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Author Topic: 55 MPH vs. 65 MPH Back to Topics
AustinLee3

Veteran Author
Tennessee

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Message Posted: Dec 14, 2013 12:58:24 PM

According to different sites, the more you go over 55 miles per hour, the less gas mileage your car will be getting. Does this apply to all vehicles?

I drive a 2003 Cavalier automatic and its estimates get about 21 city to 30 highway. I've been getting 26 city and 32 highway. I've noticed that when I would normally do 55 miles per hour that my car was getting about 30 miles to the gallon, but when I started doing 65 miles per hour on my normal route to school, I've been getting about 32 - 34 miles to the gallon.

To me, it seems as if my car gets better gas mileage when I'm doing 65 miles an hour. Yes, it's wrong and unsafe to speed, but it's just something that I have noticed.
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HotRod10
Champion Author Wyoming

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Message Posted: Apr 22, 2014 8:26:16 AM

"Common sense. The faster your engine is running, the more gas you will burn."

Apparently even "common sense" can be completely wrong. Try running car engine at 4000 rpm in the driveway, out of gear, for an hour, then run the same car on the highway at 2000 rpm for an hour. Let us know which one uses more gas. DUH!
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twt
Champion Author Virginia Beach

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Message Posted: Apr 22, 2014 5:19:55 AM

Common sense. The faster your engine is running, the more gas you will burn. DUH!
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Camry05
All-Star Author Indiana

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Message Posted: Apr 21, 2014 11:15:20 PM

I think my car might give better mileage at 85, have to try it out ;-)
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drgeeforce
Sophomore Author Los Angeles

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Message Posted: Apr 21, 2014 6:56:41 PM

My 2010 GL350 BTC gets 32MPG at 62mph and at 60-55mph I get the finger. Yes, I do drive on the right lane.

[Edited by: drgeeforce at 4/21/2014 6:57:59 PM EST]
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GrumpyCat
Champion Author Alabama

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Message Posted: Apr 21, 2014 2:25:22 PM

2000 Toyota Avalon gets better MPG at 65 than at 55.
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marielev
Rookie Author Wisconsin

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Message Posted: Apr 21, 2014 11:22:31 AM

I get 34 @ 70 mph
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scottmccarrick
Champion Author Boston

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Message Posted: Apr 21, 2014 6:12:10 AM

i get my best mileage at 68-72mph. at 55 i get shitty mileage
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drydem
Sophomore Author Maryland

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Message Posted: Apr 21, 2014 12:31:18 AM

For superhighway driving Trucks, SUV, Jeeps, Hummers have *large frontal areas* and *high air resistances* so they are less fuel efficient at higher speeds than a car with a *small frontal area* and *low air resistance* like a Honda Insight, a Toyota Prius.

For the city, the main problem is curb weight and the loss of energy after doing a complete stop, a lighter curb weight vehicle like a Geo Metro or a Honda CRX loses less energy after a full stop than a heavy vehicle like a Ford Crown Victoria or a BMW 600 series. Hybrids like the Prius can lessen this lost via regenerative braking and electrical motors but there still is a loss and the most fuel efficient solution is just to have a lower curb weight.

Conventional cars like the 2003 Cavalier use an Otto cycle engine which have a "sweet spot" - an RPM range where the internal combustion engine (ICE) is operating most efficiently - i.e. it is outputting the most horsepower per ounce of gas when engaged at the highest gear your transmission can handle. Otherwise your ICE will idle between 600 to 900 RPM.

With respect to fuel efficiency - before computers - the shell fuel efficiency contestants had a vacuum gauge display on the dash board which was hooked up to engine to figure out what was the engine's *sweet spot." The driver manipulated the accelerator pedal to get the the maximum compression rate on the vacuum gauge while adjusting the transmission gearing to get the proper speed for that power level.

The best MPGs posted for a Cavalier on the website forum of cleanmpg is 41 mpg (manual) and 37 mpg (automatic). Most recommend over inflating the tires or using low rolling resistant tires, adding a scangaugeII to help you with modulating the accelerator (e.g. xgauge LOD = 70 to 80 ), and driving at a lower speed.

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HotRod10
Champion Author Wyoming

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Message Posted: Apr 20, 2014 9:16:24 PM

Despite what all the aerodynamics "experts" here have said, different vehicles DO have different "sweet spots", and air resistance is highly variable. Yes, IN GENERAL air resistance increases with increased speed, but fluid dynamics is hopelessly complex. Calculating wind resistance on a shape as complex as a car, with varying surface conditions, depending on whether it's clean, waxed, dirty, caked with mud, etc. is a complete impossibility. Fluid resistance vs. speed is not by any stretch a linear increase; it doesn't even always increase at all. Research a phenomenon called a drag crisis.

That's the long way of saying, if your car gets better mileage at 65 mph, and it's a safe speed to drive, don't let some naysayer talk you out of it by telling you that it's not possible or that it defies the laws of physics - it doesn't. They just don't understand fluid dynamics as well as they think they do. Real experts know enough to not make blanket statements, especially when it comes to air movement over complex shapes.
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drydem
Sophomore Author Maryland

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Message Posted: Apr 20, 2014 5:21:02 AM

On a flat surface slower speeds give the better fuel efficiency because there is less energy expended overcoming wind resistance. A 2003 Cavalier automatic will get better fuel efficiency at 45 mph than at 55 mph or 65 mph when driving on a flat road surface without stopping.

Every internal combustion engine has an initial warm up cost to start the vehicle from a cold start. This cold start cost is higher in colder temperature (winter time) than in warmer temperatures (summer time). Short trips increases share of the the start up energy cost to the driving energy cost. Longer trips mitigate the start up energy cost.

The greatest energy lost and drag to fuel efficiency is doing a complete hard stop - because all the energy use to get the vehicle moving is lost as heat via the brake pads.

The reason your car appears to get better fuel efficiency at higher speeds is because it is being driven on the superhighway where it does not have to do a complete stop for a long time. The fuel efficiency advantage of a slower speed on a local road is lost because there are many places where the vehicle must make a complete stop. There are multiple techniques in hypermiling to getting higher fuel efficiency which involve modulating a vehicle's speed to minimize the number of complete stops it must do before it gets to its final destination

To validate these concepts. This Summer drive your Cavalier in the middle of the night on local roads when all the traffic lights are turned off (blinking yellow) so you don't have to stop and there is no traffic so you wont get in anyone's way for +60 minutes at 25-35 mph. Pick a relatively flat route for this driving test circuit. Use a fuel consumption monitor like a ScangaugeII to determine your fuel efficiency for that trip. If you do the following you should see your fuel efficiency for that trip jump up about +20% or more.

HTH

Walter

[Edited by: drydem at 4/20/2014 5:30:23 AM EST]
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OilerFan
Champion Author Tulsa

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Message Posted: Apr 20, 2014 1:57:17 AM

It does not apply to all vehicles. It has to do with engine efficiency as well as transmission gear ratio.
And the truth is, that the magic number almost certainly is not 55. That's just the number that is most appropriate to drive for a highway. The truth is that most cars get up to about 35 mph and hit a nice balance between relatively low rpms and being in the high transmission gear; essentially travelling as efficiently as the vehicle is capable.
Therefore, in almost all vehicles, we know that around 35 mph, most are in their high gear by then. That is the number, not 55mph.
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dieselpower2014
Veteran Author Atlanta

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Message Posted: Apr 20, 2014 12:46:24 AM

the faster you go the more rpm's the motor turns. The less mileage you will get. Its the same with any car or truck.
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mybigtruck
All-Star Author San Jose

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Message Posted: Apr 19, 2014 11:42:10 PM

The 18-wheelers around here all cruise at 60mph. Must be a good reason for it.
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Maintroll
Champion Author Lexington

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Message Posted: Apr 19, 2014 6:58:21 PM

This is complete myth with overdrive and cruise control .
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forresj
Champion Author Wilmington

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Message Posted: Apr 19, 2014 3:33:12 PM

"....When on trips I check MPG with every tank of gas. I get the same MPG when driving at 65 MPH that I do driving at 80 MPH. Once the car is moving the weight will push the car down the road and get better mileage...."

Ridiculous. This will only apply if we live in a vaccum in space.

[Edited by: forresj at 4/19/2014 3:33:56 PM EST]
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forresj
Champion Author Wilmington

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Message Posted: Apr 19, 2014 3:30:55 PM

Every vehicle has a sweet spot. It greatly depends on the gear ratio. What may be true with one car is not the same with another. One car is better streamlined than the other. Find your sweet spot. So yes, genernally if you go faster beyond your sweet spot, you'll start to see poor fuel economy.

I usually drive just below 2000 rpms.

[Edited by: forresj at 4/19/2014 3:35:24 PM EST]
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randy3116
All-Star Author Tennessee

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Message Posted: Apr 16, 2014 5:35:29 AM

unless i excessively speed; my overall ampg remains in the mid-30s
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Houckster
Champion Author Atlanta

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Message Posted: Apr 16, 2014 2:17:43 AM

QUICKIE68 writes: When on trips I check MPG with every tank of gas. I get the same MPG when driving at 65 MPH that I do driving at 80 MPH. Once the car is moving the weight will push the car down the road and get better mileage.
_________
QUICKIE68 has discovered a whole new law of physics.

THE REALITY: The faster a car goes, the more resistance it will meet. THERE IS NO WAY AROUND THIS. The more resistance the car encounters, the harder the engine must work and the more fuel it will consume. THERE IS NO WAY AROUND THIS.

[Edited by: Houckster at 4/16/2014 2:20:34 AM EST]
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pghbill
All-Star Author Pittsburgh

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Message Posted: Apr 16, 2014 12:34:32 AM

forget the 55
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dontuknowOH
Champion Author Ohio

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Message Posted: Apr 15, 2014 11:14:35 PM

Our vehicles get the better mileage at (different for each) speeds when cruising, the engine RPMs are different in top gear, and many other variances in design makes for different economy cruise speeds.
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Quick68
Champion Author Columbus

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Message Posted: Apr 15, 2014 9:08:58 PM

When on trips I check MPG with every tank of gas. I get the same MPG when driving at 65 MPH that I do driving at 80 MPH. Once the car is moving the weight will push the car down the road and get better mileage.
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PatAZ
Champion Author Tucson

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Message Posted: Apr 15, 2014 8:55:29 PM

I go what the speed limit is and don't worry about it.
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cDagasGo
Champion Author Maryland

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Message Posted: Apr 14, 2014 6:35:32 PM

Over 55 seems to get better mileage.
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deant99
Champion Author Ontario

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Message Posted: Apr 13, 2014 9:17:15 PM

65 for me but cruise helps either, except on steep inclines
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dgsteven
Champion Author Los Angeles

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Message Posted: Dec 30, 2013 3:55:37 AM

I prefer 65 mph
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GBMAX
Champion Author Phoenix

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Message Posted: Dec 29, 2013 7:32:56 PM

65 seems best for me
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BartandLisa
Champion Author Newfoundland

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Message Posted: Dec 29, 2013 5:46:32 PM

Depends on the vehicle's engine more than the speed itself.
My car revs at 2500 when cruising at 75mph, while my father's is barely 2000 rpm. At 55 mph, fuel mileage is no better.
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Floridaman2013
Champion Author Atlanta

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Message Posted: Dec 29, 2013 8:51:57 AM

Most automatic transmissions today are 6-9 speeds and with OD. Engines are very fuel efficient so 55 is not an ideal speed for today's vehicles. As other people have pointed out here, there way too many variables that can affect fuel mileage today but going 55 is the least for the newer cars.
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the1roadhog
Champion Author Atlanta

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Message Posted: Dec 29, 2013 8:33:54 AM

Good luck in getting the folk to do 55mph
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cv
Champion Author Raleigh

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Message Posted: Dec 29, 2013 7:12:00 AM

2000 rpms seem to be optimum for my truck and that is 65mph.
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hyeglenn
Champion Author Fresno

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Message Posted: Dec 28, 2013 10:25:28 AM

65 plus.
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Vin63
Champion Author San Bernardino

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Message Posted: Dec 27, 2013 10:22:09 AM

My Duramax is right around 1700 RPM - 65 MPH; the Audi A3 TDI is 1900 RPM - 64 MPH.
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SmoknFord
Champion Author British Columbia

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Message Posted: Dec 26, 2013 11:53:23 AM

Every vehicle has a 'sweet spot', mine is just under 100 kmh. Anything above that then mileage starts to suffer. But... at that speed it makes for boring driving ;)
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the1roadhog
Champion Author Atlanta

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Message Posted: Dec 22, 2013 7:31:16 AM

In '74 it was 55. Don't care to go back to those days.
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kennyman
Champion Author Alberta

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Message Posted: Dec 22, 2013 2:01:33 AM

I don't care. I like the 70MPH speed limit.
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GrumpyCat
Champion Author Alabama

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Message Posted: Dec 22, 2013 1:03:08 AM

Believe my 2000 Avalon got best MPG at 67 MPH. Always got 30+ at 67 or faster. never got more than 27 MPG at 60 MPH.

My 2009 ML320 Bluetec got 31 MPG at 60 MPH over 1000 mile average. But at 70 MPH it gets 29 MPG. Not worth the savings.
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OceanArcher
Champion Author Mississippi

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Message Posted: Dec 21, 2013 11:31:55 AM

Find the best speed for your car, and deal with it ....
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Deno
Champion Author Pittsburgh

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Message Posted: Dec 19, 2013 10:13:54 AM

My old Chevy (05 Impala) gets its best MPG at 62-67 mph, regardless of terrain.
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dontuknowOH
Champion Author Ohio

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Message Posted: Dec 17, 2013 8:32:10 AM

At the time I owned an older car which had a V/8 engine with distributor type older Ign. system. I could attach a timing light and receive the info of knowing at what cruise RPMs the engines timing was advanced the most degrees, rather the light would show the engine's set advance low end RPM timing curve.

The curve changes with varying heavy engine loads on all vehicles old/modern to prevent pinging/knocks/harmful piston/ring/valve damage (plural).

Low end tuning accounts also for pep and fuel consumption, most is done by the ECM, but older engines could be bumped by a good mechanic for each driver habits thus gaining an edge for economy or performance. Emission clamps are in effect today making the engines follow a planned course.
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mojo803
All-Star Author Columbia

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Message Posted: Dec 17, 2013 12:45:02 AM

1600-1800rpm at whatever speed
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nonsuvdriver
All-Star Author Toronto

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Message Posted: Dec 16, 2013 1:51:28 PM

Little better mileage @ 55 but that speed is too slow these days
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randy3116
All-Star Author Tennessee

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Message Posted: Dec 16, 2013 11:23:25 AM

my car averages aout 5% better mpg at 65 than 55
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BobP1950
Sophomore Author Chicago

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Message Posted: Dec 16, 2013 9:38:20 AM

I have a 2009 Ford Focus that i swear gets better mileage at 65-70 mph with the cruse control engaged. Upwards of 35-38mpg.
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ggg452
Champion Author Manitoba

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Message Posted: Dec 16, 2013 9:06:56 AM

Driving slower is not only cheaper but also safer...what's the hurry?
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mincemeat26
Champion Author Atlanta

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Message Posted: Dec 16, 2013 8:50:48 AM

hi
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OceanArcher
Champion Author Mississippi

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Message Posted: Dec 16, 2013 8:47:15 AM

General traffic flow dictates my speed on the highway - except in Texas ...
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Dale Jr.
Champion Author Illinois

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Message Posted: Dec 16, 2013 8:43:35 AM

Wow. I believe everyone here knows the faster you go the less mpg's your vehicle gets.
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twt
Champion Author Virginia Beach

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Message Posted: Dec 16, 2013 6:39:45 AM

What a Nimrod.
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MertieMan
Champion Author Lexington

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Message Posted: Dec 16, 2013 5:17:20 AM

Only a rookie could have thought this up, and the answer is a vehement NO.
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pawnkingfour
Champion Author Georgia

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Message Posted: Dec 15, 2013 10:16:24 PM

I found that to be true, and driving between 45 to no more than 55MPH gives me more mileage per gallon of gasoline.
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