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Old 09-28-2013, 06:46 PM   #1
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Default Hood protector gas mileage

Has anybody done any testing on if a hood protector reduces mileage much? I put one on our Chevy before our last trip, and it we did appear to get less mileage than in the past. It is hard to pin it on protector, though, because this was also the first long test of the van with the solar on it and the larger tires.

We did notice less air out of the vents with same fan speeds, and bug reduction was only moderate, nothing spectacular.

I think I may take it off on the next run and see if anything noticeable shows up.
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Old 09-28-2013, 11:01 PM   #2
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Default Re: Hood protector gas mileage

I would have your speedometer calibrated to accomadate the larger tires, I don't think the bug screen would change the milage noticeably but larger tires would.
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Old 09-29-2013, 12:35 AM   #3
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Default Re: Hood protector gas mileage

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Originally Posted by sebtown
I would have your speedometer calibrated to accomadate the larger tires, I don't think the bug screen would change the milage noticeably but larger tires would.
GM doesn't show that size in the tire size flash, so we are out of luck on that. We do have our Scangauge set to the exact speed and odometer, and that is what we do to check mileage.
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Old 10-11-2013, 09:11 PM   #4
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Default Re: Hood protector gas mileage

We just got back from a quick 3 day trip to southern Minnesota. Not a long trip at about 85miles each way, and quite (very?) windy both ways, headwind out, tailwind back, both with some crosswind. I took off the hood protector and put the tire pressure back up to 65/80 psi. We were pretty light as it was a short trip, but did have water and 3/4 tank of gas on board.

It was so windy, it was a bit hard to tell, but it sure looked like the mileage was pretty much back to where it was before the protector, lower pressure on big tires, and solar. It is very handy with the Scangauge to do relatively short segments to see what the mileage is under different conditions without doing fillups and such. The air out of the vents also came back to normal. I think we will leave the protector off and do a bit more testing at the different tire pressures. It is a bit smoother ride at 60/72, and handles about the same.
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Old 10-12-2013, 12:09 AM   #5
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Default Re: Hood protector gas mileage

Larger diameter tires will cause your odometer to indicate that you traveled fewer miles than you actually did, making it look like a reduction in mileage.

The odometer calculates the miles traveled by multiplying the number of tire revolutions by the circumference of the factory specified tires. If the factory recommended tire size was 225/75R16 which has a diameter of about 29", the circumference would be about 92". For every revolution of the tire, the vehicle would advance 92". The vehicle will advance 1 mile (63360 in) every 689 revolutions (63360/92).

If you increase to a tire size with a diameter of 31", the circumference will increase to about 98 inches, so now the vehicle will advance 98" per revolution, 6" more than with the factory recommended tire. For those same 689 revolutions the vehicle will advance 67522 in or about 1.07 mile. When the odometer indicates the vehicle has traveled 100 miles, the vehicle will have actually traveled 107 miles. If you calculate your gas mileage based on the odometer reading, you will be basing it on fewer miles than you actually traveled resulting in poorer mileage than what you actually achieving.
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Old 10-12-2013, 01:15 AM   #6
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Default Re: Hood protector gas mileage

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Originally Posted by AmCruiser
Larger diameter tires will cause your odometer to indicate that you traveled fewer miles than you actually did, making it look like a reduction in mileage.

The odometer calculates the miles traveled by multiplying the number of tire revolutions by the circumference of the factory specified tires. If the factory recommended tire size was 225/75R16 which has a diameter of about 29", the circumference would be about 92". For every revolution of the tire, the vehicle would advance 92". The vehicle will advance 1 mile (63360 in) every 689 revolutions (63360/92).

If you increase to a tire size with a diameter of 31", the circumference will increase to about 98 inches, so now the vehicle will advance 98" per revolution, 6" more than with the factory recommended tire. For those same 689 revolutions the vehicle will advance 67522 in or about 1.07 mile. When the odometer indicates the vehicle has traveled 100 miles, the vehicle will have actually traveled 107 miles. If you calculate your gas mileage based on the odometer reading, you will be basing it on fewer miles than you actually traveled resulting in poorer mileage than what you actually achieving.
As I mentioned earlier, since GM doesn't seem to offer an odometer flash for the bigger tires, we have our Scangauge set to read the correct miles. It was set based on a 50 mile gps and mile marker run, so it is very, very accurate. IIRC the tire diameters were 31.5 up to 32.7 or 3.8%. The odometer from the factory read almost 2% high, so no it reads about 2% low, but we never went by the odometer anyway, and always used the calibrated Scangauge. Our big concern with the no available flash for the bigger tires, was that the engine and transmission properties also are changed with the flash, and could affect things. We had some limited tests last year on the bigger tires, but not enough to know for sure what was up, that indicated no major change in mileage, and that was at the higher pressures like we used today. Things got harder to evaluate now because the solar went on the roof and the deflector on the hood because then with the bigger tires, and the bigger tires at lower pressure, we wound up with too many variables at the same time. We seemed to be seeing lower mileage on a previous trip so I decided to see if it was really less or not. This last test tried to get rid of some variables so we get better information. As of now, my guess is that the tire size is probably not much of an issue, but lower pressure may be. I think the deflector was probably a larger problem than any of the others. The solar probably is also not much of an issue.

I didn't mention it before, but we also did limited test trying to confirm some things we had noticed before, and that was mileage on rolling hills. I had always known that it got worse mileage in hills if it was on cruise control, especially if it would get into downshifts going up the hills, when you could keep it from downshifting if you were driving it yourself. What had appeared to be going on that I really wanted to test was that it seemed to get considerably better mileage in rolling hills than on the level. Many times we would see our average mileage jump a bunch going downhill, more than it lost going uphill. Couldn't understand that one. We tried it on this last time out on segments of a rolling two lane road. It looks like, based on this small test, we lose about 5-7% mileage when we go on the cruise control in the hills, which was expected. When we compared the mileage on the rolling hills compared to the level, it did almost 5% better in the hills, or about .5 to 1.0 mpg. We also found that we only got that gain if we let the van coast down the hills (in gear). If we just went over the hill and feathered the throttle to hold speed, or get a bit of extra speed to go up the next one without downshifting, the gain was very much smaller, or gone. Watching the instantaneous mileage on the Scangage showed it going to 999 when going downhill with the throttle closed, and in the 30-40mpg with it slightly open. When we watched the fuel delivery on the Scangauge, it was obvious why. They shut all the fuel off when coasting, and they unlock the converter, so you use no fuel at all and you coast better. So whenever you coast downhill, all the engine inefficiency of combustion is gone, and the wheels are mostly unlocked from the engine, letting it roll better. These things never happen on the level, so you have 100% of the time with the combustion inefficiency and drivetrain loss. Since the engine stays closed loop going up hill (from what we saw), it runs about the same efficiency as on the level, so all you do is store energy and get it back going down the other side (carb engines didn't work this way), which theoretically gives you the same mileage as on the level. Having it shut off completely some of the time gives you the extra mileage. Definitely not what one would expect, but it explains why the best mileage we ever got was on a very hilly road in northern Minnesota that you expect to kill the mileage, not help it.
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Old 10-12-2013, 12:21 PM   #7
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Default Re: Hood protector gas mileage

Interesting observations re: hills. Different motor in my van but I've noticed the "freewheeling" down hills and the 999 mpg. My van really picks up speed on some downhills - more than the vehicles in front of me - and I have to brake more than than the traffic ahead even though I keep a very large gap in between my van and the other vehicles in front. Weight and gravity I guess.

Three fill-ups ago it was up hill as soon as we left the service station and up and down for a couple of hours. Some steep climbs too. 4 and 6 mpg on the ScanGauge uphill climbs. At the time I commented to my wife that the mpg for this tank would be very poor. It turned out that for that tank of fuel the MPG (16.26mpg) was average for the trip and slightly better than average for this years (16.05mpg) travel.
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Old 10-12-2013, 03:44 PM   #8
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Default Re: Hood protector gas mileage

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Originally Posted by markopolo
Interesting observations re: hills. Different motor in my van but I've noticed the "freewheeling" down hills and the 999 mpg. My van really picks up speed on some downhills - more than the vehicles in front of me - and I have to brake more than than the traffic ahead even though I keep a very large gap in between my van and the other vehicles in front. Weight and gravity I guess.

Three fill-ups ago it was up hill as soon as we left the service station and up and down for a couple of hours. Some steep climbs too. 4 and 6 mpg on the ScanGauge uphill climbs. At the time I commented to my wife that the mpg for this tank would be very poor. It turned out that for that tank of fuel the MPG (16.26mpg) was average for the trip and slightly better than average for this years (16.05mpg) travel.
I guess it does make sense, and is also confirmed, I think, by the way they drive the vehicles in the hyper-mileage competitions. In those competitions, they accelerate at wide open throttle (less pumping loss), leanest mixture (obvious), to a predetermined speed, then they shut it off and coast to another predetermined speed before repeating the procedure. The only time they are putting in any energy is at full output, so the basic inefficiencies are a smaller % of the total load and fuel use, and the rest of the time they use no fuel.

I have watched our air fuel mixture on the 6.0 gasser, and you have to push it very hard to get it to go off of the leaner setting, so climbing moderate hills doesn't richen it up and use more fuel, like a carb engine would. Don't know how that applies to a diesel, which probably is even better, as they always run with excess air, I think.
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Old 10-12-2013, 11:37 PM   #9
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Default Re: Hood protector gas mileage

Sorry but I'm having a lot of trouble warming up to the logic/math/physics that doing hills (at least any sort of significant hills vs. just minor rolling hills) can benefit fuel mileage, especially if as Markopolo says you have to brake a bit (or a lot) on the downhills, which I also have to do (a lot). Bear in mind that it is very difficult especially on the Chevy Roadtreks to fill the tank all the way and to do it with any consistency, and hence "mileage on the last tankful" results can be deceiving. And then there are the variables of wind, and starting vs. finishing elevations; eek! We've had gas mileages all over the place, but very consistently it suffers badly in hilly terrain. But as the big folks say YMMV!

Disclaimer: My 09/10C190P Roadtrek has a mousey 4.8l motor, and just a 4 speed tranny.

Regards, Dick
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Old 10-12-2013, 11:49 PM   #10
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Default Re: Hood protector gas mileage

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Originally Posted by dicktill
Sorry but I'm having a lot of trouble warming up to the logic/math/physics that doing hills (at least any sort of significant hills vs. just minor rolling hills) can benefit fuel mileage, especially if as Markopolo says you have to brake a bit (or a lot) on the downhills, which I also have to do (a lot). Bear in mind that it is very difficult especially on the Chevy Roadtreks to fill the tank all the way and to do it with any consistency, and hence "mileage on the last tankful" results can be deceiving. And then there are the variables of wind, and starting vs. finishing elevations; eek! We've had gas mileages all over the place, but very consistently it suffers badly in hilly terrain. But as the big folks say YMMV!

Disclaimer: My 09/10C190P Roadtrek has a mousey 4.8l motor, and just a 4 speed tranny.

Regards, Dick
I totally agree, if you have to brake, it all goes out the window. And it only works if you totally close the throttle going down the hills. It is pretty hard to do with traffic around because of the speed variations. It also doesn't work if you are to the point of getting downshifts on the (as you say, significant) uphills. Coasting down the other side will help, but not enough to make you break even with what you would get on the level sections. Your 4.8 probably has to downshift, or go to power mixture, considerably more often than the 6.0 does, so that would hurt you.

We do all our mileage checks off of the "current trip" on the Scangauge, because as you say, you can't go by the tank fill method and be anywhere near accurate. The "current trip" is nice because you can reset it at any time, so you can do as many tests as you want without filling the tank. Going across Michigan a while ago, we did the tests at different speeds from 50 to 75 in 25 mile segments just to see how it looked. It was as radical as you would expect, varying from 17.6 down to 13.5mpg.
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