My car get better gas mileage doing 70-75 mph

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My 92 Corolla is getting 35-38 mpg cruising 70-75 mph. If I go up to 80 mph, that number is at 35 mpg. I think the EPA number for this car is 30 mpg hw maximum. I am using Michelin tires, which could explain the increase as my 02 Honda used to get 29 mpg when all 4 tires were Michelin. The 02 Honda gets only 24-25 mpg hw now because the 2 front tires are Yokahoma tires with more aggressive tread pattern.

So my question if the "drive speed limit of 60 mph because every 5 mile over that is a big waste of gas" a big government conspiracy lie to scare us into slowing down to grandpa pace?
 
I'm not really sure. On one trip I did (300 miles) in my Saturn I got 42 mpg (my highest ever) and I was doing between 75-80mph the whole way.
 
One of my old cars (186 c.i. 6, with triple S.U. carbs, big valve head, big cam and free flowing exhaust) had a definite "sweet spot" at 80 MPH.

You'd be sitting in it on the highway, glance down and see the speedo at 80. Back off to 70, and 15 minutes later it was 80 again (confirmed by my Dad when he took it for a trip).

If you let it have it's head, you scored 30MPG (Oz). A sold 2-3MPG more than if you obeyed the speed limit.
 
"So my question if the "drive speed limit of 60 mph because every 5 mile over that is a big waste of gas" a big government conspiracy lie to scare us into slowing down to grandpa pace?"

In a word, no. At highway speeds overcoming air (wind) resistance becomes a very large factor. The power to overcome air resistance increases roughly with the cube of the speed.

If you have studied rudimentary physics you will understand what this means. If not, you may just have to settle for conspiracy theories.
 
What RPMs are you running at 60 mph, 70 mph and 75 mph?? I have always believed that the higher gearing (lower RPM) for better gas mileage was not the correct route to take. In a lab environment for EPA results and marketing, you will achieve 30 mpg running at 60 mph and 1600 RPMs in the highest gear (auto/man trans). In the real world outside of running on a dyno or the Bonneville Salt Flats, running a constant 60 mph gets less mpg due to more throttle to maintain the speed and transmission down/up shifting to make it over any hills. Why??? Lower RPMs is lower on the hp curve. My 13hr/750 mile trip (each way with stops for the kids) proved this as the trans rarely down/up shifted @ 70 mph (higher on the hp band and speed limit was > 55 mph) and was shifting more often at 60-65 mph. I also think momentum for hills play into it also.

I've noticed this for years. When I used to commute on the Taconic 500 in NY (rush hour commuting was 70 mph on rolling hills, bumper-to-bumper driving like drafting on the back straight away at Talladega waiting for the "big one") I would drive in 5th gear. At 55-60 mph (relaxed driving on the weekend) I would drive in 4th to prevent lugging the engine and downshifting, keeping the engine higher on the hp band (also, faster response when I had to accelerate to pass or get away from a "blue hair"). My gas mileage in 4th was not much lower then my 5th gear 70 mph commuting which were both equal or higher to 55 mph in 5th. From what I recall, Taconic 500 gas mileage was 28-32 mph with a few city miles in the mix; '88 Z24 & '92 Civic Si (both 125 hp engines).
 
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As wgtoys said, vehicle shape is the main factor. Japs such as Toyota often have good shapes offering less wind resistence. Actually this morning I paseed by a Prius and really looked at its lines. Wow. Talk about smooth.

The old limits of 55 mph or 60 mph were based on the old cars which had horrible shapes that encountered a lot of wind resistance at lower speeds. Therefore they were correct and made sense couple of decades ago. Turth be told, there are still many vehicles that sacrifice efficiency for better looks.

Therefore the most efficient speed is different for different vechiles and 55/60 limit is just a generalization.

Also, 4cyl OHC engines are supposed to be more efficient at higher RPMs (there were many discussions about this). My smaller Civic's 1.5L engine runs at higher RPMs than bigger 2L Mazda and still gets excellent 40ish mpg (of course it is small, light, etc.) So high RPMs for cars like Corolla are actually better (up to a certain point of course).
 
My car gets 38 mpg at 55 mph, in summer, with 5w30, but it's really hard to keep it at that speed.

The engine is a 2.5L 4 cylinder, side cam.

Higher speeds increase your risk of dying, in a collision, so there's a risk cost associated with driving fast that may or may not offset the value of saving time.
 
Try controlling that car at 80mph after blowing out a tire.

A tire can blow for many many reasons, not just from age, lack of tread....but, one can hit something in the road and have it go too.

That said, it can be very easy to reach higher speeds and maintain them in a little car, you have much less wind resistance then those with trucks do.....but, just keep in mind what oilyriser stated....accidents are much more deadly, harder to avoid at higher speeds.
 
Originally Posted By: Ramblin Fever
Try controlling that car at 80mph after blowing out a tire.

A tire can blow for many many reasons, not just from age, lack of tread....but, one can hit something in the road and have it go too.

That said, it can be very easy to reach higher speeds and maintain them in a little car, you have much less wind resistance then those with trucks do.....but, just keep in mind what oilyriser stated....accidents are much more deadly, harder to avoid at higher speeds.


Mad Max and Mad Max 2 (Road Warrior) should be required viewing in all Driver's Education classes.
 
"So my question if the "drive speed limit of 60 mph because every 5 mile over that is a big waste of gas" a big government conspiracy lie to scare us into slowing down to grandpa pace? "

Couple reasons (some mentioned above)
1. Potentially MPG may decrease due to higher RPMs, but it depends on gearing and engine.
2. MPG WILL decrease due to higher air drag (car aerodynamics & wind dependent)
3. Inability to control car during emergency situation or inability of OTHER drivers to handle your travel speed.
4. Energy released during the crash is exponential to your speed (IIRC from physics class). The faster you go during the crash the more chances somebody is going to be seriously hurt or die.
 
My friend's 89 Cutlass 2.8 got better mileage at 80 than it did at 60, but it was largely because of the circumstances of her driving. Wyoming was very hilly and at 60mph, the gearing was too tall and the car downshifted on every hill. At 80mph, it didn't downshift.

If driving on flat ground, 60 would have yielded her better mileage than 80.
 
This topic comes up quite often. All of the folks that I PERSONALLY know who claim this occurs with their vehicle do NOT keep accurate, detailed records of their driving/fuel consumption so am suspect of their reports. At 50 years of age I have driven/owned a fair number of vehicles, NONE of which delivered the fabled "best mileage at 80 mph". Sadly, my personal vehicles have all strictly obeyed the laws of physics regardless of how much I wish they would be rebellious.

Since aero drag increases with the square of velocity, it is absurd to think that it somehow requires less power to go faster. The only way a vehicle can get better fuel economy at 80 mph than at 60 is if there is something wrong with the fuel delivery system in said vehicle. I do believe that this happens sometimes.
Joe
 
I have a 2002 Subaru WRX with the EJ20 (2.0 liter) turbo 4. I've taken a few trips from New York to Reston, VA recently. I do believe there is some validity to this theory. The car is EPA rated for 20 city/27 highway. One trip I set the Cruise at 65mph the whole way. On the trip down I pulled in about 29.8mpg. The next trip I decided to lock in the cruise at 72 mph. For that drive I pulled in 30.8mpg. I know the figures are close but I did follow the same route/gas refill routine both times. I fill up the night before and drive down the next morning. When I get to VA, I go to the office and then on the way to the hotel I refill the tank.

My car has the optional turbo boost gauge. I do notice that if I drive at 60-65 mph which is about 2400-2600 rpms the engine sometimes needs a little more boost to maintain that speed vs. 70-72mph which is 2800-2900rpms.

The engine produces it's peak horsepower at 6500 rpms and the max torque around 4000 rpm. So, I do believe the engine performs better (perhaps more efficient) at higher speeds.

I really haven't tried a long cruise at 80 mph however. I'm a little worried about the state troopers locking in on a WRX going over the speed limit.
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Just my :2cents:

Ed B.
 
On one of my motorcycles, I get the best mileage at 65. I get 50+ mpg on a 1200cc touring bike that makes near 100 hp. I get 40-45 at 55-60. The numbers are as consistent as the day is long.

It is true that drag increases with the square of velocity, but that is being multiplied by a (typically) very small number (the drag coeffecient), so it's effect is small until the velocity gets to be larger. However, the larger issue for many vehicles is where the engine is in terms of RPM and it's peak torque at different RPMs. My '92 Buick Roadmaster got 24 mpg at 70 (which was 1700 rpm) and 22 at 60 (which was 1500 rpm or so). Why? I dunno. Gearing. Peak torque on that was 2200 rpm.

Another example, at the other extreme, is how much more horsepower it takes to go from 190mph to 200mph. It can take 30 HP or more for some motorcycles.

Gearing and engine efficiency *combine* with drag to produce the most efficient travel.
 
Originally Posted By: Jon
On one of my motorcycles, I get the best mileage at 65. I get 50+ mpg on a 1200cc touring bike that makes near 100 hp. I get 40-45 at 55-60. The numbers are as consistent as the day is long.

It is true that drag increases with the square of velocity, but that is being multiplied by a (typically) very small number (the drag coeffecient), so it's effect is small until the velocity gets to be larger. However, the larger issue for many vehicles is where the engine is in terms of RPM and it's peak torque at different RPMs. My '92 Buick Roadmaster got 24 mpg at 70 (which was 1700 rpm) and 22 at 60 (which was 1500 rpm or so). Why? I dunno. Gearing. Peak torque on that was 2200 rpm.

Another example, at the other extreme, is how much more horsepower it takes to go from 190mph to 200mph. It can take 30 HP or more for some motorcycles.

Gearing and engine efficiency *combine* with drag to produce the most efficient travel.


Peak engine efficiency does not occur at peak torque, that's an urban legend.

Peak efficiency at full throttle occurs at about 85 to 90% of peak torque rpm.

At less than full throttle, peak efficiency rpm drops way down from that.

For most cars, actual controlled testing instead of anecdotal stories shows they will develop best fuel economy someplace between 35 and 50 mph in their highest gear.

That even holds true for a 400 hp C6 Corvette with a manual transmission that has the engine turning 900 rpm at 40 mph.
 
Originally Posted By: SuperEd73
I know the figures are close but I did follow the same route/gas refill routine both times.


Wind can make a difference, too. I've gotten GREAT gas mileage driving south in the winter..but the wind was blowing from the north.
 
My gas mileages always gets exponentially worse once I go over 75mph, around 60-70 my gas mileage is above average with about 31-35mpg, depending on how much traffic, weather conditions, amount of variation in speed, etc. I'm just using the cheapest tires on my car, but with a current alignment/balancing and correct air pressure in all of the tires. When I drive over 75 I struggle to break 30mpg. Also keep in mind that my car has a fairly low drag coefficient of .33.
 
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For most cars, actual controlled testing instead of anecdotal stories shows they will develop best fuel economy someplace between 35 and 50 mph in their highest gear.

That even holds true for a 400 hp C6 Corvette with a manual transmission that has the engine turning 900 rpm at 40 mph.



Why did you have to bring all that up?
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Slower pays. No two ways about it. Now some configurations are going to be effected less by wind resistance ..so the penalty is less noticeable/has lower impact. The thing is, most of you that report better mileage aren't comparing it to itself in the other mode. You go on one trip @ 80mph and get 2-3mph more and consider that proof that you're doing better ...but you never do the same trip at 55 to see the other side of the coin.

I could get 30 mph out of my BMW driving it in geezer mode (up from 24 typical-and it ran like stink the next day). I could get a 2.8 Citation to get 35 in geezer mode ...placing an egg between my foot and the gas pedal. I could get over 40 out of a Chevette ..same deal. These were all runs of over 300 miles per day over a few years; using 50 as my top speed as opposed to 65 (this was the Reagan federal highway speed enforcement years of 55mph limit). I didn't get to do this often, time was usually at a premium.

Fuel economy, outside of actual operational conditions, is primarily effected by miles per warm up cycle. 75-80 yielded the BEST mileage that I ever saw out of my minivan; nearly 25mph. It was merely due to the 400 mile/one warm up situation that would normally have included 8-15 warm ups the rest of the time it was in use. Even my wife's 25 mile one way commute produced lower numbers (around 20mph). I'm sure that if we had the patience to do the whole 400 miles @ 55mph (or even 65mph) the yield would have been even better ...but I80 in central PA is ONE BORING ride if you don't have company in terms of traffic. 90mph may make the drive interesting enough to keep your attention.
 
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