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Old 03-08-2011, 01:50 PM   #21
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Default Re: MPG, Fuel Economy, - the definitive resource

Actually if you stay off the freeways you can plan a route shorter in distance but not necessarily shorter in time. Distance not time determines fuel savings.
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Old 03-08-2011, 04:33 PM   #22
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Default Re: MPG, Fuel Economy, - the definitive resource

So, in the rest of the forum, there have been threads discussing all sorts of things, and someone inevitably jumps in and asks about fuel economy issues, usually in some electronics upgrade or storage thread. Then folks start talking about fuel economy.

Now we have a whole section devoted to all considerations of fuel economy, and it all comes down to two things?
1) Drive slower
2) Shortest route

What about tire pressure, tire size, RPM range, fuel grade, extra weight, trailers, temperatures, humidity, air-dam under bumper, specific class B, specific motor, RV brand, Chevy-vs-ford,...? Maybe all those things are insignificant, compared to speed and distance.

If so, we now have three threads we can send folks to, when they start mucking up a good "Refrigerator Install" thread, asking fuel economy questions.

Any thoughts??
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Old 03-09-2011, 03:09 AM   #23
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Default Re: MPG, Fuel Economy, - the definitive resource

Somewhere on here I had a discussion about tire size, going larger diameter, to reduce rotations
per mile, and to try increase mileage, but I'm darned if I can remember where it is.
Feel free to resurrect it if you can locate it.
I sent some emails and did a bit of research, probably prior to last Spring when I got the new
set of standard sized tires. I believe the costs of switching to a slightly larger tire size wasn't
recoverable over a reasonable amount of distance and time. Nor was it recommended or safe.
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Old 03-09-2011, 06:38 PM   #24
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Default Re: MPG, Fuel Economy, - the definitive resource

Quote:
Originally Posted by Mike
Somewhere on here I had a discussion about tire size, going larger diameter, to reduce rotations
per mile, and to try increase mileage, but I'm darned if I can remember where it is.
Feel free to resurrect it if you can locate it.
I think I found it. I did a search for "tire size MPG".
viewtopic.php?f=9&t=1258

Quote:
Originally Posted by Mike
I sent some emails and did a bit of research, probably prior to last Spring when I got the new
set of standard sized tires. I believe the costs of switching to a slightly larger tire size wasn't
recoverable over a reasonable amount of distance and time. Nor was it recommended or safe.
Were the larger tires that much more expensive than the stock size?

Did you notice any MPG improvement, when you switched to Michelin tires?
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Old 03-09-2011, 09:23 PM   #25
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Default Re: MPG, Fuel Economy, - the definitive resource

Quote:
Originally Posted by Photog
Quote:
Originally Posted by Mike
Somewhere on here I had a discussion about tire size, going larger diameter, to reduce rotations
per mile, and to try increase mileage, but I'm darned if I can remember where it is.
Feel free to resurrect it if you can locate it.
I think I found it. I did a search for "tire size MPG".
viewtopic.php?f=9&t=1258

Quote:
Originally Posted by Mike
I sent some emails and did a bit of research, probably prior to last Spring when I got the new
set of standard sized tires. I believe the costs of switching to a slightly larger tire size wasn't
recoverable over a reasonable amount of distance and time. Nor was it recommended or safe.
Were the larger tires that much more expensive than the stock size?

Did you notice any MPG improvement, when you switched to Michelin tires?
I sent letters to local GM Dealers, as well as GM Canada, describing what I wanted to do.
The main issue was having to not just buy the larger tires, but also the appropriate larger rims, and I believe
there were some other concerns regarding turning clearance inside the wheel wells. When I looked at the info
I'd gotten back, the mpg increase would be marginal, I don't recall exactly how much, but it was less than a 3% improvement.
I decided to go with Michelin AT/2 (more aggressive tread pattern than MS2 series) with OEM specs. I like them, they
ride nice, I try to keep them at 80 psi rear and around 65 psi front, for improved stability and mpg reasons. I think they
help, although again, no idea by how much.
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Old 03-09-2011, 10:30 PM   #26
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Default Re: MPG, Fuel Economy, - the definitive resource

Trying to guess if you will get better, same, or worse mileage from a tire size change is pretty much a lesson in futility, unless you have seen very similar combos before.

The energy needed to move the van down the road doesn't change, only the wheel and drivetrain rpm. How the mileage comes out depends on where in the efficiency curve of the engine and drive train you are. If it were just a matter of big gears/low rpm, you would see much different gear ratios in things. If your rpm drops enough that your torque multiplication isn't enough, you need more throttle, which may put you at a different place on the fuel map, change spark advance, change cam valve timing, cause it to come out of lockup, downshift etc. If any of these things happens, even a small % of the time, you can lose mileage. When they make the monster top gear 6 speeds, they count on the transmission to only use those gears when at very low engine outputs, downhill, or whatever. The other gears take up the rest of the situations, and are closer to what you see on the 4 speeds. I haven't talked to anybody that has the new Chevy 6 speed and compared to the 4 speed, but I would guess on a 9600# Roadtrek, it is out of 6th gear a lot of the time. I would it expect it to get a bit better mileage than the 4 speed, but not huge differences.

Bigger tires, in general, also have higher rolling resistance, so that can cost you.

I can't tell you how many friends have done the "how much further they move per wheel revolution" calculation, and then expected that much improvement in mileage. Let's say they were all pretty disappointed.
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Old 03-09-2011, 10:40 PM   #27
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Default Re: MPG, Fuel Economy, - the definitive resource

Mike,
I noticed the 235/85R16 was mentioned. It is 1" larger diameter (3.3%), or 1/2" taller sidewall. 31.5" tire that fits the same wheel size as stock.

I would not think this would rub in the fender well. It would be more narrow, which might be a good for MPG.

With ours being lifted, I am looking at a 33" tire. We may need to have the axle re-geared to 4.56:1 (currently 4.11:1). If we can fit the 33" tires, I will do a number of MPG and mountain driving test, before considering the re-gear. If it all works out well, we may not install 4X4.

Booster,
I agree with you. From my research, good mileage comes from driving at an RPM somewhat below maximum torque. I'm not sure how far below max-torque. So, the final gear ratio (tranny/axle/tire) needs to be correct to have the engine in the optimum RPM range, at the desired highway speed.

The 1 ton vans may not be setup at factory, to run in the optimum RPM range, since there are other performance factors to be considered. Lower gear rations make it easier for the engine to haul around a heavier load, as it pushes the RPM range up closer to max-torque, at cruising speed.
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Old 03-11-2011, 04:04 AM   #28
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Default Re: MPG, Fuel Economy, - the definitive resource

About 20 yrs ago, bought a '77 hightop van,Dodge 360 camper. It got 14-16 mpg. Sold it for Asia travels.
About 10 yrs ago, bought a '77 hightop Chevy 350 camper. It got 10-11mpg. Tried everything-- bigger alloy wheels/Michelin tires, various carb/ air filter mods, even an overdrive transmission...... It still gets 10-11.
About 2 years ago, got a '96 Ford 351 EFI. It carries a lot more weighty equipment, and gets 14-16mpg. I attribute this to the EFI, what seems a great transmission, and a lower profile roof design.
The PO of the Ford told me he got 18mpg.... he was from Canada-- lower speed limits-- it makes sense. If I made myself drive this van 55mpg for a whole tank, I'd probably get 18,too.
So, the lesson here is you can spend a heap of $$, and not see any improvement. The ONLY thing that improves the mpg in any of these vans is..... drum roll........ go slower.
Meanwhile, keep tire inflation correct, don't drive with full water tank(s). My dad told me drive like there is a raw egg between your foot and the gas pedal...... easy does it.
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Old 03-11-2011, 04:58 AM   #29
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Default Re: MPG, Fuel Economy, - the definitive resource

Quote:
Originally Posted by shezonit
The PO of the Ford told me he got 18mpg.... he was from Canada-- lower speed limits-- it makes sense. If I made myself drive this van 55mpg for a whole tank, I'd probably get 18,too.
Possibly bigger gallon, too?
1 Us gal = 3.8 L versus
1 Imperial (Canadian) gallon is around 4.5 L. = More gas per gallon.
I can get around 20 mpCg on a flat road, on cruise, at around 100 kph (62.5 mph), with no wind or hills.
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Old 03-11-2011, 05:48 PM   #30
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Default Re: MPG, Fuel Economy, - the definitive resource

Quote:
Originally Posted by Mike
Possibly bigger gallon, too?
1 Us gal = 3.8 L versus
1 Imperial (Canadian) gallon is around 4.5 L. = More gas per gallon.
I can get around 20 mpCg on a flat road, on cruise, at around 100 kph (62.5 mph), with no wind or hills.
Now that just makes things confusing.
Don't the Canadians measure fuel economy buy the number of liters per 100km? (I know, more confusion. ).
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