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Old 03-02-2008, 06:50 PM   #1
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1998 National Speed Limit?

What was the national speed limit in 1998 for highways? My 98 explorer was rated at 14/19 city/highway from the DOT back then. I'm trying to figure out how it rates now compared to then. With the most recent changes I made to the Explorer, I'm getting 29 MPG @ 55 MPH! That is the peak efficiency speed, if I travel slower the mileage drops, if I go faster it begins to drop at 58mph, steadily to 70 mph, then above 70mph it takes a nosedive in mpg's. @ 75 I'm getting around 20 mpg. Before all the changes I made it would only get about 23 @ 55 mph. I'm trying to figure out how ford decided the average hwy mpg rating, at what speed. I know I've improved a lot over ford's design, but I don't know exactly how much.

JOE
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Old 03-02-2008, 07:39 PM   #2
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What was the national speed limit in 1998 for highways?
JOE
I think that the 55mph limit was repealed in either 1994 or 1995. Part of the "Republican revolution". The Republicans were trying to push some things back to the States, including speed limits. Some western states had "unlimited" speeds on some roads during daylight hours, for example.

I agree with this idea - I would rather economics than law dictate speeds. When gasoline prices rise high enough people will drive more sensibly. We'll see more conformity towards sensible speeds because people cannot waste.

I am not sure if this principle works overseas because governments abuse speed limits as a method of revenue generation.

I'm not too surprised that your Explorer runs best at 55 miles an hour. There has to be a "most efficient" speed. Makes sense that Ford would juggle everything, especially gearing, body shape and motor internals so that MPG would peak when the overdrive was on and you were doing the former national speed limit.

You could lower this speed by changing the rear end gearing or perhaps modifying some of the engine parameters. You might even do better because the vehicle will probably (but not certainly) have less drag at somewhat slower speeds. I have suspected that shapes are "tuned" to have a certain amount of drag at certain speeds and that drag is not linearly increasing with respect to speed but this is speculation.

My Yaris seems to peak from about fifty to sixty miles per hour. Over seventy, which I've hit at times, the Scanguage II shows a distinct drop in mileage. Since our highest limits are sixty five I do not indulge that foolishness too often.

I'm seriously considering getting that old lap top that someone gave me to "log" so that I can poll the OBD-II system. I think such data would be useful to determine a best speed mileage combination for my car, one based on data rather than observation of instantaneous scanguage readings.

Gene
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Old 03-02-2008, 07:43 PM   #3
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Here is some safety research on repeal of the National Speed Limit...

"here were significant increases in total crash rates on urban (60?65 mph) Interstate segments (confounded by extensive ongoing highway construction on these highways), and in fatal crash rates on high-speed (60?65 mph) rural non-Interstate segments. The following variables were unaffected: total, fatality, and injury crash rates on rural Interstate segments; fatality and injury crash rates on urban Interstate segments; total and injury crash rates on high-speed non-Interstate segments. These results show an adverse effect on crash occurrence for subsets of crash types and highways, but do not show a major overall effect of NMSL repeal and increased speed limit on crash occurrence on Utah highways."

source

As far as oil consumption - it's hard to say whether people driving faster or people driving gigantic SUVs and other substitutes for Station Wagons which were eliminated by raising CAFE averages for passenger cars are responsible for our current unrealistic rates of oil consumption.

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Old 03-02-2008, 07:46 PM   #4
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I've noticed that a '94 Tempo I've driven a lot is "notchy" as it were around 55 and 65mph, kind of annoying since 55 is 88kph and 65 is 105kph and our limits are at 80 and 100. By notchy, I mean it seems to be tuned to those points and runs smoother, and is less affected by small grades and gusts. It seems difficult in fact to maintain speeds slightly either side of those, always twitching the gas to slow or speed up.
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Old 03-02-2008, 08:55 PM   #5
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I've noticed that a '94 Tempo I've driven a lot is "notchy" as it were around 55 and 65mph, kind of annoying since 55 is 88kph and 65 is 105kph and our limits are at 80 and 100. By notchy, I mean it seems to be tuned to those points and runs smoother, and is less affected by small grades and gusts. It seems difficult in fact to maintain speeds slightly either side of those, always twitching the gas to slow or speed up.
One wonders if you could obtain a "chip" for the 94 Tempo that is optimized for the slightly slower speeds? I do not think that you can "tune" using a laptop since in those days ECUs were pretty primitive in comparison to more modern systems.

Possibly you could obtain slightly smaller tires or "fool" the system into thinking you have larger tires than the car uses now. This would adjust the set points to roughly where you'd need to be and would make driving easier and mileage a bit better. Of course this would also bollux up your speedometer....

I'm surprised and disappointed that Ford did not optimize for metric markets since there are far more folks using metric than using imperial these days. While I make fun of Fords because they do things like catch on fire or only seem to pass emissions checks at Ford Dealerships it would be nice to know that a US company isn't so damned stupid as to neglect their export market or make our Canadian neighbors use imperial measurements.

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Old 03-03-2008, 06:27 AM   #6
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I'm getting 29 MPG @ 55 MPH! That is the peak efficiency speed, if I travel slower the mileage drops, if I go faster it begins to drop at 58mph, steadily to 70 mph, then above 70mph it takes a nosedive in mpg's.

JOE
Ok, someone has to know where the information is...

There was some reasoning put into the 55MPH speed limit, but I cannot say that it was completely thought out. We're not breaking the sound barrier, but there are "force barriers" of sorts out there. I've ran into stuff that I've read that does talk about speeds above 60 MPH having some kind of relationship with cars, etc. Might also have to do with the shape of the vehicle.

Similarly, we've always noticed that the ram air starts to "activate" on production motorcycles at that point too. Becomes more pronounced over 70MPH.
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Old 03-03-2008, 09:39 AM   #7
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I noticed on my 350Z there's a much wider range of driveability and speed vs mpg. I can drive at 55mph and get approximately 33 mpg, and at 80 mph I'll get around 25 mpg. So I normally just set the cruise for 74 or whatever is closest to the traffic so I don't have to keep accelerating/decelerating etc.

I think the car and the truck are showing the same speed breaking point at which the mileage drops except the truck magnifies it and makes it much more obvious. The design of the 350z is not so much sleek as it is more designed to sort of punch a hole in the air and lift it (the "hole") up high enough for the rest of the car to drive through.
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Old 03-22-2008, 02:10 PM   #8
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I quite agree, the Benz's cruise acts up once in a while, and I try so hard to keep it at 55-60, but whenever I look down it's inched back to 70, where the little V-8 actually has to try. The only other speed it likes to stay at is 100mph. Hopefully once i get the new spark plugs, o2 sensor, and cruise working I'll be able to see what sorts of gas mileage its' capable of.
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