I've finally found the secret to increase mpg

[wintechsw]

Background: 1. My experience is 31000km in a base II model with Lifetime Average of 20.4 km/litre or 57.5 mpg(Imperial) in Oz, mainly long distance driving at 110kph speeds on Cruise Control uphill downdale. That's 48mpg for US measuring. 2. I have a ScanGuage II installed and have one of 4 settings to display LOD which is the engine loading. My observations are that the consumption falls dramatically when the LOD is 79 or greater. So my key to Economical motoring is to keep the LOD figure below 79 at the maximum speedo reading. 3. I went for a 330km trip yesterday over fairly flat terrain, sat on 110kph(113 on the speedo) and averaged 5.2l/100km. On my return trip, after refuelling, I set my speedo at 106(actual 104kph) and the consumption average dropped to 4.7l/100km. The LOD stayed below 79 for most of the way, occassionally going over that figure for uphill gradients. 4. Coincidentally the displayed engine RPM stayed below 2000rpm whilst the LOD was below 79(not a surprising fact), but it still fluctuated wildly for level running. At 110kph, the RPM varies greatly from 2100rpm to 2500 rpm at every SGII refresh.

My maximum freeway speed will now be set at Cruise Control 106kph.

[krousdb]

Originally Posted by wintechsw

My maximum freeway speed will now be set at Cruise Control 106kph.

What you have discovered is that aerodynamic drag increases exponentially as your speed increases. At 106kph, you have not yet found the sweet spot. Try 88kph. You will be amazed.

[spinkao]

Krousdb is right. You are also correct that the engine is most efficient when working under certain load; Try to read this very interesting article by Hobbit:

Prius throttle control

If 88km/h is sufficient for you, you will get amazing results at that speed (assuming flat ground, no wind). Your consumption at that speed can get as low as about 3.2-3.5l/100km average, sometimes even lower. I regularly manage to repeat that feat when traveling some 55km at that speed on the highway from my apartment to my parents' house. I also try to travel behind a large truck if possible, in order to try to reduce air resistance. Of course, I always keep a safe distance behind him, which compromises the efficiency a lot, but safety should always be at the first place.

Using this technique and well-known hypermiling tricks in the city, I regularly get 4.2l/100km per tank, including short commuting trips through the city, which are a real consumption killer. You can get significantly better than that if you can avoid short trips, which I, alas, cannot.

[wintechsw]

Originally Posted by krousdb

What you have discovered is that aerodynamic drag increases exponentially as your speed increases. At 106kph, you have not yet found the sweet spot. Try 88kph. You will be amazed.

Yes I agree the consumption mpg increases with reduced speed, but the tradeoff is the time difference for the trip which is from 7 mins to 45 mins of increased travelling time.

[krousdb]

Originally Posted by wintechsw

Yes I agree the consumption mpg increases with reduced speed, but the tradeoff is the time difference for the trip which is from 7 mins to 45 mins of increased travelling time.

Agreed. We all have to make that decision. I would save 20 minutes a day if I increased my speed to 106kph. But saving 20 minutes does not mean that I will earn more money. It just means that I have some spare time on my hands. My choice is to give up some spare time so that I can spend less of my hard earned money supporting OPEC. It works for me.

[JimStPete]

Originally Posted by wintechsw

Yes I agree the consumption mpg increases with reduced speed, but the tradeoff is the time difference for the trip which is from 7 mins to 45 mins of increased travelling time.

[donee]

Hi wintechsw,

You have rediscovered SHM - Super Highway Mode. Do a search on SHM here, and on CleanMPG.com for complete details.

[richardp]

What's a ScanGuage II ?

[direstraits71]

Originally Posted by richardp

What's a ScanGuage II ?

ScanGaugeII - Trip Computers + Digital Gauges + Scan Tool

Aftermarket instrumentation that allows you to see additional information about the performance of you Prius. Search "scanguage" on this forum for a flood of information.

[Swanny]

The biggest difference I've seen in the last few tanks of gas is due to the fact that I've held the accelerator steady on the highway, don't use cruise, let the car speed up and slow down as appropriate (I keep it at 5.0 or lower, thus speed is around 100 km/h).

I saw an increase in economy from 5.4 - 5.7 down to 4.8 - 5.1 L/100km. Give that some thought, just focus on keeping your consumption on the highway below 5.0 and don't worry as much about the speed.