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03 Forester 2.5X 4 Speed Auto
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674 Posts
Discussion Starter · #1 ·
003 subaru forester 2.5x / automatic

If I get in my car and drive it (let it run for 30-45 seconds until the fluids are circulating) I get 17-18mpg.

However... if I start my car and let it really warm up for 7-8 minutes. Enough to let the engine reach operating temperature and the piston slap to go away, I'll get 22-23mpg.

And I'm not reading off of any display. I'm calculating this with the trip meter and how many gallons it took to fill up my car.

I'm not opposed to getting into a cozy car on a cold day.. But I'm wondering now, what would cause me to get such a difference in fuel economy just by warming up the car first. To be honest I figured the warm up would cut my fuel economy since the car is sitting there sucking gas but going no where. Makes me wonder if there could be some sort of underlying issue?
 

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03 Forester 2.5X 4 Speed Auto
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674 Posts
Discussion Starter · #2 ·
Spark plugs have 8,000 miles on them. All fluids are fresh. I'm a freak when it comes to that sort of stuff. Coolant has 8k on it, diff fluid has 3k on it, engine and transmission fluid were changed 600 miles ago. Air filter gets cleaned every other oil change (about every 7-8k). It's a k&n filter. All oils used are synthetic
 

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2004 FXT 4EAT
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259 Posts
Well, there could be some skews in your calculations, and how you drive during those tanks. You're telling me you do the exact same method of warming up the car, take the exact same trip in the exact same traffic during the exact same weather conditions as the other method of warming up the car? And where you fill up and what time of the day you fill up could also cause variations.

Tbh, I've had multiple tunes, drive city and hwy, and make pulls here and there and CANNOT break 20 mpgs and never had below 18 in the past couple months.

But how your getting 17 then 23 is beyond me. (Other than variating how you calculate it)
 

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03 Forester 2.5X 4 Speed Auto
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674 Posts
Discussion Starter · #4 ·
Same drive. Just to/from school and work with a grocery run every Friday.

Didn't pay too much attention to driving style. I just drove like normal. Figured that would get me the most realistic results.

As for warming up my car: I have a timer. Its within 60 seconds of every other warm up.

Same grade of gas, same gas station. Usually fill up in the evening.


Even tried flipping it again and re-doing my tests and got the same result. Miles driven divided by gallons used. Using the trip odometer and filling my tank to the top.


I've actually been keeping track of this over the past several months. Was part of my decision making process to decide whether to get a remote start for my car. Or some sort of engine pre heater(gas cost vs. Electrical cost to warm the engine)
 

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MY06 MY06 & MY10 Forester 5MT/DR + 4EAT (Sports)
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54 Posts
Gidday All

In my MY06, I get between 11.5 - 12.5 L/100 kms (20.5-18.8 miles per US gallon) around town, but sometimes better than this if the tank is used doing a lot of freeway or major arterial road driving.

On the open road, this gets far better, at around 7.5 L/100 kms (about 31 miles per US gallon).

These figures are calculated on total fuel used since I bought the car about 10,000 kms ago, and specific tank usage. My average usage has been 11.22 L/100 kms overall (20.96 miles per US gallon). I do a lot of short trips around town ...

I have kept detailed log books on all our cars for at least 35 years.
 

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Smooshed FOTY 2011
2005 Lifted 2.5 XT 5-Speed MT Dual-Range
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5,732 Posts
Cold engines are inefficient, but cold transmissions are also very inefficient. The engine may not be what is causing better milage, but your transmission. When cold, transmissions are not running as efficiently as they could be, meaning you are getting drag on the drivetrain. Automatic transmissions take a lot longer to get to operating temp than engines do. And if you get in an automatic, and start driving it right away without allowing it to sit and warm up, you could essentially continue to cool the transmission (especially on cold days) as it doesn't have a thermostat, and continuously pumps fluid through the lower part of the radiator. So without getting the transmission up to temp, driving on a cold day may even yield not even getting the transmission to warm up to the proper temp.

In fact, a lot of the newer cars these days are coming with transmission fluid heaters that head up the fluid as quickly as possible to get the transmission to optimal operating temp. People always think a cool automatic transmission is the happiest, but that is not the case. They need to be in an optimal temp just as the engine does.

That being said though. When I had my old 4EAT XT, I couldn't get better than 18 mpg in the city no matter if I floored it everywhere or drove like a school teacher. So I just drove the pants off of it, haha!
 

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2009 Foz XT Limited Auto
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369 Posts
Going off the gas mileage readout in my SH XT, when the engine is cold its quite thirsty (20+L/100km instantaneous). But if I let it idle, say, 3-5+ minutes instead of 30 seconds (lets suppose I'm cleaning the windows from the snow), it doesn't drink as much (can get it around 15L/100km).

I dont think the key is to let it idle 7-8 minutes, but maybe 2-3 minutes would be more reasonable.
 

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2011 Forester 5 Speed Manual
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1,612 Posts
I had an old 4runner whose stick shift even with fresh oil felt like I was shifting in peanut butter when it was really cold. I'm not surprised engine and or transmission temperature affect fuel economy.
 
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