It never WAS the optimal speed, that was just political posturing. Yes, slower speeds means less wind drag, but fuel economy is a function of the overall vehicle and powertrain design and calibration. That’s why I’ve had cars that got better mileage at 80 than they did at 55.
And has been pointed out, driving at speeds much different from everyone else results in more accidents. It’s dangerous. Those clowns trying to hypermileage at 48 on an expressway posted at 70 should get the heck off and onto a surface street.
“chilehed”, you had cars that that got better mileage at 80 then 55 mph ?
Please, name them. Any Ramblers in the bunch ? That’s one of the few cars I know of that could go down hill at 80 mph and no faster then 55 mph up hill. Then and only then might it be possible.
Nothing trumps economy more then air resistance. Even a Corvette looses as much as 18 % efficiency from 55 mph to 80 mph. If you owned car(s) that did better at 80 mph, you need to submit these findings to GM or Toyota tout de suit.
After driving Ford E-150 vans more than a million miles on a 340 mile/day route and experimenting with speeds and mileage it seems that those trucks actually used more fuel when the speed was intentionally kept at or below 55 mph compared to 65 +/-. That was long ago and pre-EFI but the 4.9L 1 bbl engines with 3.40 final drives apparently fell into some ideal situation just above 60 mph. On interstates, driving at 75+ mph the mileage dropped dramatically. 17.5 mpg was fairly predictable for an end of the month average while driving somewhat consistently at 65 mph between towns. On the other hand, the 1990 S-10 with a 2.8L TBI engine would run at the 28 mpg+ range when driven at <55 on a federal parkway, 23 mpg at <65 mph. On interstates driving at 75 +/- the mileage dropped to well below 20 mpg. Aerodynamics, gear ratios, tire friction and other factors can make for significant changes in mileage. Experimentation will be needed for any vehicle in any particular climate and driving condition and driving style it would seem. But as some point speed becomes quite expensive.
Can you assume I’m not a character in a Charles Bronson Movie and address the question? There are a few roads left in the country where the speed you can drive varies from a few miles under the posted limit to a few miles above. Please assume I don’t want to kill myself - because then I would get 0 mpg - and would just look to modify my driving within a reasonable range. Sheesh!
Also, note that I cannot adjust the incline of the roads around here - it is simply beyond my powers. I realize that going up or down a hill affects your mileage. I’m really not (quite) that stupid. Please assume that my home and work have a fixed elevation difference and I want to try to save some fuel by driving those roads better. On my behalf, I promise to seek out commutes which are downhill both ways.
Your mileage will differ by car model!? Get outta here! Well, that explains all those funny numbers and abbreviations in all those car ads. Note, btw, that my OP did include my make and model and ask if there was a way to determine it for an individual car. And yet, a few of you posted to tell me that it was different for each type of car. Remember: reeding (not only the heads lynes) is fun-duh-mental!
A careful reading of the replies provides you with a complete answer. 55 is not, and never was, an optimal speed. MPGs decrease pretty much uniformly with speed for most cars.
So, to answer the OQ (original question), it turns out that the the “slowest speed in highest gear”., as several have posted, .is the right answer.
Higher gears get better mileage than lower gears, but higher speeds increase wind resistance. The higher gear has a bigger effect than the wind resistance. But once in your highest gear, there is no higher one - duh! - so you should minimize your wind resistance within that gear. Minimal wind resistance is the lowest speed. (For the mathematically inclined, the increase in MPG due to the gearing is approximately linear, while the increase in wind resistance is approximately quadratic.)
My only remaining question, probably for another thred, is how do you identify how many gears you have and when you’re in the top gear (for automatic transmissions).
“Higher gears get better mileage than lower gears, but higher speeds increase wind resistance”. @SundayDriver–this isn’t always the case. Truck drivers may often shift down a gear or two to maintain speed when faced with a strong headwind. Lugging an engine in high gear even at low speeds is less economical than downshifting to a lower gear on a steep hill. If you ride a 10 speed (or even a 3 speed) bicycle up a hill, going to a lower gearing ratio lets you maintain your speed with less effort that leaving the bicycle in its highest gear ratio. The same is true with an automobile engine going up a hill.
@Triedaq: You may be confusing gas mileage with maintaining speed. If a truck’s engine does not have enough power (or maybe that’s torque) to maintain a certain gear, due to heavy headwinds or upgrade or any reason, then he (or she) must downshift. The same applies to riding a bicycle uphill. That is, the rider simply isn’t strong enough to push the bike uphill while in the highest gear. However, in both cases, you are not travelling as far on a fixed amount of energy (that is, you have to expend a lot more energy to move the same distance then you would expend in a higher gear).
Let’s say you encounter a hill and are ascending the hill at 30 mph in high gear with the accelerator on the floor. If you shift to a lower gear and maintain the same speed with the accelerator only half way to the floor, you are traveling the same distance up the hill but using less fuel.
I suspect that you will not be able to maintain the same speed at a lower gear with the pedal halfway to the floor. But I must consider this point further (and discuss with my professors).
Gas mileage peaks at between 35 and 60 mph for vehicles. After that, ALL gas powered vehicles begin to show a significant drop in mileage. Most cars do better in the 55 to 60 mph range…
dagosa, did I say BETTER at 80? … yeah, I guess so. That was an overstatement, I should have said the same… It was an '81 BMW 320i. Yeah, no one will believe me, but I put a LOT of miles on it between Detroit and Miami (roughly ten round trips) and it got pretty much the same mileage at 80 as at 55.
Mileage isn’t just about wind drag, it’s also about how the engine’s designed. If the engine’s more efficient at the higher speed you can end up with better mileage.
“chilehed” An 81 BMW ? The only way to tell for sure is to compute your mileage regularly over an interval where the speed averaged 80mph. Then do the same over an interval where average speed was 55 mph. And, make sure he terrain was exactly the same. Otherwise, with all due respect, it’s just a memory from days gone by when " the older we are, the better we were". Great cars in their day though, I agree.
The 55 mph speed limit may be good for gas mileage but is a collossal economic waste of manhours lost. The state of Montana went along with enforcing it and a sheriff could stop you on the spot and fine you $5!!! for speeding and if you also paid on the spot he would wish you “Happy Trails” and you were on your way while the sheriff heading back in the other direction.
A neat way of keeping federal highway funding for a thinly populated state where driving 55 mph would put you to sleep.
Does it work to tell the cop who pulls you over for doing 85 is because everyone else is doing it, “I’m just keeping up with the traffic?” Will he let you off? Think NOT!!! Optimal speed is 60-65, they can pass me on the left or on the shoulder if they want that is their problem. HAMMER DOWN!!!
