Speed to achieve optimum mileage - What is too slow?

Cars are tested at 50MPH for highway fuel mileage because that is where they are supposed to be the most fuel efficient

'Sright. After 50Mph/80Kmh wind resistance starts to make a difference.

The only common factor with your two cars though is the engine. Different mass, drag, frontal area, maybe diffeing traffic and road conditions and most importantly, different driver. Your car with you in it is the only thing that will determine what is best for your situation. Cheers

I posted it year ago:

hardware: -2002 Impreza 2.5TS, 90k miles -Yakima racks + 3 pairs of skis with poles mounted in the less aerodynamically efficient way possible -car loaded with 2 adults and 7 years old boy and all the gear for

route and time frame: -Chicago-Denver-Santa Fe-Springfield Missouri-Chicago (3400 miles) -December 25 2005 to January 1 2006

scenario #1: -Iowa -heavy rain -very strong front-side wind -car speed 75-85 mph -20 MPG

scenario #2: -Colorado -no wind, beautiful weather -car speed: 85-95 mph -22 MPG

scenario #3: -Santa Fe -frequent mountains trips -heavy acceleration on the mountains roads -relatively slow driving -23.5 MPG

scenario #4: -Texas -car speed: 80-85 mph -VERY strong tail wind -27MPG

scenario #5: -Missouri and Illinois -car speed: 65-70 mph -no wind -26.5 MPG

bottom line: -MPGwise: 70mpg and 80mph are totally different.

I believe the lowest speed that allows you to use the highest gear is the most efficient speed.

Maybe not quite that low (idle speed in highest gear is probably gonna 'lug' the engine and lead to overheating or other problems) depending on what you mean by "allows you to use". Plus, there is likely more passing power nearer the peak torque RPM. I think there are 'histograms' or other calculations to help with those determinations - but 10-20% below the peak torque point might be a good place to begin experimenting. Aerodynamic drag really begins kicking in around 50mph or so IIRC. Its' a square function so high speeds are always bad for efficiency. In the real world, probably best to keep up with the average speed of traffic using a gear 'proper' for that speed. Proper again somewhat subjective but likely to be halfway or more between idle and peak torque RPMs.

Carl

Not always. I have had a LOT of Toyotas, and have found for the 1.5-1.8 liter models, right around 70 MPH seems to be optimum (I always fill the tank, reset the trip meter and make a note of the mileage)

On a Corolla 1200, 70 MPH rendered 38 MPG On a Corolla 1.8, " " " 36 MPG On a Corolla 1.6 ("G" series, or 'performance' engine) 70 MPH rendered 34 MPG

In a Tercel with an AT, 70 MPH rendered 44 MPG! (and I sold it to buy a Supra that gets 21..but..Who Cares?!?!? ;)

In an '87 (new) Corolla 1.6 "F" series engine, 75 MPH with the AC on all the way rendered 46MPG!!!!

Meanwhile, my studoes at 50-55 (Remember the National Speed Limit?) rendered the WORST fuel economy in 5th gear, around 24 MPG!!!! Or, in other words, just what the EPA sticker said I would get...

Here's my rule of thumb: Under 60 MPH, leave it in 4th, or remove the OD on an AT. Over 60, use the highest gear, and hover between 60 and 70.

You have to remember, these are Japanese cars, the Japanese are very fuel concious, their National Speed Limit is 100 KPH (62 MPH), and NOBODY drives the speed limit, anywhere!

I have an automatic, unfortunately. Actually the EPA city mileage for the manual tranny in '97 was 1 MPG lower, and highway was the same, which was a good part of the reason for the choice.