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Optimal Speed for Optimal MPG?

12K views 20 replies 15 participants last post by  Aglie  
#1 ·
Have there been any tests or data found to see at what speed (or RPM) the Fiesta gets the best MPG? I found that between 60-70 MPH I can stay around 38 MPG (that's with some city driving, too), but anything higher than 70MPH on the freeway hurts. This is assuming that everyone is using E10 gas when performing these tests. Any input or ideas?
 
#3 ·
I can't speak for everyone but I found that at 66 mph on I30 from Little Rock I was showing 46.0 on the computer. That seemed to be the "sweet" spot for me. When the trip finished up with combination driving it came to an average of 39.6 mpg.
 
#4 · (Edited)
If you are really interested the best fuel economy, look into the clean mpg forum. They managed to pull 63 point something or other for 2600 miles. It involves some pretty radical changes to your driving though. Even if you don't use all of their techniques you might still take away some things that will make your driving more efficient and your mileage better.

If you decide to pursue it, try one new technique at a time so that you can see what it does and so that you get it down pat before you start on the next. Also, get an Ultra Gauge or Scan Gauge.

CleanMPG Reviews the 2011 Ford Fiesta - CleanMPG Forums
 
#5 ·
The best MPG you will get on the freeway is probably what the Minimum speed limit is.

On flat roads your best MPG will be when your at the lowest RPM in the highest gear. Not sure what speed that is in manual but in the automatic that is right around 40 MPH.

You can get 60 MPG or higher driving at that speed... but everyone will hate you!

When driving 55 on the local highways in Michigan I can get around 51-52 mpg (my trips are never long enough to keep my MPG that high though)
 
#21 ·
The 5-speed can maintain a steady-state at perhaps 40 mph if it is flat and straight. I think 60 mpg is probably possible under such ideal conditions. But you can get excellent (40+) gas mileage anywhere from 20 mph to 60 mph in appropriate gears if it is steady-state and flat. Hills and curves are not necessarily bad for gas mileage as long as you don't have to brake; but it is far harder to make sense of instantaneous gas mileage. Over 60 mph the 2011 5-speed's gas mileage starts to decline, such that it's hard to hit 40 mpg at 70 mph, and over 70 mph things get quickly worse. The 2012 has adjusted gear ratios and may do better (or worse).
 
#7 ·
But going to a wider tire adds more rolling resistance so it will not give you better mileage.

I have no problem getting 40+ mpg on the highway in my auto SES hatchback keeping it around 65 mph average. I keep my tires at 35 lbs.
 
#8 ·
Going to a taller,vs. wider tire/wheel combo should, in theory have the effect of increasing your gearing and thus your MPG. Of course YMMV.
 
#10 ·
A 205 tire isn't that much taller than a 195, it may drop my RPM,s
by say 100 to 200 at speed. That might get me an extra 1 mpg if
i am lucky. The width is probably not even measurable as far as
rolling resistance is concerned.

The speedo difference is probably or maybe 1 mph difference????
 
#12 ·
It extends your warranty.:D (for mileage at least) I plus sized my tires on my Mazda so the circumfrence was about 6% more. So, for every 100 miles actually driven, the odometer thought I had driven 94 miles.
 
#13 ·
Stock Tire - 195/50R16
Compared1 - 205/50R16
Section Width: 7.67 in 195 mm
Section Width: 8.07 in 205 mm
Rim Diameter: 16 in 406.4 mm
Rim Diameter: 16 in 406.4 mm
Rim Width Range: 5.5 - 7 in
Rim Width Range: 5.5 - 7.5 in
Overall Diameter: 23.67 in 601.21 mm
Overall Diameter: 24.07 in 611.37 mm
Sidewall Height: 3.83 in 97.282 mm
Sidewall Height: 4.03 in 102.36 mm
Radius: 11.83 in 300.48 mm
Radius: 12.03 in 305.56 mm
Circumference: 74.36 in 1888.7 mm
Circumference: 75.61 in 1920.4 mm
Revs per Mile: 878.7
Revs per Mile: 864.1
Actual Speed: 60 mph 100 km/h
Speedometer1: 59.0 mph 98.3 km/h
Speedometer Difference: - Speedometer Difference: 1.689% too slow
Diameter Difference: - Diameter Difference: 1.67%
 
#14 · (Edited)
I posted a little table to compare the tires but it is being moderated....

It boils done to about "1": RPM will drop 1%, speedo will drop 1 mph at 60 to 59, and Odo will read 1.67% off....all well within the tolerance limits of the instruments. In fact those who have noticed that their speedo is a bit optimistic would now read on their speedo a number closer to what the radar gun or gps claims. As for fuel mileage will a 1% RPM drop (about 30 RPM at speed) make any difference? I kinda doubt it. As of rolling resistance I imagine there are lots of variables there but I doubt the very minor size difference will account for much.

This is all comparing a 195 to a 205, so that is a 1cm difference in width and 5mm difference in sidewall height.

I suspect if you strictly used your instruments to compute mileage all these numbers are a) very small and b) would balance out. Of course real mileage has to be computed by real distance (which is hard to know without a well calibrated odo (this change could make it more or less accurate depending on where it started)) divided by real fuel consumed which for most of us requires some averaging over many fill ups using the pump number.
 
#16 ·
depends on how you measure it...if you are using real mileage and real fuel burned then the gauges don't play a part. If you use real fuel but rely on the gauges for distance and you see an increase well that is probably a real mpg increase as the odo will read lower miles as a result of the tire swap....

I do not believe the fuel consumption numbers in the car are even a real measurement but calculated so they will be affected by the other things like the odo and speedo being wrong.


All in all I doubt any changes that could be seen would be very meaningful with such small change in tires and the vagaries of measurement in both the pre change reference and the post change deviation. 1% is just too small to see unless you have a lot of data or very precise measurements. Actually I would maintain that there are so many variables that even a 10% change would be hard to be certain of with out a lot of data. People that see fractions of MPG over a couple fill ups are probably just deluding themselves. It is like dieters who think they have lost some fraction of a pound based on one or two scale readings.
 
#19 ·
I have been finding that 45 to 50 MPH has been the sweet spot for maximum fuel efficiency with a steady foot and coasting though when neccessary. Roads with speed limits of 45 and long streches of road with synchronized stoplights here and there every mile or so has worked great for me.
 
#20 ·
i live around wwwwwaaaayyyyy to many hills, but @ 60 mph on a straight away I can sustain 45mpg according to Ultra Gauge. 60 is probably the slowest I can go on i395 and i95 without getting shot at :-\