In order to do my next post I need a answer to this…I suppose its 65??0r 55? also may depend on waht kind of car it is?
Depends on the design speed of the engine, gear ratios throughout the drive train, and tire size, not to mention a host of other variables. Two visibly identical vehicles sitting side by side may have two radically different “ideal” speeds.
This being a railroad forum, I’d be remiss if I didn’t suggest infinity, as in riding on a suitable railroad car instead of being driven down the highway…
Aerodynamicly speaking?
You don’t need to worry much about Cd, gear ratio numbers, etc., except as general guides. Ceteris paribus, you’ll get the best continuous mileage when the engine is running with minimum admitted fuel to hold it at the rpm that corresponds to its torque peak. (Ignore downgrades for a moment). Now, if you had a properly-designed CVT and engine-control electronics, the transmission would constantly adjust to keep rpm ‘right’ and the injection system would adjust to give appropriate best economy. The ‘next best thing’ is a whole bunch of gears, as is found in heavy road trucks, which can keep the engine relatively close to its torque max (keeping inside the powerband is a very different thing).
Going downgrade, best economy involves using neutral and either idling the engine or cutting it off. In many cases, the same applies to normal driving without a regenerative transmission: you use the engine to get up to speed, then cut it off and coast down, then restart it and accelerate again. (Don’t try this with an ordinary car, for a variety of applicable reasons, unless you know what you’re doing!)
The situation can get more complicated if accelerations can’t be made at max torque, for example if you have an automatic transmission with ‘only’ 3 or 4 speeds in the planetary or gearbox, or a fixed ratio through the final drive that doesn’t allow operation at the torque peak at a desired road speed. It also gets more complicated if the engine isn’t ‘happy’ accelerating its speed with an applied load – many diesels are in this category, and will give huge clouds of overfueled smoke if required to do this, although they can hold a given speed remarkably well under the exact same conditions.
Of course, the short method of testing is to cover the same route or course multiple times, at different speeds, and see which speed and method of driving gives best economy. I can say with some assurance that the speed and technique vary greatly from vehicle to vehicle, and also can vary
So pollution controls reduce milage?
That is actually coming back, Honda for sure and I think either toyota or GM will have a car either 2005 or 2005.5 that at cruising speed with shut off cylinders randomly. Whats old is new again.
The “politically correct” answer is 55 mph and it seems to make sense that it would take more fuel to reach and maintain a higher speed.
But tree68 is correct and aerodynamics is important too.
I once owned a car that consistantly got 35 mpg at 80+ mph and only 30 mpg at 65. (I was lucky I never got a speeding (or any other) ticket while driving it. Ironically I have gotten speeding tickets while driving slower in other vehicles. Maybe thats justice) It was a small car with a small efficient engine and excellant aerodynamics.
The ACTUAL ‘politically correct’ answer is somewhere in the neighborhood of 62.5mph (which is also the ‘magic ton’ in the metric system). That’s based on truck speeds, but modern car speeds often accord with it. 55 was a goofy number from a goofy time. You’ll find that politicians with somewhat better common sense ran 55 up to 65 in most places … and that was NOT because the oil lobby told 'em to.
I remember reading in the '60s about one of the Porsche Speedsters, which got better fuel mileage the faster you drove it (I believe the pattern persisted well up over 80mph). This is not uncommon in high-performance vehicles, and also in some larger vehicles that have high momentum (making them relatively insensitive to environmental and grade decelerations at constant engine rpm).
One very amusing thing: I had a 1988 Lincoln Town Car with the 3.54 axle and their version of ‘high performance’ 302. This had the Lincoln digital dashboard with a display of instantaneous fuel economy. Driving on Interstates (mostly at night for somewhat obvious reasons) I discovered there was a ‘magic range’ from about 80 to 84mph where my fuel mileage would dramatically increase, even beyond the steady-state ‘best average mpg’ at around the mid-sixties. I figured this corresponded to the engine making max torque in the overdrive gear of the AOD transmission. Interesting how that speed seemed like really rocketing back in the early 1990s…
…Like so many other posts above have indicated we have so many conditions that effect fuel economy. Cd of the automobile…speed…road surface…tire pressure…type of tires…wind…amb. temperature…driver…weight of automobile…gear ratio…transmission final dr. ratio [OD]…engine tune…windows up, windows down…fuel quality…engine type and size…lubricating oil type and viscosity…engine temperature…and the list goes on…almost endless if one is seeking absolute numbers.
So should we have “Fast lanes” were people pay higher tolls to do 80MPH?
and at what speed would people go from Car to Train to make it worthwhile?
I, of course, think that ‘fast lanes’ – paid for exclusively by toll-funded debt, of course, and not out of any general funding – would be a delightful thing to have. Do I think they’re either socially or politically/legally practical? No.
Everybody around here goes at least 79mph in a 70 zone already – having “high-speed” lanes with zero tolerance at 80 would cause more problems than it would solve. 80 is too slow for justify my paying a toll anyway … particularly if that’s used to justify a lower radar enforcement limit on conventional roads, as it almost surely would be in some states.
Now, an intelligent highway permitting automated cruise control, and optimized lane-change notifications to allow seamless passing at ‘autobahn’ speeds … THAT would be worth paying a toll (electronically, of course, no plazas) for.
Now, the second point has a certain amount of worry associated with it – if that speed increase applies to CAR speed, it would pu***he horizon for LD trains outward considerably. I remember being disgusted to find that my father got from northern New Jersey to Washington by car in time to meet the Metroliner he’d dropped us off (in Manhattan!) to catch. Real people’s trips are point to point, not station to station … especially not inconvenient station to unsafe station, leaving and arriving at weird times in the middle of the night. If car trips are made faster and/or more convenient (or easier for people who don’t like driving quickly) it’s going to cut into train utilization much of the time, except to destinations where it would be inconvenient to have a car.
I assume you’re not talking about something like an “Iron Highway” carrying cars at high speed parallel to (presumably congested) Interstates or other routes. I have seen places in Europe (Austria notably) where this sort of thing has worked – but it usually involves avoiding either substantial amounts of driving reduction or parts of the road system that sane people woul
No it’s 55 mph. Any correspondece between “politically correct” and reality is a coincidence.[:D]
Yeah – what I meant was the number that made the maximum sense for politicians to use when cobbling up artificially low speed limits for ‘safety’ under the guise of ‘fuel efficiency’. It would have helped to have ‘plausible denial’ with ACTUAL ‘fuel-efficiency’ maximization (on a ton-mile or passenger-mile basis) rather than having to be alarmist over a fuel crisis to ram the scheme down American drivers’ throats (or perhaps up some other part of their anatomy)
If only there were ACTUAL, rather than perceived, correspondences between reality and what politicians view as ‘politically correct’ … !
If there was no speed limit how would small states and towns get any revenue[}:)]
They’d figure something out, just as they so often did in the early years of automobiling. Speed traps are not the only kind of trap – one of my least favorite scams is the “California Roll” method of enforcing stop signs to the letter of the law. One also suspects that the old Mexican trick of pulling out in front of you in an old car with no lights … and no insurance … would find gleeful adoption.
I don’t think it would ever be practical to abolish ALL speed regulations in the USA – certainly not within town areas – provided only that they be in accord with actual traffic conditions (which I know to be a requirement in NJ, and I think in California – other states, let’s hear from you!). The problem for me with the ‘double nickel’ involved an artificially low speed limit, which was then thrown as a plum to local folks (Linndale cops, you know who you were!) with a nice, typically Democratic stick behind it (deprivation of highway money) for the states who didn’t make their quota of drivers toeing the line. The place I live now – Shelby County, Tennessee – was notorious in the late '80s and early '90s for massive delays and Fascist enforcement: you could hear the CB start to crackle before you got to Jackson westbound or Forrest City eastbound with the relayed reports of activity. I personally witnessed these folks leave highway barricading up on two completely repaired lanes on a Thanksgiving weekend (causing truly monumental traffic delays in the process) – the only reason being so they could reap the benefits of double fines in the nominal 45-mph speed zone from all the out-of-towners forced to pass through on I-40 and I-55…
This enforcement stuff went on even as the highway folks either continued to build and spend to make 70mph design-speed highways, or cut back on the designs to put in cheap curves and bridges, or to keep substandard or temporary roadwork in the finished scheme because it was ‘good enough’ at the new lower speed limits.
To get back on
My magic range is about 2000rpm on the diesel. That is 58 in third gear, and 72 in OD.
Adrianspeeder
Electronic Fuel Injection and control systems pretty much eliminates any real Ideal speed since the on board computer is constantly adjusting the fuel/air/timing controls to get maximum from each gallon of gas. I get better milage in my truck at 75 on cruise control than at 65 at foot control.
55 was a random number in the mid seventies. The fuel savings at 55 vs 60 were pretty miniscule for the average car. Mass and momentum have a more detrimental effect on cars than speed. A 2000 lb VW was less mass to pu***hru the air and less interia to vercome cornering and stopping. A 5000 lb Cadillac with a 5L V-8 still got crappy milage because it had so much Mass to push around.
During WW2 the national speed limit was 45mph, but that had far less of an impact on fuel savings than the srtict travel limits that were imposed. WW2 is where we get the phrase “Is this trip really necessary?” meaning is it that important that you have to travel and use fuel. Public transport was also heavily emphisised during WW2 which also had a far more dramatic impact on fuel savings than an arbitrary speed limit. Remember that 45 sounds slow today, but in 1940, on the just then completed super modern Pasadena Freeway, the speed limit on this new exotic high speed marvel was …45.
…I can’t speak for California speed limits in WWII but eastern US used a speed limit of 35 MPH…and of course gas and tires in the automotive field were rationed so people needed to save as much fuel as possible.
EFI won’t directly affect the torque peak that governs top fuel efficiency – but VVT can. Remember that the engine will likely produce its relative maximum hp for a given fuel flow at torque-peak rpm. That doesn’t care what the fuel injection rate from the computer is – which is why ‘magic speeds’ work so well.
Note that if the torque curve is flat and broad, the engine will make ‘best’ economy over a wider range of speeds, which means that the economy can be achieved by balancing fuel feed against load at a given road speed – naturally, this means that even if the gear speed that corresponds to measured torque maximum is, say, 68mph, the engine may still deliver reasonable economy if driven at 55mph in that gear.
When I shut down my store in Studio City, I rented one of those Class 5 cab-forward box trucks to move the stock and furnishings to the store in Beverly Hills and the warehouse in Culver City. The truck was rented from a facility in downtown LA , adjacent to the Santa Monica Freeway. The truck had no 5th gear, so it was something of an adventure to run the engine up to the speed required to do 55 on the freeway. I drove the truck from there to Studio City (over Cahuenga Pass), loaded up and went over the Hollywood Hills to S. Beverly Drive, went back over the Hills, came back down to Slauson & La Cienega, then drove back downtown to turn the truck in. I was a bit concerned they might overcharge me if I didn’t make sure the fuel tank was filled, so I went to an adjacent station – charging a relatively high price for diesel – to fill it up.
Filling the tank ALL THE WAY TO THE TOP OF THE NECK consumed 1.53 … I don’t remember now whether that was gallons or dollars. There is no physical way the truck could have consumed less than that for what translates into over 60 miles of driving, much of it up steep grades with a reasonably full box of exercise equipment (treadmills and weight machines). Much of this involved careful acceleration and use of the clut
Most cars I have driven manage somewhere around 3000 rpm at freeway speed. For years truck diesel engines were designed to peak torque at 1800 rpm and 1200 ft/lb torque with a range of 350 to 425 Hp, with lighter class trucks at 2000 rpm, 800 ft/lb and 250-350 Hp. This was an agreement kept with manufacturers of transmissions and driveline components ( remember, a truck’s components are all made by different manufacturers). In the early nineties, this all changed with new emissions regulations and the addition of electronic engine controls along with many other design changes to enhance reliability. This forced manufacturers of transmissions and drivelines to completely redesign all of their products.
These same engines in offroad or marine applications would be rated as high as 600 Hp.
With these changes, engines now produce a standard (for the sake of driveline component manfrs.) 1800 ft/lb of torque at 1200 rpm at up to 525 Hp. The lower speeds obviously forced driveline mfrs. to change gear ratios offered, but the significantly higher torques forced the developement of new technologies in gear tooth profile. The funny thing is the higher torques and horsepowers were a side effect of lowering rpm and electronic engine controls to meet emission control laws, and the new engines do this with better fuel economy than before. So, in effect with the slower speeds, mechanical and thermal efficiencies have risen