Speed for maximum fuel efficiency by car

[purecomedy]

The old adage was that 55 mph maximized fuel efficiency in vehicles (that is why the US changed highway speeds in the 70's). Over 30 years there have been a number of changes in vehicles, here are a few:
1. Aerodynamics
2. Engines, use of fuel injection
3. Transmissions - more gears, taller top gear (overdrive)

I believe that these factors have changed it so that 55 mph is likely not the most efficient speed anymore. My questions are:
1. What is the new 55 mph for cars in 2006
2. What is the absolute best fuel efficiency speed for specific vehicles. Given that cars and trucks of different sizes and shapes are going to have different aerodynamics, wheel sizes, transmission ratios etc. the maximum efficient speed should be different per vehicle. Is there an equation that you can plug in certain parameters to calculate it for your vehicle?

[YARDofSTUF]

It varies from car to car.

[mountainman]

I still think 55-60 is the best. At least it was on our Xterra on the Colorado Road Trip.

[Illini25]

I still think 55-60 is the best. At least it was on our Xterra on the Colorado Road Trip.

Yeah, that range of speed is optimal for most cars. I usually dont see a dip in mpg until I hit 75mph....then it will slowly decline.

[Unholy]

I'd say whatever your speed is @ 3K rpm on your final gear.

[Illini25]

I'd say whatever your speed is @ 3K rpm on your final gear.

I can't remember off the top of my head, but for my car...that would be about 90mph, LOL. At 65, im at around 1800rpm's

[Unholy]

I can't remember off the top of my head, but for my car...that would be about 90mph, LOL. At 65, im at around 1800rpm's

Damn you and your low revving high torque American Motor Vehicle.

[YeOldeStonecat]

It varies from car to car.

That's what I'd say...so many variables here.

Engine, gearing, wind resistance, rolling resistance, those all change quite a bit from vehicle to vehicle. Engines vary quite a bit....each type having its own sweet spot thats optimal for power output versus consumption. That has to be factored into the vehicle its powering.

[twwabw]

Hey YOSC.... what's the gadget on your dash? Sat radio? GPS?

[twwabw]

It varies from car to car.

Unless you have my F150 4x4... lol. It's the vehicle my wife drives, and she never speeds; never has her foot in it. Result? 13.5 mpg. When I drive, even on Freeways @ 80 mph, always into it- 13.5 mpg. I think I'm noticing a pattern here . (except towing the boat = 7.1 mpg... ouch!)

My Caddy actually does it's best between 70-80 mph. I avg between 25-26 on the highway.

[Bastid]

I can't remember off the top of my head, but for my car...that would be about 90mph, LOL. At 65, im at around 1800rpm's

i was going to say, the Suburban would be hitting about 95-100 if that were the case...

[morbidpete]

looks like sirius radio

[YARDofSTUF]

[quote="twwabw"]Unless you have my F150 4x4... lol. It's the vehicle my wife drives, and she never speeds]


I hear that, best fuel efficiency on the firebird is at 0 mph. Probably about 75 for the altima.

[fastchevy]

Probably 70-75 for me...gotta love that 6th gear

[purecomedy]

My Honda Accord V6 (not American) is around 2200 rpm @ 75 mph I think.

I guess I was looking for more of a scientific answer than "it depends" or "it's complicated".

I think that there should be a formula that looks at just a few factors

1. Rolling resistance of tires
2. A formula that describes aerodynamics by speed
3. A formula that describes the way an engine and transmission perform, including efficiency etc. It can probably be relatively simple, we know that we'll be in the top gear so it is really just a question of engine efficiency and amount of energy actually making it to your wheels.

I expect 1 to be relatively constant at any speed, aerodynamics should probably penalize performance on a square of velocity basis and the formula to describe the way an engine produces power to the tires I have no idea (as long as you are driving in a reasonable range I think this efficiency number should be around 25%).

[Illini25]

Hey YOSC.... what's the gadget on your dash? Sat radio? GPS?

Who?

Sirius Sportster plug and play unit. I just detach from the car docking station and slap it into a boombox. Works great!

[twwabw]

Who?

LOL- Must have been a little forum dyslexia...... hahahaha. Saw YOSC's post, and I guess I translated that to yours! Anyway, I kind of liked having the Sat radio in my car for a few months (built-in) but it seemed kind of a waste being tied only to my car. Does it transmit to your radio, or do you have to hard-wire it to an input?

[Illini25]

LOL- Must have been a little forum dyslexia...... hahahaha. Saw YOSC's post, and I guess I translated that to yours! Anyway, I kind of liked having the Sat radio in my car for a few months (built-in) but it seemed kind of a waste being tied only to my car. Does it transmit to your radio, or do you have to hard-wire it to an input?

That sort of unit transmits to my car radio just because it's a plug and play unit. When it's in the boombox, it's hard wire.

[Lefty]

Each car is different, in high gear its around 4k for me. I save a little gas by coasting too. If you know you have to slow down/stop I coast up to it and save a little more fuel. When driving in this state in the less congested areas I can go 100 miles without hitting the brake at all.

[purecomedy]

Each car is different, in high gear its around 4k for me. I save a little gas by coasting too. If you know you have to slow down/stop I coast up to it and save a little more fuel. When driving in this state in the less congested areas I can go 100 miles without hitting the brake at all.

The impression I am getting is that depending on engine size you will have quite a different rpm to get to around the 55 mph range. At the end of the day it appears to be aerodynamics that determine the final few mph (near 55 mph) for maximum efficiency. I think an internal combustion engine is about as efficient at 2000 rpm for a larger engine as 4000 rpm for a smaller engine just because fuel injection pretty much makes the engine work quite well. I'm sure an engine at idle (eg. 600 rpm) is not efficient (if I had to guess it is hard to get enough air).

[joecool169]

Pay attention to your tachometer. The slowest speed you can drive and stay in overdrive, (which on many cars is actually the converter locking up) is going to give you the best econemy. Now this also differs from car to car because some engines do run more effecient at a slightly higher rpm. Also note that coasting as was already posted does up fuel mileage quite a bit. Most newer honda's actually shut off the fuel injectors while coasting so you are using no fuel at all. Other manufacturers may also do this but I am not aware of it. Most of the time the slower speed is going to give you better mileage though, as long as you don't go overboard with it.

[purecomedy]

Hondas are confusing because they use variable value timing. Didn't know about the injectors turning off, but I know the engine can kick into a less aggressive mode with the variable value timing.

[joecool169]

Hondas are confusing because they use variable value timing. Didn't know about the injectors turning off, but I know the engine can kick into a less aggressive mode with the variable value timing.

Yes they actually use the regular valve timing when you are driving normal. The valve timing is then advanced when you put your foot into it hard. It gives you the effect of having a wild cam without having a wild cam.

[purecomedy]

All I know is that the strangest thing with my Honda is when I coast it sort of brakes rather than glides as most vehicles do. Hard to explain unless you've driven it before.

A good example is that I can go about 60 km/h (~40 mph) down a pretty steep hill and I will stay at that speed rather than accelerate. Obviously, if I kick it into neutral instead of drive I pick up speed easily.

[piston149]

All I know is that the strangest thing with my Honda is when I coast it sort of brakes rather than glides as most vehicles do. Hard to explain unless you've driven it before.

A good example is that I can go about 60 km/h (~40 mph) down a pretty steep hill and I will stay at that speed rather than accelerate. Obviously, if I kick it into neutral instead of drive I pick up speed easily.

In order to coast, you have to put the car in neutral. Otherwise you're not coasting, you're engine-braking, which is useful if you're driving a heavy truck down a steep hill. If you had to use brakes on the wheels in neutral,
the brake pads would get redhot and shiny and would become useless. You wouldn't want that obviously.

But coming back to your original question, for most subcompact cars, the optimum speed is the speed at which the engine gives the maximum torque at the highest possible gear. That is usually 55 mph by design. You can design the engine and the gear ratio to optimize the fuel efficiency at a range of engine rpm/car speed combinations. Check out http://www.fueleconomy.gov

[purecomedy]

In order to coast, you have to put the car in neutral. Otherwise you're not coasting, you're engine-braking, which is useful if you're driving a heavy truck down a steep hill. If you had to use brakes on the wheels in neutral,
the brake pads would get redhot and shiny and would become useless. You wouldn't want that obviously.

But coming back to your original question, for most subcompact cars, the optimum speed is the speed at which the engine gives the maximum torque at the highest possible gear. That is usually 55 mph by design. You can design the engine and the gear ratio to optimize the fuel efficiency at a range of engine rpm/car speed combinations. Check out http://www.fueleconomy.gov

I agree that engine breaking happens and can be helpful. The part that confuses me is why my car seems to have an extreme amount of engine breaking while my dad's Chevy Silverado seems to have almost none (and it could be towing something and actually would benefit from engine breaking).

I agree with your theory, maximum torque in the highest gear should be where the engine runs most efficiently with 1 big caveat....if the car was not subjected to any resisting forces. My car claims to hit maximum torque at 5000 rpm, I can't recall ever getting past 4000 rpm! I think you need to compare the slope of the torque curve to the slope of the air resistance curve. If increasing your speed by 1 mph increases air resistance more than it increases engine efficiency then you are actually losing.

[joecool169]

I covered it all in my post but no one was reading I guess.

[YARDofSTUF]

I covered it all in my post but no one was reading I guess.

You answered too quickly, this is the internet, we need 5 pages of debate before the correct answer is acceptable.

[joecool169]

Well move a long then, I'll copy and paste when we get to page 5.

[YARDofSTUF]