Prius MPG Test!

[Wayne]

I recently did some fuel efficiency and speedometer accuracy tests on my Prius, and wanted to share the results. I've attached a detailed explanation of my tests for anyone who might be interested, but consider this an "executive summary".

Fuel Efficiency vs Speed: First, I wondered just how much of a hit you take in fuel efficiency the faster you drive. As with ALL cars, the faster you drive, the lower your MPG will go. However, since the Prius has significant advantages over a normal car at slower speeds, you will see a wider swing in MPG as you increase your speed. Still, at the fastest, a Prius is still more fuel efficient than a normal car, so you can speed and still save money.

All that said, I'm still a big fan of treating the speed limit as a "limit". As I did the higher side of these tests, I found myself getting caught up in the competitiveness of speed, and found myself yelling at people who got in my way. I forgot how relaxing it was to be passed, instead of working to pass others!

"Very generally speaking", you will get roughly 1 MPG of change for each MPH you drive faster or slower than normal. If THAT doesn't encourage you to drive slower, nothing will.

Remember to check out the attachment for my specific methodology, but here are the results in a nutshell: You get about 5 MPG better mileage by driving 65 instead of 70. You can get a whopping 13 extra MPG by driving 55! Of course if that isn't the truck speed in your state, that's too slow. You will lose at least 4 MPG by driving 75 instead of 70. And you will lose a whopping 12 MPG by driving only 10 miles per hour over at 80. Of course getting almost 40 MPG at 80 still ain't bad, but you can do so much better!

My tests, my conditions (sorry about the formatting):

MPH MPG CHANGE % CHANGE
55 63.1 13.2 26%
65 54.8 4.8 10%
70 50.0 ref ref
75 45.7 - 4.3 -9%
80 38.1 -11.9 -24%

Note I found almost a 17 MPG variation between 65 and 80. Wow. Go 65!!!

Air Conditioner Hit: The A/C is electric, so there is no immediate drag on the engine. However, the energy pulled from the batteries has to be replaced, which means the car has to stop using the electric engine for part of the torque and use it instead as a generator to recharge the batteries. I found little difference in MPH on the leg out, but -4.6 MPG on the leg back. The round trip average between the two tests was between -2.4 and -2.9 MPG lower, so expect a hit of a few MPG when you use your A/C.

Speedometer Tests: I was going to list these results in a separate thread, but I see little need. The results were good, and as expected. Accuracy varied more by what precise speed I chose to engage cruise than anything else. There were the usual variations due to hills and valleys, but generally speaking the speedometer was always within 1 digit of the actual speed, favoring the higher side, as the US mandates.

Attached are all the specifics. You will need Excel. Tabs at the bottom of the page will lead you through the file. The chart will be the first thing up. For those who are Excel-challenged, I've also provided the chart in GIF format. Enjoy!!

[Wayne]

Wow -- an hour later and it gets bumped off the first page with only 35 reads! I would have thought this info was of MUCH more interest. Oh well...

[roach52osu]

Originally posted by Wayne@Oct 23 2005, 06:11 PM
Wow -- an hour later and it gets bumped off the first page with only 35 reads!  I would have thought this info was of MUCH more interest.  Oh well... 

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I thought this was very interesting, to see the real numbers.

Although I am still waiting for my Prius but around here most of my driving will consist of about 8 miles of city and 25 miles of freeway per day with averages on the freeway of about 60 (65 mph speed limit 55 trucks and construction, all the time). I will however take some trips that take me from 65 to 70-75mph limits/driving. I wondered how these speeds would impact the mileage but it doesnt seem so bad.

My parents just made the trip I will make from time to time of about 900 miles each way and limits from 65-75 and they averaged about 45 mpg on a brand new prius on the way down (ohio to northern florida). I am impressed with this and happy to see your results are similar to what I was hoping for. Now if I can only get my gas guzler sold soon so I am ready for my new Prius...

Thanks for the info Wayne

[NuShrike]

Change your board preferences to list more articles per page.

Interesting results.. I've tried the same before for myself on the way home from work (12am-ish), with not much cars around, and I've found strange results with mine where sometimes I got better MPG at > 121Km/h than at 118Km/h or lower. It may have been the terrain, but it's difficult to find a nice predictable track of freeway to run around in SoCal.

[altaskier]

Thanks for taking the time to make these careful measurements!

I would have expected fuel use to scale with the kinetic energy of air displaced, at least at the high velocity end. Therefore it's of interest to re-plot your data as fuel used as a function of velocity squared. Excluding the 55 mph data, the fit is OK...[attachmentid=693]

[tomdeimos]

Results look pretty reasonable for my car at around 80 degrees, except for some reason I get less at 55 mph. I don't see temperature listed, but the effect is big.

My general rule is 55 in winter = mpg of 70 in summer, this from reading someones graph posted a year or so ago. Foget what temps I looked at but I think I figured summer at 80 degrees and winter at 10 or so.

I also notice a jump up in mpg when it rains light so the air is humid but roads aren't wet enough to add drag.

[hdrygas]

Very nice data. I only get up to highway speeds occasionally, but I am driving a bit slower now. The limiting rate is making people around me crazy!

[Wayne]

Originally posted by tomdeimos@Oct 23 2005, 08:00 PM
I don't see temperature listed

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The temperature was listed in the methodology section of the spreadsheet. It was 68-73 degrees.

[tumbleweed]

Wayne, this is good information, I'm glad you took the time to do this. I actually was hoping it would show no great penalty for speed because sometimes I drive to fast, but I'm not surprised at the results. I ordered a spoiler from Brian at BT Tech and plan on doing some before and after milage tests when it gets here.

[tullynoon]

Cool stuff. I'm going to give it a try too at temps from 80 down to 30 degrees and compare my data. Also, thanks for the spreadsheet.