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-   -   ~10% jump in FE... summer fuel? (

Christ 04-20-2009 10:13 PM

~10% jump in FE... summer fuel?
I know it's too early to tell for sure, but I think I just got a FE boost by summer fuel...

I've been to a few diff stations lately, and the last two tanks (last one isn't in my log yet) have been 29+...

I feel, with my previous goal of 30 mippigs, that I've somehow "cheated" though...

There is a chance that I've gone up another 2 mippigs from the addition of LRR tires and overinflation, but the first 29.7 tank occured with a left rear tire at 28 and the rest at 35 or less... the rims/tires are almost the same weight as OEM, but I've been hauling 500-1500 lbs of crap around for almost every trip now.

I guess my question here is whether or not the increase of around 10% in my FE seems like a right figure for going from winter gas to summer gas, and if the timing is right?

Those LRR tires and the rim swap came right before the first high tank, so I'm not 100% sure whether they even made a difference... Darn the timing.


SVOboy 04-20-2009 10:14 PM

Is there still a difference between summer and winter fuel in your area? It might just be the good weather.

Christ 04-20-2009 10:20 PM

Jeez, that was quick...

I can't really say if there is a diff or not, I though it was a universally applied characteristic, although I never really saw less fuel economy in the winter with any of my previous cars... (always standard variation, never enough to discern a pattern).

I'm going to blame it on the tires, for now, just to say so. I can tell you this - at 50 PSI, those Michelin MXM4's are HARD. I never had the notorious minivan "clunk" in the front end, until I aired these up to 50 PSI... now I hear it once in awhile.

I just wonder if it's coincidence that I went from 27.7 to 29.5 in a single tank, w/ the LRR tires installed just after a fill up.

Either way, I'm still tentatively waiting on the 30MPG mark. When I can hit 30MPG reliably in the van, I'll start building the smaller engine/MVB trans for it, and consider the RWeD hybrid system as well.

tasdrouille 04-21-2009 07:13 AM

There is not much difference in the BTU content of gasoline winter blend and regular gas. AFAIK <5%. Diesel is an other story though, ~15%.

Nevyn 04-21-2009 08:32 AM

If you check my fuel log, there was a big jump when I put LRR's on the rear of my car and aired up - I bet it's a combo of the tires and the decent weather. All the rain (and a lot of travel loaded up) has pushed my tanks back down.

MetroMPG 04-21-2009 10:09 AM

I'll second Tasdrouille's point: likely too big a jump to account for by a change from winter blend. The difference in energy content between summer & winter gasoline is less than 2%, according to here.

Christ 04-21-2009 05:59 PM

So I'd be right in saying it was the rims and tires then? Would varying loads between 500 and 1500 lbs account for any fuel loss, or would that be negligible?

I'm still hoping to see 30+ soon. That will make it more viable based on fuel savings (over EPA) to start building the turbo 2.0 that I want. (1.8 if I can find/make the proper sleeves)

Christ 04-21-2009 06:36 PM

Another question, slightly OT for the thread:

Which has more effect, reducing frontal area, or reducing Cd by the same percentage?

Say we take a bullet, with a Cd of .24, and a frontal area of 50 ft^2.

In one test, we reduce the bullet to 45 ft^2, a total of 10% reduction in A, but Cd stays steady at .24

Now in the second test subject, we reduce the Cd by 10%, to .216, but leave the A the same.

Which reduction would have the most effect? My initial thought is reducing A having more effect comparatively, all other things equal.

NeilBlanchard 03-27-2010 04:50 PM

Bumping this older thread -- I think I got my first tank of "summer fuel"! The FE went up noticeably, and it is actually much cooler again, so we'll see what the results are after it gets warm...

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