Old "Urban Legends" about vehicles: True or False?

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Kit Sullivan

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You know, people randomly say and do things with the explanation that "It gives better gas mileage" or something, and sometimes I am not sure if it is true or not. Here are a few I have always questioned.

1) "Windows open gives better mileage than running the A/C."

Well, I can see that maybe on older cars, Like our Mustangs with thier anvil-heavy and innefficient "York" compressors, that may have been true to some degree...but what about newer vehicles? I think the lowered aerodynamics of an open-windowed vehicle may actually be less efficient than the newer light-weight and efficient A/C systems. What do you think?

2) "Tailgate up or down?" I know "Myth Busters" did a famous episode on this, but I have never been too impressed with thier methodology, and certainly do not trust thier results as gospel. Any opinions on this?

3) "More air pressure in tires gets better mileage" This makes sense I suppose: Higher-presssure in tires equates to less rolling resistance, which should lead to better mileage. But, will it be noticeable? I know a severely under-inflated tire can lead to a loss of fuel economy, but over-inflating has to follow the law of "diminishing returns" to some degree. How much over-inflated do you have to go before it becomes significant? I know there is a tire-expert around here that can give a dissertation on this!

4) "Higher octane gasoline gives better mileage" Well, we all know that that is definitely not true, but...SO MANY PEOPLE are still under that impression. I stopped giving answers to that years ago.

5) "A giant free-flowing exhaust gives the best performance." Another one that is proven totally incorrect, yet so many just don't get it. Backpressure serves a function.

6) "Chrome is faster." Well...That's true, of course!

 
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I can have an my experience on 1 and 3.

The AC ON in my 06 expedition has an effect on the MPG, and windows Open has an effect also

I think the AC puts a load on the engine, and the windows do kill the arrow dynamics acts is like a big air catcher.

with the AC off and windows closed I get the best MPG's 16 city and 22 highway.

As for the tire pressure, My wife's 2010 Prius had a huge affect, 33 PSI rear, and 35 PSI front is the factory recommended, I have them set at 40-43 all the way around, and we have about a 10 MPG increase, it went from 50 city to almost 60 city, we bought the car in Aug 2009 and I started doing this in Aug of 2010 and have gotten consistent results.

just my 2 cents on this topic :D

 
You know, people randomly say and do things with the explanation that "It gives better gas mileage" or something, and sometimes I am not sure if it is true or not. Here are a few I have always questioned.

1) "Windows open gives better mileage than running the A/C."

Well, I can see that maybe on older cars, Like our Mustangs with thier anvil-heavy and innefficient "York" compressors, that may have been true to some degree...but what about newer vehicles? I think the lowered aerodynamics of an open-windowed vehicle may actually be less efficient than the newer light-weight and efficient A/C systems. What do you think?

2) "Tailgate up or down?" I know "Myth Busters" did a famous episode on this, but I have never been too impressed with thier methodology, and certainly do not trust thier results as gospel. Any opinions on this?

3) "More air pressure in tires gets better mileage" This makes sense I suppose: Higher-presssure in tires equates to less rolling resistance, which should lead to better mileage. But, will it be noticeable? I know a severely under-inflated tire can lead to a loss of fuel economy, but over-inflating has to follow the law of "diminishing returns" to some degree. How much over-inflated do you have to go before it becomes significant? I know there is a tire-expert around here that can give a dissertation on this!

4) "Higher octane gasoline gives better mileage" Well, we all know that that is definitely not true, but...SO MANY PEOPLE are still under that impression. I stopped giving answers to that years ago.

5) "A giant free-flowing exhaust gives the best performance." Another one that is proven totally incorrect, yet so many just don't get it. Backpressure serves a function.

6) "Chrome is faster." Well...That's true, of course!
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Kit, I agree with what you said except the last part of number 5. There is no purpose for back pressure. In fact "back pressure" is a term coined to describe the effects created by a less than optimal exhaust system design. If the flow is sufficient and the velocity is maximized the system is optimal. That is why a too large, or too small, primary, collector, or restrictive muffler results in less performance.

Chuck

 
Another "myth" is that the engines from the 1960's would only last 60k miles before needing an overhaul, if you were lucky, you could get to 100k miles.

Myth true or false: synthetic oils and changes every 15k miles is better than dino oil and changes at every 3-5k miles.

 
another good one is that premium cleans your fuel system. I've also heard that about race and airplane fuels.

as far as the exhaust goes, I really don't care, I'll give up a few ponies as long as it sounds good.

 
Another "myth" is that the engines from the 1960's would only last 60k miles before needing an overhaul, if you were lucky, you could get to 100k miles.

Myth true or false: synthetic oils and changes every 15k miles is better than dino oil and changes at every 3-5k miles.
Click to expand...
True IF, the filter has the capacity to be effective for 15,000 miles. If not, the filter needs to be changed at about 7,500 miles, it varies depending on the filter capacity. Anecdotal information, I've used synthetic oil/lubricants in street cars, race cars, lawn movers, generators, compressors, etc. with great results. The ordinary filter gives up before the synthetic oil does. Chuck

 
1) My 1999 Jeep Grand Cherokee take about a 2 mpg hit with the air on. I don't see anything notable with the windows down.

2) Who cares? If your driving a real truck and you think your tailgate is a MPG issue then you probably need to rethink what you drive.

3) Yup, keeping the tires on the jeep a little high helps. I drive 80 mi a day (40 each way) for work and I can always tell by the MPG meter that I need to check the tires.

4) I believe it helps (a little). I always seemed to get a little further with my GSX1100 on Avgas than I did with 87 pump.

5) If that were true "tuner" exhaust wouldn't be popular. Every exhaust system can be "tuned" to improve performance. It's very overlooked aspect of performance builds.

6) If your Google.

 
Well I know 1 fact red cars are faster hehe


I once did a 700km trip in a later model commodre in 45+c heat with a working AC then the same trip same conditions 3 weeks later with no AC and the windows down. The car used more fuel with no air and the windows down than with a working aircon. The increase in drag must of been greater than the load a modern AC draws

 
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How about this one: Tailgate a tractor trailer and you can get better MPG by drafting them. I believe mythbuster did an experiment on this one as well.

 
Most vehicles in recent years(little weary of saying all) cycle the AC Compressor on and off anyway during normal driving. That way it continues to circulate the oil throughout the system. That was part of the problem with older AC Systems. The compressor cyles on alot of vehicles when ever the environmental control system is running. That way all of the seals stayed oiled and don't dry out. Amongst other reason like windows fogging but it is built into programing.

One of the cool reasons performance parts like the Killer Chiller works so well.

 
On the subject of item five (5), I do believe there is a purpose for back pressure at least on my '71 4V quench Cleveland.

It is mentioned in Des Hammill's book about Ford's 335 series engine. My custom exhaust is patterned after one used on

the Boss 351 and has numerous kinks and bends that seem to serve no purpose.

I do know my car runs better on hot dry/air and not cold/damp air even when warmed up.

mike

 
On the subject of item five (5), I do believe there is a purpose for back pressure at least on my '71 4V quench Cleveland.

It is mentioned in Des Hammill's book about Ford's 335 series engine. My custom exhaust is patterned after one used on

the Boss 351 and has numerous kinks and bends that seem to serve no purpose.

I do know my car runs better on hot dry/air and not cold/damp air even when warmed up.

mike
Click to expand...
This is a good read on exhaust optimization and explains why no back pressure is needed. Too large of a pipe diameter causes the flow too slow down to less than optimal. When the smaller, correctly sized pipe is used, the flow speeds up and performance increases. I believe this is why people continue to say some back pressure/resistance to flow is needed http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/

The second issue you described both contain "offsetting penalties". Hot air is less dense, dry air is more dense and cold air is more dense, moist air is less dense. This is an overview of air density and performance. http://www.bankspower.com/galesinsidernews/show/43-Air-Density---The-Key-To-Making-Power

Chuck

 
Too large of a pipe diameter causes the flow too slow down to less than optimal. When the smaller, correctly sized pipe is used, the flow speeds up and performance increases.

Chuck
Click to expand...
So it's like with sewage pipes?

It's better to have smaller pipes in the street so when they fill up the stream will cause a vacuum (suction) which leads to better scavenging of the pipes of the houses above?

Is that what you mean?

But if they are too small in diameter they will overflow and push back, thus flooding my cellar.

I had this discussion with the workers that put in new sewage pipes in our road and that's what they told me when I asked why they did not use even bigger diameter.

 
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Tailgates up or down... depends on the make and year model of the truck. Anything older than a '94 Dodge Ram, "tailgate down" or removal altogether will provide slightly less drag - which is why the tailgate nets and tonneau covers worked so well. Auto manufacturers started programming better aerodynamics into the trucks (ever notice the factory aero packages on tailgates?).

I had a tonneau and a tailgate net for my first '85 Nissan 4x4, and would observe the area of the soft tonneau just ahead of the tailgate being pushed against the tailgate itself at speed. With the tailgate removed, there was less noticeable drag on the highway than with the tailgate in-place with no tonneau.

With my Ram and its soft tonneau - not so much.

Aerodynamics is huge. My '82 Mustang with its sugar-scoop-like headlight buckets used to get decent gas mileage. On the 250 mile trip segments (between Alamogordo, NM and SLC,UT), it would typically use about 5/8-3/4 of a tank. When I put headlight covers on, I noticed I only used 3/8-1/2 tank on the same 250 mile trip.

So yeah - aerodynamics makes a huge difference.

 
My favorite is 'new or new rebuilt' engines are 'tighter' and 'run hotter'. This is especially interesting when it is suggested to someone as the reason their rebuilt engine 'runs hot'.

 
"Tighter." I can buy into that as well. My 2nd '85 Nissan popped a head gasket (with a nice chunk removed from the block and a chewed up #3 piston skirt, but no damage to the block miraculously), so I rebuilt the whole thing.

I would normally shut off the engine with the clutch in and coast down the alley to the mailbox. Shortly after I rebuilt the engine, I did this but accidentally let the clutch pedal out too soon while still rolling... which locked up the back wheels and skidded for about 3 feet. It had never done that before (the skidding part) when I'd made the mistake prior to the rebuild.

That tells me the engine definitely had much better compression - and I'm sure that it was a bit tighter thanks to the rebuild going back to actual torque specs. Probably doesn't qualify for what you're suggesting, though. ;)

It never ran any hotter, though. Ran a bit cooler, as a matter of fact.

 

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