thicker oil than the book calls for and did not notice a big difference, but I usually don't do that until a few thousand miles down the road. I would say just keep track of it for a while and see what the average is during summer and then during winter....and then change oil wt if you need to, but do what the mfgr says at least during warranty
Unless you have a test track and control all the variables I don't think you can expect perfection in milage evaluation. The 5W should give slightly better than the 10W but I would guess its less than 1 mile per gallon. I doubt that the difference in brands played a great deal. My guess is that the oil variations played no more than 1 mpg. Just my opinion. For instance when I drive to NC from Pa. the trip is 380 miles. Even under virtually same conditions-same oil and same type of weather the milage can vary about 2 MPG. And bear in mind this is consistant very few stops. That milage sounds terrific for the GrandAm. Is it the 3.4? I used to be able to squeeze out 33 to 37 with the 3.1 GM V-6.
Al--- No it is not the 3.4, It is GMs Global Ecotec 2.2 L engine.Great performance for only 140 HP. And I was lucky enough to find a Grand Am with a 5 speed. Bill
Last winter I was using 5w30 in my Firebird, and when I switched over to 10w30 the mileage remained the same. When I compare the last month running 5w30 to the first month running 10w30, the weather was just as cold too (we had a colder than average spring) so it's not as if the switch to 10w30 was offset by warmer weather either. On all cars that I used to swap back and forth with viscosities like this, I never noticed a difference in MPG.
Would the fuel economy be better when it's cold or hot? Cars are faster in colder temps so I'm guessing that more dense air requires more fuel so in the cold it should be less efficient since it uses more fuel. What do you guys think?
You'll always get better fuel economy in warmer weather compared to cold weather, because the engine warms up quicker and the oil doesn't stay thick nearly as long. My car takes a couple of extra miles of driving to reach operating temp in the winter compared to summer. My gas mileage is about 0.5 MPG better in summer too, and that's with using the AC also. People that do shorter trips than me will see an even bigger drop in mileage in the winter.
I have always noticed cars perform better in cold temps and get a little better gas mileage in hot temps. Theoretically the 5w/30 should get better gas mileage, but it is usually such a small difference it is tough to figure it given all the variables in normal driving. I have never used 5w/30 myself, I have used 10w/30 though, and when I have changed from 10w/30 up to 10w/40,15w/40 or 20w/50 I have never been able to see the difference in the gas mileage, and I figure every tank of gas I use. Good luck with the car! Happy good mileage to you!
Bill- I have heard some good things about the Ecotec. I think that class of engine is the way to go. Good fuel economy. I have a 2L Sentra that puts out a couple of HP more but your 2.2 with the 5 SP and maybe a few more pounds would be comparable to mine. And mine is no slouch-good choice. [Cheers!]
I really doubt it is the oil. Too many other variables, like the others said. 0w30 and 5w30 oils are only required to show an average of 1.5% mileage improvement over the baseline (I think its a 15w40) oil, according to SL spec... and 10w30 only 0.8% improvment. So there is only 0.7% difference between 5w30 and 10w30. Surely not worth the lower protection and durability, IMHO. But when a mfr sells millions of cars it all adds up, that's why they choose it. Here is the specifications:
My experience is that, with all items being equal (such as same type of base oil, same outside temps, same driving conditions, same load), there is little difference between 5W30 and 10W30. The best I have been able to determine is that with 10W30, I get about 0.5 mpg decrease in gas mileage with 10W30 in my Nissan Frontier, 2.4L, I4, DOHC. I choose 10W30 for reasons posted elsewhere.
I guess it depends, depends, depends ... I'm sure some larger, lower-RPM motors may not be sensitive to oil weight/type, but my Honda 1.5L I4 certainly seems to be. Last summer, I was running a mixture of 10W30 and 5W30 Valvoline Synpower in my car when I switched to Red Line 10W30. It was in the middle of a heat wave and my mileage went from approximately 45mpg to 42-43mpg. This was an immediate change with no other variables I could pinpoint. The polyol formula that resists thinning at higher RPMs and temps seemed to also produce more drag. So, I have switched back to 5W30 in an attempt to gain some of that mileage back. Been keeping track of the mileage this year and the average is still about 42mpg since April despite some really hot weather spells. I guess we'll see by the end of the summer. Of course that brings me to the heat. The car seems to run much leaner in the summer which means more fuel economy and less power. Just the opposite is true in the winter. Once the car is up to operating temp, the denser air charge provides more power and terrible mileage. Sure, in the winter I switch to different tires and the occasional snow storm leads to nightmarish 5-30mph commutes which last for hours. Still, those are sudden events and you can still see a gradual change in fuel economy as the ambient temperature changes with the seasons. Oh, and I seem to get better mileage using the A/C at highway speeds (70mph and above) than driving with the windows down. [Wink]
Bror, Regarding AC. I told the same story to people in the past and they laughed, but I had the same experience with my '86 Burb. I suspect it has to do with wind drag, but it could also have something to do with matching source (engine power) to load (car) and the AC may provide a better power match.
Molakule, I really think it's the drag. I had a '76 Chevy Caprice as a kid and I was fun-racing a buddy of mine on the highway. My car was always a little faster than his Pontiac but I simply couldn't pull away from him with the windows open. On a hunch, I closed the widows and the car almost lunged ahead. I figure any drag on the motor is a bad thing for wheel horsepower or mileage so I can't imagine engaging the A/C compressor as beneficial.
Howdy Bror, You had air conditioning? in 76? Just kidding! I did too, lived in Fl, compressors were harder on engines back then too, newer ones seem to give less resistance. see ya Have a good weekend! Rando
Yeah I noticed a huge increase in fuel economy driving at 85 through the burning desert with my 160degF stat, M1 10W-30, and the AC on vs the windows open. I tried it for 200 miles with the windows open, 27.4mpg. Then windows shut, A/C on, a nice 60degF inside my camaro, 34.4mpg. This was repeatable too [Smile]
When I had my 98 Formula I found that when I drove with the windows up and the AC off, it would get 30-31 MPG on the highway. On the same long trip (from Toronto to Memphis and back) if I drove with the windows still up, but with the AC on it would drop the MPG to 27-28. I noticed the exact same mileage penalty with the 95 Trans Am I had before that one. I never tried a long trip with the windows open and the AC off though.
Yep, and you'll get even more mileage with shut windows and no a/c... [Smile] It's simply due to the fact that it takes more horsepower to overcome the drag of open windows than it takes to drive the a/c compressor. And because at highway speeds, air resistance is primary source of engine load. I just gained ~3mpg at 80mph by reducing the drag of my car. I have already removed all a/c components since it broke and my car never gets above normal temp, so I replaced the center grille with one from a non a/c model (upper 2" closed), completely blocked off both left and right grilles, and then blocked off the slit above the oil cooler that is there for a/c models. Voila, free money when driving on the highway. Coolant temp still stable, even at 100F in traffic, no change in oil temp either. I figure I could also block off the oil cooler and brake ducts in the winter for more gain as well. BTW, my car has MPG display computer and I found I lose 2-3mpg just from open windows.
Sorry Jason, but I'd gladly pay for the extra gas when I'm driving dirt roads with 115 degree heat and 90% humidity. Give me A/C anyday.
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