The Insight Theory of Relativity (5spd):

[dan]

I’ve been working on this theory since I posed the aerodynamic question a while back and got such a wide variety of answers. Briefly stated: Any modification that affects any property that will affect MPG will disproportionately affect those striving for high MPG more than those who don’t.

I am not talking about a corollary to Murphy’s Law, or some perceptual illusion arising from staring at little LEDs or traveling at mind numbing speeds.

Rationale: One’s ability to stay in lean burn seems to be biggest factor in determining whether one averages MPG in the 60s or the 80s. Anything that keeps one out of that “window” hits those folks that try to spend more time in it more than those that don’t. Examples: a/c use, tire swaps, hills, rain, warm air mods, opening windows, pizza delivery billboards, etc.

Implications: Observations about the effects of modifications are only applicable to those of similar driving style. Example: someone who drives around at 65-70 MPH may notice a 3% drop in MPG with a new type of tires, while someone else who keeps things down in the 55MPH range might suffer a 10% drop in MPG with the same tires, all other things being equal.

I imagine that many of you already get this concept. I bring it up because it took me a while to get, and judging from some of the discussions it is not always self-evident to everyone right off the bat.

I invite your comments and observations, after all this is only a theory.

[figgy]

I agree with that.

It's the finesse at the bleeding edge to get the stratospheric mpg results.

I see the same kind of thing if I have nursed it incredibly carefully for my trip to town, and then have to turn around to park or do a 3-way turn and watch my trip mpg slip away into the void...

[Holicow]

Interesting, and true for the most part.

I think there is a peak of efficiency at 45-55 mph, and most effects will hit those who travel at higher speeds more than the slower speeds. Things like: any aerodynamic drag, wider tires, tire pressure, etc.

[dan]

Holicow-

That is true for all vehicles (including the Insight), but I am trying to illustrate an opposite tendency in the Insights.

What I am trying to say is that if you are routinely traveling at higher speeds you have already given up most of your lean burn window, and will not be as adversely affected by small compromises (increasing drag, etc.) as someone who really tries to stay in there as much as possible, and will be kicked out of it more frequently.

[dan]

Figgy-

I had to think about your example a little bit more. Your example (I think) is an illustration of how a great MPG can be quickly diluted by a very low MPG. What I am proposing a slightly different mechanism.

My theory is completely based on the ability to stay in the lean burn window. I may not be explaining it clearly, so if is not clear keep challenging me so I can clear up misconceptions. (If my theory is valid!). Maybe someone has a better way of explaining it...

Hi Dan:

___When you brought up the lean burn window and higher loss of fuel economy in the RFG thread for those pushing that aspect of our Insight’s last week, I thought you were on to something. To me, the Insight is all about its/my ability to maximize fuel economy, whereas to many others, it is about a fun scoot around town or a totally unique automobile along with the sensual experiences that go along with it more then likely.

___I am sure if you tagged Rick Reese’s, Chisight’s, Billy’s, and or my ideas as to what works and what doesn’t for fuel economy, performance, and or comfort it would be somewhat different then what Holicow, Sparky5501, Guillermo, Willie, or Hambone would offer on the same subject.

___As you have observed and observed correctly I might add, the tire threads as just one example drive me nuts because I see a loss of 10% and probably and I cannot fathom how anyone could accept that large a hit no matter if the tires levitated the Insight with 0 road noise, total road isolation, and had perfect traction in all-driving conditions. I also see how those on the other side of the fence considering driving 50 - 55 mph for hours on end w/ 50 + #’s in the RE92’s and installing/removing warm air mod’s/intakes would be absolutely livid of that pace all for the sake of ones fuel economy …

___If you only knew some of the interesting techniques I use for that extra .2 to .3 mpg per commute

___Fortunately for all of us, the Insight is such a unique offering that can and does satisfy both camps wants and needs in a single package.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email:12fnc4mu]Waynegerdes@earthlink.net[/email:12fnc4mu]

[Yves M.]

Hi Dan,

I think that you are absolutly right.

I have an other theory for you:
The OEM tires are the ONLY Perfect Tires for the Insight's best MPG. Any other tire will not have the similar MPG. Here is why:

a) The tire size:
165/65R14 tires have the rim wider than the tire. Put the tire on the ground face down and the tire is on the rim not on the tire like any other car. The tire can be thought as being "square"
The sidewall is about inline with the rim and when you hit a bump, the tire can not deform because it would change the tire form and would create more tire air pressure. So the tire will follow (up/down) the road surface instead of deforming to the road surface (creates a harsh ride) for lower rolling resistance.

I have 185/60 and those tires are more round, when I put them on the ground face down, they are on the tire not the rim. Like any other car's tires. They have a softer ride even with more pressure than the oem since the tire being not square but round, it can deform without increasing much the pressure. It has more room the deform. More deformation means more resistance to roll.

b) The tire tread depth.
According to tirerack.com the new OEM depth is 9/32 inch . Less than any other tires of the same brand (10/32 or 11/32). The tires I bought have a depth of 12/32. So more tread depth means more thick rubber to flex and more resistance.

c) Off course the tire air pressure.
It needs to be higher than the generic 32 psi. From 35 to 44, it gives less tire deformation while driving so it has less resistance. The tires needs to have a 44 psi cold rating.

ADDED, forgot:
d) The sidewall thichness. My 185/60 are about twice as thick and feels harder. The OEM are slim and soft, easier roll resist.[/img]

[dan]

Oh how I wish we could post simple graphs and diagrams without resorting to bandwidth gobbling jpeg…

Hi Yves-

Your tire theory “a” is an interesting one that I had never considered. I don’t know enough tire physics to evaluate it, but (I think) I understand what you mean. I wonder, do Priuses have the same problem with scuffed rims as Insights do? In other words; are their tires “square” like ours?

[Holicow]

Hey dan,

I don't disagree and I think we are saying the same thing. I just point out that the MPG's for those who generally drive above the speed for max efficiency will be more affected by alterations than those who generally drive at lower speeds (city vs. freeway).

Most "drag" sources will have increasing effect with speed.

Or maybe I am way off. One or the other...

[dan]

Hi Holicow-

I’m not disagreeing, but I am not sure we are saying the same thing;
“MPG's for those who generally drive above the speed for max efficiency will be more affected by alterations than those who generally drive at lower speeds…Most "drag" sources will have increasing effect with speed.”

Yes, I agree, this is true for all cars. Particularly aerodynamic drag that increases exponentially (as opposed to rolling resistance [which I believe increases linearly {?}]).

I am trying to describe an effect unique to lean burn. “MPGs for those who generally spend as much time as possible in lean burn mode (The hypermilagers; those that generally drive at lower speeds*) may be disproportionately influenced by factors that affect their ability to remain in lean burn mode. In some cases this effect may be greater than the rule you site.

Consider hypothetical: **

Able: 83.8lmpg / Av. Highway Speed: 56mph / % Highway Driving: 99 / Ave speed: 55.8 mph / % of time in lean burn mode~80(?)
Baker 55.6lmpg / Av. Highway Speed: 72mph / % Highway Driving: 70 / Ave speed: 37.4 mph/ % of time in lean burn mode~3(?)

They both buy new tires with 1% increase in rolling resistance…They do this because they value less tram-lining over extreme mpg.

New observations: (Trip A)

Able: Trip A: mpg: 80.46 (4% decrease) All other factors the same except: Av. Time in lean burn mode: ~75% due to inhibited ability to remain in “lean burn window” related to 1% increase in rolling resistance of tires.
Baker: Trip A mpg: 55.5 (1% decrease) All other factors the same (He is still in lean burn mode ~3% of the time he suffers only the rolling resistance factor related decrease in mpg).

Able was effected disproportionately (3% more) by the change than Baker.

They both live in Billings, Montana winter comes…

Able’s Trip A: 74mpg (9% decrease)
Baker’s Trip A: 52 mpg (4% decrease)

Able takes a bigger hit than Baker related to inability to spend as much time in lean burn window than usual.

They both do warm air mods…

Able gets a larger benefit than Baker because it increases the time he spends in lean burn mode.

OK, I think the horse is dead now. I’ll shut up. Are we saying the same thing?

*I do not mean to imply that those who attain fantastic MPGs do so exclusively by driving slowly!

**These are all hypothetical numbers. I have no hard facts to back them up. I am proposing them to illustrate my theory.