How speed affects MPG
- Thread starter SuperFudd
- Start date
Palmaris
New member
Why don't you want to lower speeds? Cars are already crashing into each other in huge numbers in the speeds we have set now. More speed = more danger to the participants, and that is probably the only reason why the speed limit is still set to an average of 65 mph. All cars are not really crash tested at higher speed than what we drive now. At much higher speeds, there is no way to make the crumple zones work effectively.
At that point, they may as well try and maximize the car's economy for those speeds as well.
sx sonic
New member
The Germans are an excellent example of how speed is not necessarily linked to danger. Large stretches of roads with certain lanes having unrestricted speeds. So there're cars doing 150mph+ in one lane with cars doing 60 or 80 mph in the next lane, yet they still have a lower accident rate than America as a whole, and significantly lower than most U.S. cities.
What they do differently is they're very strict on driving mannerisms and what it takes to get a license. Left lane camping, not signaling, not passing in a fast lane, and forcing a passing car to hit the brakes (basically cutting someone off) are huge no no's.
Things that are hardly enforced over here. He!! alot of American drivers don't even know It's illegal to camp in the left lane on interstates and highways, plus It's really ignorant and pointless.
Boil it down and the causality of accidents is almost always the result of poor root driving behavior. Speed exasperates this but isn't usually the root cause.
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Swordsmith
242hp/252 ft-lbs BNR EFI
I remember a study done back in the 80s on highway speed. Conclusion was that there was no particularly safe speed to drive, there wasn't the link you might expect between lower speeds and fewer accidents, higher speeds more accidents. Rather, the safest speed was the one that most people were travelling. If, on a given stretch of road, most people were going 75 mph, then that was the safest speed. You got more accidents, per capita, if you went 80, still more going 85, and so forth, and yes, the faster you went, the greater the chances of an accident. BUT the interesting thing is, you also saw more accidents per capita of people going 70, still more going 65, even worse at 60 and so forth, in the same proportion. This was true despite the fact that the speed limit on that stretch of road was 65. Look at that closely, driving 65, the speed limit, was as likely to get you in an accident as driving 85.
I avoided using the word dangerous there, because no, it wasn't quite as dangerous to drive 65 as 85... the consequences of the 85mph accident were worse for all involved. But driving 75, and thus probably not getting into an accident at all, is certainly safer than either.
I avoided using the word dangerous there, because no, it wasn't quite as dangerous to drive 65 as 85... the consequences of the 85mph accident were worse for all involved. But driving 75, and thus probably not getting into an accident at all, is certainly safer than either.
Palmaris
New member
This is all true.
But Swordsmith just pointed it out. If you crash at 85 mph, it causes more damage than if you crash at 65mph. The car was designed to keep you as safe as they can in crashed up to around 65mph. The crumple zones are set to absorb a maximum impact predicted at that speed. So if you are going faster, you will use up all of your crumple zones and more when you have an impact. This means that the passengers will absorb the impact as well, and this is not what you want.
The cost to design a car to absorb faster impacts likely goes up at an exponential rate, same as the energy created by the speed increase.
Scot
Athletic Supporter
One thing some people forget, or just haven't had the chance to learn: as aero, powertrains, and rolling resistance approach optimum levels of efficiency - there is still one overriding rule to consider.
Kinetic energy.
Simply put, (if we negate friction and resistance of air) it is the energy needed to move a mass (m) at a velocity (v).
Look at the relationship between mass and velocity. There's a square on the velocity - and it's that square that ultimately limits our top speed, kills moviestars in their Ferraris and makes the prospect of a meteor hitting the Earth a devastating idea.
Consider an example:
Let's say our Sonic weighs 1250 kg, and we're driving 30 km/h.
(0.5)(1250)(8.3^2) = 43.4 KJ
Now, double the mass (perhaps close to a Silverado?).
(0.5)(2500)(8.3^2) = 86.8 KJ
You can see how the mass doubles the energy.
Double the speed!
(0.5)(1250)(16.6^2) = 172.2 KJ
:sadbanana: Just by going twice the speed (60 km/h), a Sonic-sized vehicle takes as much energy to propel as a SIlverado-sized vehicle at 30 km/h.
But what you should really be interested in is that just by doubling the speed, it now takes FOUR TIMES as much energy to push the car down the road.
This is why brakes fade and need to be designed bigger on vehicles intended for racing - when you slow from high speeds, you're dissipating a lot of energy, which turns into heat that needs to be dissipated.
Also, this is why I would much rather hit a wall at 20 mph instead of 40 mph!
[Taking a deep breath]
One reason some manufacturers like to "lug" the engine at highway speeds is fairly straightforward:
1. Less RPM means less fuel injected on a per revolution basis, i.e. if an engine is spinning 50% faster, there are 50% more injections.
2. By running an engine below its most efficient area / below peak torque you can increase fuel economy by reducing pumping losses. Less RPM means less exhaust mass flow which gives your lower backpressure AND you have to open the throttle farther to make the same power (that you would running closer to peak torque). Since F.I. engines run closed loop fuelling, this doesn't use any more fuel: It lowers the restriction on the air coming into the engine.
Look at "displacement on demand": that's how it delivers fuel gains - running very close to WOT really lowers pumping losses and increases fuel economy.
One last disclaimer*
I've been doing this kind of work for about 20 years now with the last eight specializing in new methods for optimizing fuel economy.
Kinetic energy.
Simply put, (if we negate friction and resistance of air) it is the energy needed to move a mass (m) at a velocity (v).
Look at the relationship between mass and velocity. There's a square on the velocity - and it's that square that ultimately limits our top speed, kills moviestars in their Ferraris and makes the prospect of a meteor hitting the Earth a devastating idea.
Consider an example:
Let's say our Sonic weighs 1250 kg, and we're driving 30 km/h.
(0.5)(1250)(8.3^2) = 43.4 KJ
Now, double the mass (perhaps close to a Silverado?).
(0.5)(2500)(8.3^2) = 86.8 KJ
You can see how the mass doubles the energy.
Double the speed!
(0.5)(1250)(16.6^2) = 172.2 KJ
:sadbanana: Just by going twice the speed (60 km/h), a Sonic-sized vehicle takes as much energy to propel as a SIlverado-sized vehicle at 30 km/h.
But what you should really be interested in is that just by doubling the speed, it now takes FOUR TIMES as much energy to push the car down the road.
This is why brakes fade and need to be designed bigger on vehicles intended for racing - when you slow from high speeds, you're dissipating a lot of energy, which turns into heat that needs to be dissipated.
Also, this is why I would much rather hit a wall at 20 mph instead of 40 mph!
[Taking a deep breath]
One reason some manufacturers like to "lug" the engine at highway speeds is fairly straightforward:
1. Less RPM means less fuel injected on a per revolution basis, i.e. if an engine is spinning 50% faster, there are 50% more injections.
2. By running an engine below its most efficient area / below peak torque you can increase fuel economy by reducing pumping losses. Less RPM means less exhaust mass flow which gives your lower backpressure AND you have to open the throttle farther to make the same power (that you would running closer to peak torque). Since F.I. engines run closed loop fuelling, this doesn't use any more fuel: It lowers the restriction on the air coming into the engine.
Look at "displacement on demand": that's how it delivers fuel gains - running very close to WOT really lowers pumping losses and increases fuel economy.
One last disclaimer*
I've been doing this kind of work for about 20 years now with the last eight specializing in new methods for optimizing fuel economy.
rodslinger
Member
No you won't. EVER. No amount of gearing shenanigans can overcome the other laws of physics. Wind resistance being the big one.
sx sonic
New member
$ub$onic
The Sticker Guy
Sure you can, It's easy. Hybrid truck that relyes mainly on the electric motor
Touché
ahtoxa
New member
Yes. Agreed. It that was the case, don't you think manufacturers would be all over it? Meeting CAFE regulations in the next few years is a BIG deal and is a requirement. This is mostly why many are turning to producing smaller turbocharged motors, a-la BMW and upcoming Mustang.
SonicBoy24
New member
HollywoodTurbo
New member
Wind has a lot to do with MPG too.
I usually average 59mpg at 60mph and rpms stay pretty low. With a strong headwind that will drop to mid 30s with no change in RPM and with a tailwind my mpg will jump up to the low 70s. I've cruised at 75 with a good tailwind and was averaged 60mpg. Gotta take advantage of the 'good days'
I usually average 59mpg at 60mph and rpms stay pretty low. With a strong headwind that will drop to mid 30s with no change in RPM and with a tailwind my mpg will jump up to the low 70s. I've cruised at 75 with a good tailwind and was averaged 60mpg. Gotta take advantage of the 'good days'
