Optimal speed for best mpg??

When I fill up with gas and drive around for maybe 100 miles I get some 51-52 mpg. But as I continue to drive identically the same as I started the mpg starts dropping and goes down to about 45. Why?

 

My best mileage to date. I have 3800 miles on my 05 Prius. Took this photo yesterday in Key West, Fl. and filled my tank in Miami,Fl. My ave. mpg for Ft.Myers to Key West,Fl and return is 56.6 mpg at 70 Mph on the interstate and the speed limit from Fl. City to Key West & return.

 

Does the grade change, even a tiny bit? The Prius is extraordinarily sensitive to gradient - roads I used to think were flat actually aren't, and it's the difference between 45 and 55 mpg.

 

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Assuming, for the sake of argument, that 45 mph is "THE" sweet spot, there is a fairly simple relationship between increase of speed and resulting increase of drag.

The drag increases as the square of the velocity. If 45 mph is the starting point, then a 10% increase (4.5 mph) results in a 21% drag rise. That is (1.1)^2 = 1.21 (hence 21%). Similarly, a 20% velocity increase results in a 44% drag rise, and so it goes. Rounding to the nearest 5 mph speed, one can use the following table to visualize the approximate effect of speed increase above a chosen "sweet spot", being 45 mph here

45 mph = sweet spot
50 mph approx 23% drag rise
55 mph approx 49% " "
60 mph " 77% " "
65 mph " 109% " "
70 mph " 142% " "

Above applies based on airspeed, not wheel speed. If driving 50 mph into a 20 mph headwind, the drag rise is 96% over 50 mph road speed. And so on.

Finally, as road speed increases, there is a non-linear rise of wheel and mechanical resistance. Although not anywhere near as large as aerodynamic effects, it is part of the debt payment which is withdrawn from the fuel tank.

If all other variables are fixed, then the relationship between velocity a fuel efficiency is straight forward. If there is consensus regarding the velocity for maximum fuel efficiency, then any greater velocity must reduce the mpg (Assuming the speed is greater than that for which the ICE must run).


OTOH, maybe not. :roll:

Denny

 

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What "calculas" is that? Wind resistance goes up as speed squared (the kinetic energy of the wind you're pushing out of the way). When moving at constant speed, you're just fighting against wind resistance, friction of the tires on the road, and internal friction of bearings etc, with wind resistance playing an increasing role at increasing speed. Faster gives poorer mileage, so it's just a tradeoff between economy and actually getting someplace.

Because the Prius doesn't have a set of fixed gears there should be no particular gearing "sweet spots." The engine has an efficiency curve (you get more power per fuel mass at moderately high power output) but the HSD computer tries to optimize that as much as possible.

 

sorry my sweet spot is more like 25 mph

 

I drive in the I-15 south traffic to San Diego each morning. The speeds are about 10-30 mph. I thought low speeds were supposed to be gas efficient but I am only getting about 41 mph.

Is there any driving technique to getting the most out of my mpg at these low speeds? Try and stay at a constant speed, or ??

 

ya keep foot off brake.

every time you hit brake it kills mileage. stop and go traffic is the acid test for economical driving.

i frequently get the same kind of driving conditions and ya sometimes im getting 75 mpg and other times its 45 mpg. it all depends on how well you can anticipate. but at least its 45 mpg. think of what a regular car would get and you WILL SMILE

 

<div class='quotetop'>QUOTE(tim_in_jax\";p=\"81718)</div>

Actually, the reason behind the logis is that most people will gradually speed up as they get comfortable. This of course makes fuel mileage drop, and cruise control will beat "manual" speed control in this manner. However, if you are actively working to maximize fuel mileage, then it's quite easy to beat the cruise.

 

Fuel Ecomony

Thanks everyone for the driving tips. Looks like I will have to beat to a different drummer, minimal breaking, in heavy traffic.

 

we have a 95 prius, with 15,000 miles. both my wife and i get only about 35mpg around town. is that normal? certainly doesnt ring true with epa numbers. we get 49-50 on the freeways, which seems fine to me.
i keep wondering whats up with the "around town" mileage. anyone have any thoughs?

 

I'm averaging between 38 & 41 mpg (3,500 miles on the care). My driving on a average day is 13 miles freeway 13 miles local. The point of this post is, being in L.A., I usuallly drive with the driver's window open and the passanger window about 1/3 way down (not a great fan of A/C). How much do you think the open windows is effecting my mileage?

 

phil: at freeway speeds having the windows open will lower your mileage. how much, its hard to say. at street speeds the effect will be minimal. i also like to ride with the windows open at speeds of 40 mph or less. faster than that i rather have the windows closed because of other cars on the road making too much noise.

running the air conditioning reduced my mileage about 1-3 mpg depending on how hot it was. most here feel that having the windows open will lower the mileage more than that. i dont think that is true and it if is, i doubt that the difference is much. but lets face it, air conditioning feels better, is much quieter, and with the cabin air being filtered, probably more healthy for you