My car get better gas mileage doing 70-75 mph

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Originally Posted By: brianl703

Wind can make a difference, too. I've gotten GREAT gas mileage driving south in the winter..but the wind was blowing from the north.


I don't think it was the wind, it was because you were driving down hill.
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(that's what my father always said: driving down hill when going south and up hill when going north [on the globe])

I neglected to say that my car has an average mpg reading, not an instant mpg reading. On my 13hr/750 mile trip using cruise control on a several hour portion of the trip where the road was fairly flat, the average mpg decreased when driving between 55-65 and increased when driving at 70. I don't know at what speed over 70 it will start to decrease; I guess I have to plan a trip out west to drive on one of those 75-80 mph posted interstates.
 
Engine and drivetrain friction are pretty much linear with respect to speed.
Wind resistance is proportional to the square of the speed.
I don't see how greater drag and friction increases gas mileage.
 
I drive slow, but not road speed. I try to maintain a constant airspeed in my car. Going against the wind, I slow down; with the wind, I speed up. Over my lifetime, the extra time spent slowing against the wind will be made up by going faster with the wind.

If a round trip has one half against the wind, and one with the wind, you will use less gas if you slow down when going against the wind and speed up when with the wind, than if you do the whole trip at a steady speed.
 
Originally Posted By: wgtoys
"So my question if the "drive speed limit of 60 mph because every 5 mile over that is a big waste of gas" a big government conspiracy lie to scare us into slowing down to grandpa pace?"

In a word, no. At highway speeds overcoming air (wind) resistance becomes a very large factor. The power to overcome air resistance increases roughly with the cube of the speed.

If you have studied rudimentary physics you will understand what this means. If not, you may just have to settle for conspiracy theories.



So, true! With my GMC Yukon XL 6.0L at 65MPH i get 20.8 MPG at 70MPG I get 18MPG at 75MPG it drops to 16MPG.
 
Added power to oversome wind resistance may be true but there are other factors that effect speed vs. mpgs. Areodynamics, wind direction, wind speed, road surface condition, weather, temperature, humidity, altitude, weight, rolling resistance, engine displacement, engine efficiency, and drivetrain gearing (to name a few) will be factors beyond the basic physics of acceleration/deacceleration and moving a mass through a non-vacuum space.

Not all cars will achieve higher mpg's at speeds above 55mph; it depends on the vehicle. I don't want to pick on your Yukon, but I think the results would be different if you compared a 2wd Yukon with passenger tires in Kansas on a cool day (and making it a low-rider too
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) to a 4wd Yukon with all-terrain light truck tires in Denver on a hot day.
 
I do lots of four hour trips and have noticed the TL gets best mileage at 65mph. I've gotten 35mpg on a 280 mile trip with no stops. The GN on the other hand gets best mileage at the lowest speed it can hit OD with the convertor locked which is 55mph. I have a feeling if I got a chip that locked the convertor sooner it would only get better mileage but a 3,800 stall convertor doesn't get good mileage unlocked even on level ground.
 
Originally Posted By: ProfPS
Added power to oversome wind resistance may be true but there are other factors that effect speed vs. mpgs. Areodynamics, wind direction, wind speed, road surface condition, weather, temperature, humidity, altitude, weight, rolling resistance, engine displacement, engine efficiency, and drivetrain gearing (to name a few) will be factors beyond the basic physics of acceleration/deacceleration and moving a mass through a non-vacuum space.

Not all cars will achieve higher mpg's at speeds above 55mph; it depends on the vehicle. I don't want to pick on your Yukon, but I think the results would be different if you compared a 2wd Yukon with passenger tires in Kansas on a cool day (and making it a low-rider too
grin2.gif
) to a 4wd Yukon with all-terrain light truck tires in Denver on a hot day.


True! I have had 3 of these vehicles all configured differently and the mileage was different on each based on all of those factors. The best was a 2002 Suburban 5.3L 4wd which got 21.2 mpg at 65 mph in 2wd setting. All had 3.43 gearing and 4L60 transmissions. The mileage was consistent in year round temps.
 
One of you physics enabled types do a comparison based on drag coefficient, please? Give an index to how much difference the spread is for two different vehicles. I imagine that we'll find that the "increases by the square" impact is a whole lot more for some than others. What may make 5% penalty for some slick configuration ..may be over 20% for a rolling brick.
 
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Originally Posted By: Gary Allan
One of you physics enabled types do a comparison based on drag coefficient, please? Give an index to how much difference the spread is for two different vehicles. I imagine that we'll find that the "increases by the square" impact is a whole lot more for some than others. What may make 5% penalty for some slick configuration ..may be over 20% for a rolling brick.


I forget the actual CD of my TL but I remember being very surprised at it being close or equal to the C5 Vette. The Buick is a brick. The Buick also has tons of low end torque while the Acura needs to rev a little. Acura gets worse mileage below 60mph while the Buick gets better the slower I go until the TCC unlocks. I suspect if I lock it manually 40mph wouldn't be out of the question for peak fuel economy.
 
Oddly, my wife's 4.0 Wrangler appears mostly indifferent to speed. Naturally we've never done 80mph for extended periods of time, but it doesn't seem to care much one way or another. I did break 21 mpg one time on a longer trip, but I attribute it to one warm up over a tank instead of 12+.

Not in contradiction with my belief that slower is better (nor what XS650 said) ..but I believe that the power development characteristics of a given engine can surely influence the efficiency gain that one can realize with varied speed; especially when you're forced into certain confines of practical ceilings and floors in what you can manipulate in a given set of drive train semantics.
 
Gary,
that was exactly it with my Torana. Engine was much more happy at 3,700 RPM than 3,000 with the big carbs etc.
 
Also, nobody drives at the legal minimum for extended periods on the highway.
So someone getting a full tank value for gas mileage is not believable to me at 45-50 MPH. If we did drive at that speed, we would certainly get better gas mileage than at 65-75.
Everybody does the limit or more.
 
Originally Posted By: mechtech2
Also, nobody drives at the legal minimum for extended periods on the highway.
So someone getting a full tank value for gas mileage is not believable to me at 45-50 MPH. If we did drive at that speed, we would certainly get better gas mileage than at 65-75.
Everybody does the limit or more.


I'm the exception to that. For nearly two years I had a four hour trip twice weekly. It got pretty boring and since I was paying for my own gas I cruised in the truck lane a few times doing 50-55mph to see what it did mileage wise. A couple times I managed no gear shifts and the car did not even unlock the convertor once during the trip. Unfortunately that's around 1,500rpm in the TL and I saw no increase in mileage. Even a true 65mph seems rediculously slow but that seems to be the sweet spot.
 
Gear ratios have a great deal to do with it too.

Some care only have to use like 10-15% more fuel to go 70 vs. 55 while going 27% faster. Other stay about the same (my car is in this category) and other fare far worse.
 
Originally Posted By: ProfPS
What RPMs are you running at 60 mph, 70 mph and 75 mph?? I have always believed that the higher gearing (lower RPM) for better gas mileage was not the correct route to take. In a lab environment for EPA results and marketing, you will achieve 30 mpg running at 60 mph and 1600 RPMs in the highest gear (auto/man trans). In the real world outside of running on a dyno or the Bonneville Salt Flats, running a constant 60 mph gets less mpg due to more throttle to maintain the speed and transmission down/up shifting to make it over any hills. Why??? Lower RPMs is lower on the hp curve. My 13hr/750 mile trip (each way with stops for the kids) proved this as the trans rarely down/up shifted @ 70 mph (higher on the hp band and speed limit was > 55 mph) and was shifting more often at 60-65 mph. I also think momentum for hills play into it also.

I've noticed this for years. When I used to commute on the Taconic 500 in NY (rush hour commuting was 70 mph on rolling hills, bumper-to-bumper driving like drafting on the back straight away at Talladega waiting for the "big one") I would drive in 5th gear. At 55-60 mph (relaxed driving on the weekend) I would drive in 4th to prevent lugging the engine and downshifting, keeping the engine higher on the hp band (also, faster response when I had to accelerate to pass or get away from a "blue hair"). My gas mileage in 4th was not much lower then my 5th gear 70 mph commuting which were both equal or higher to 55 mph in 5th. From what I recall, Taconic 500 gas mileage was 28-32 mph with a few city miles in the mix; '88 Z24 & '92 Civic Si (both 125 hp engines).



The new Hondas that use iVTEC have a sweet spot right about 2250 RPM, which is the switch over point for the VTEC from 12 valve VTEC-E to VTEC-power mode.

I find that if I go too slow in 5th, like at 45-50 mph, then the MPG suffers. 60 MPH seems to be the sweet spot for my 2005 CR-V EX Manual. Once the warranty is up, I will be installing 6th gear from the TSX/RSX-S/Si where the spot holder is now in the tranny. There is a handful of Elements and at least one CR-V running around with the 6th gear conversion, and get over 30 mpg on the highway. Which is not bad at all for an AWD SUV.
 
I just realized what winter is doing. The oil in my rear diff is about 30 degrees C cooler in winter than summer. Oil roughly doubles thickness for every 10 degrees C near freezing (i think), so the oil in there is now 8x thicker than in summer. That must be having a bad effect of fuel economy.

If I make a cloth bag to go around the diff., that might reduce the cooling effect of the wind, and keep it warmer.
 
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