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Sudden MPG drop

3K views 36 replies 8 participants last post by  bobgg 
Regarding MPG/KPL, my 01 CVT used to clock in over 50 most of the time with: A) careful but not pedantically so driving working for me; B) the topography of my landscape is pretty hilly which is a mixed bag; C) my IMA was weak, I had no belly pan, and didn't monitor my tire pressures carefully.

When I had 2 different people use the car for many months at a time, driving it without any significant understanding or concern for the details of how to maximize efficiency, the MPG tended to range from about 43 to 48 per tank. I personally occasionally slipped down into that range, usually when hurrying (maintaining high average speeds) on long trips that included some significant hill-climbing. This

While I understand the principle of higher tire pressure being good for MPG, my own experience has been that above some reasonable number (depends on the design of the tires and the weight of the car), the returns are diminishing. In my insight, I tended to run in the high 30s and didn't see any measureable increase when I pushed it over 40. When I might have let 1 or more tires drop down into the low 20s, I do believe I noticed.

I live in the mountains of the southern end of the Rockies (NM) where snow and ice have become fairly intermittent over the last two decades, but am still fairly aware of the need for decent traction when those conditions emerge and do find that higher pressures (over mid-30s) on that care yielded some noticeable loss of traction. I have a particularly icy, shaded, windey hill which on occasion becomes wicked-treacherous and the smallest difference can matter... I have deliberately dropped the pressure in my insight to about 20 to be able to navigate those conditions with a little more control and it the extra softness and traction area definitely helped!

My Chevy Volt is quite a bit heavier and I find the difference both in MPG and in traction to be less dependent on tire inflation, but then I am running Bridgestone Run-Flats which may not suffer is much with lower pressure given their very stiff sidewalls? The Volt has much more precise instantaneous instrumentation... especially with My Green Volt monitoring the current draw and speed (and therefore miles/kWh) and I find that under similar conditions/terrain that driving style significantly dominates mpg, including acceleration/deceleration and speeds. Some of my best efficiency actually comes from climbing a fairly steep 9 mile hill from my home at 40-50 mph, driving a few miles around a fairly flat mesa-top townscape at under 25 mph and then back home (9 more miles with a 2000 ft drop) at 40-60mph.

When I commuted the same trip in my Insight, I didn't do quite as well, probably because the downhill run was pretty much 15 minutes of "idle" without a big enough battery to return more than a little of the energy from the climb through regenerative braking. Occasionally I would "hypermile" this route, but the power-steering/brakes without engine were problematic for comfort/safety. I suspect I could have done a great deal better with an MT on these routes.

Previous to my Insight I hypermiled/coasted my 84 CRX-HF the same commute up to 70mpg per tank. I don't think I ever broke 65mpg on a tank with the insight, but beat the CRX on all other routes.
 
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I don't record these (outside my faulty anecdotal memory) but a reasonable estimate is that I probably arrive home with about 5kWh/10kWh useable from a nominal 25mile RT (including some "around town". Under the best conditions, the climb consumes more than that, I regen about 1kWh on the return... so about .5kWh/mile...

The run from my home to my (former) workplace and/or the downtown location of the small mountain town is nominally about 11 miles with the first and last mile being only slightly uphill. I hit the top of the hill with more than the 40% retained by mountain mode, showing 4 or 5 (of 10) bars on the dash... depending on circumstances, I may or may not trigger mountain mode before heading back downhill...

That first/last mile on the mesa-top is too flat to coast the whole way back down, but the consumption is very limited for that short run... sometimes that is when mountain mode kicks in if I allow it and then the minimum .10 gallon gets burned... but I avoid that naturally.

Your question has me wanting to track those three (six) legs more specifically. Unfortunately here in mid-winter I never (as of this year) get a clean run without triggering the ICE in some combination of 1) almost immediately; 2) early in the climb; 3) the duration of the climb; If I (or the cold-mode) run the ICE the whole climb, it still slips behind with some high RPMs on the steep parts.

I'm looking into the battery-heating system to see if that is failing (intermittently) because the cell-by-cell performance monitored during and after use/charge doesn't show any obvious issues.
 
My experience for over a decade of checking this nearly every fill up was that it was very accurate, I chalked up discrepancies mostly to differences on just how/when I cut off the pump...

Even though it has been decades since it made sense (to me) to "top up the tank to the next round number" as my father always did "back in the day" I still find myself dropping in another fraction of a liter (.07?) now and then. I'd guess that the fill-tube is likely to hold a larger fraction of a liter if you push it.

The pump auto-cutoff sensors are likely to vary from pump to pump, even time to time?

I'm very impressed with the ability of modern computer-controlled vehicles to measure fuel consumption on the fly so accurately... it makes sense when you think of how precise the fuel-delivery via computer-injection needs to be.

Glad you are enjoying your Insight. I'm a little jealous... having flashbacks to my first year of ownership!
 
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