what speed gives you the best gas Mileage

[Atl XJ]

If you're talking about the freeway, the minimum posted speed of 45 MPH will return the best mileage.

ymmv

Try that without getting run over. I've had to limp a few cars home (over heating, would miss fire if you gave it too much gas, slipping transmission) going 45-55 mph on the interstate and, even though I stayed in the right lane, everyone was riding my ass, flashing their lights at me, etc. Truckers are by far the worst about it.

If I'm trying to get good gas mileage, I go 65-70. I'm barely even touching the gas pedal and turning under 2000 RPMs. I did that one trip and I got 22 mpg on my '95 4.0l AW4/231 XJ. My XJ is at stock height and I'm only running 235s on it though.

 

[Yooper]

If I'm trying to get good gas mileage, I go 65-70. I'm barely even touching the gas pedal and turning under 2000 RPMs. .

Not trying to be a idiot, but how are you doing under 2000 RPM's at 70 MPH?

 

[5-90]

Well...

Driving cross-country (a lot! San Jose to Indy...) I've found that getting optimal mileage isn't so much a matter of ROAD speed, but of ENGINE speed.

IOW, pick a cruise speed that keeps you running 2500-3000 rpm and you'll be fine. If you can't, pick another gear!

A few points -
An engine is most efficient when it's making peak torque. For the AMC242I6, that happens a little short of 3000rpm, but is also a "wide, flat" zone - rather than a sharp peak. You've got a little leeway there.
This trend toward lowering cruise crankshaft RPM doesn't seem to be working - just because you're engine's turning slower, doesn't mean it's using fuel effectively. Cruise RPM will ALWAYS be somewhere under 5252 (that's the point where HP passes torque output,) how much under depends upon the engine.
One thing I noticed - when I stopped using 5th gear on my 88 (Peugeot BA-10/5, 3.07 gears) I noted an INCREASE in fuel mileage/efficiency, simply because it pulled my freeway cruise RPM up from 2200-ish to very nearly 2800. A small difference, but worth noting. I think it was a "bonus" of about 3mpg, by the time it was all figured out (88XJ/4.0/BA-10/3.07/250kmiles.) When I regear, I'll select for cruising speeds hitting about 2600-2800 crankshaft rpm.
The vacuum gage installation is a good idea - max vacuum typically happens at or near peak VE and torque peak, which makes it much easier to locate in fact. Something else I'd planned on restomod - when I build the new gage cluster anyhow...
Keeping constant throttle is a good idea - IF you're at or near peak VE/torque output, ESPECIALLY if you're somewhere "hilly."
Tyres? Get a tread depth gage, and monitor wear. Measure at AT LEAST three places across the tread! If the centre is a little high, let some air out. If it's a little low, air up. When you find a pressure that gives consistent wear, WRITE IT DOWN! Not only will this give you nice round tyres with a consistent contact patch (important for efficiency!) but it will also save you a few bucks in road tyres.
Speed limits? Dunno about this one - seems to me, speed limits are set more as "revenue enhancers" than as "fuel savers," with the possible exception of yellow "advisory" speed limit signs for curves and such. Cruising speeds aren't set by law, they're set by mathematics and engineering, and neither of those ever follow Acts of Congress. I'd like to see speed limits on freeways rescinded, or perhaps go to a German model (Autobahn/U-bahn/City streets,) but that would take actually teaching people to drive, and not having driver training be "babysitting" for six-nine weeks. I've seen it, I've put two boys through it, and I'm not impressed. Once my boys got their licenses, I told them "forget everything you've learned, I'm going to teach you how to drive properly." No, they don't drive like mainiacs - they're probably the safest drivers in California...

Granted, I think the XJ could benefit from some aerodynamic improvements before it could be considered a "freeway machine," but for what it's designed to do (nearly everything,) I've found it to be surprisingly efficient.

5-90

 

[Atl XJ]

Not trying to be a idiot, but how are you doing under 2000 RPM's at 70 MPH?

Actually, I think you are right. I think by 70 I'm at 2100 RPMs. I'll have to remember to look next time I'm driving.

 

[BrettM]

I try to keep my MJ to 65 on flat highway, I get about 17mpg. My RPMs there are about 2600 if memory serves. I find that over 70 my mileage really plummets.

With my 95 Accord, I seem to get the best mileage around 75, though I haven't tested it very extensively between 55 and 65.

 

[hicksrcool426]

Actually, I think you are right. I think by 70 I'm at 2100 RPMs. I'll have to remember to look next time I'm driving.

that sounds right. im at 65 mph at about 2000 rpm, and 85 mph at 2500 rpm

 

[Alienspecimen]

You will get the best mileage if you take it easy during acceleration. This is the time when the gas mileage is the worst and its when 90% of the wear and tear happens...along with cold starts.
I have always driven stick shift and the rule of the thumb was that you get to fifth or highest gear, stay there and drive up to about sixty five miles per hour.
With the Jeep, that's all junk science. I have the computer console (which is regarded highly accurate btw) and automatic transmission and found out that you can drive on lower gear and get within twenty two, twenty four mpg range...which puzzled me, because this is the best I get on the highway.
I am going to have a long trip this weekend and will take some notes...but in a mean time, accelerate very slowly and brake lightly, don't go over sixty five ( according to the computer, there is no difference in gas mileage if you drive 55 or 65) and you will get the best mileage
P.S. A friend of mine claims an extra mile per gallon if he has the Thule coffin on top...I think it does not make sense, but you make up your mind...

 

[MaXJohnson]

lot of heresay, opinion and anecdotal evidence here...

I've found that getting optimal mileage isn't so much a matter of ROAD speed, but of ENGINE speed.

both, less drag at lower road speeds and less air & fuel pumped into the engine at lower RPM's.

An engine is most efficient when it's making peak torque.

more efficient at making power, not specific fuel consumption. Can you site any automotive authority or published study that will confirm that peak torque = peak fuel efficiency?

max vacuum typically happens at or near peak VE and torque peak

max vacuum typically happens at closed throttle. You're not likely to be making peak torque at closed throttle

Tyres? Get a tread depth gage, and monitor wear. Measure at AT LEAST three places across the tread! If the centre is a little high, let some air out. If it's a little low, air up. When you find a pressure that gives consistent wear, WRITE IT DOWN! Not only will this give you nice round tyres with a consistent contact patch (important for efficiency!) but it will also save you a few bucks in road tyres.

higher air pressure = less rolling resistance = better fuel mileage
A rock hard tire may not wear well or handle well, but it will help reduce fuel consumption.

Speed limits? Dunno about this one - seems to me, speed limits are set more as "revenue enhancers" than as "fuel savers," with the possible exception of yellow "advisory" speed limit signs for curves and such. Cruising speeds aren't set by law, they're set by mathematics and engineering, and neither of those ever follow Acts of Congress.

the former national speed limit of 55 MPH was set by an act of Congress, specifically to reduce fuel consumption and reduce traffic fatalities.

The original question simply asked what speed gives you the best gas mileage. Aero drag and rolling friction start to require a noticable amount of horsepower at speeds above 45MPH
Per the Bosch Automotive Handbook, horsepower to overcome aero drag on a sample vehicle with a cd of .35 to .50 and frontal area of 2 sq meters:
25MPH = .95 hp
50MPH = 7.4hp
75MPH = 25.5hp
100MPH = 59hp

Using figures for a Jeep Cherokee and including rolling resistance, I have calculated required rear wheel horse power at:
25MPH = 2.5hp
50MPH = 11.8hp
75MPH = 34.7hp
100MPH = 79.2hp

also per the Bosch Automotive Handbook:
10% reduction in weight = 6% fuel economy improvement
10% reduction in aero drag = 3% fuel economy improvement
10% reduction in rolling resistance = 2% fuel economy improvement

 

[MaXJohnson]

Always daft behind a 18 wheeler when you can.

unless you can draft within a distance equal to less than 50% of the vehicle wheelbase, aero drag is more likely to increase rather than decrease. Furthermore, under most circumstances, the lead vehicle benefits more from the draft than the trailing vehicle.

 

[MG_43]

Drafting a truck is the worst idea you could do, he can't see you and that can cause you large problems.

 

[jrgilchrist]

SO it is early and it;s probably that im lazy to figure it out myself but on a flat stretch of road say a highway what speed should i travel to get the most out of a gallon of gas.

After reading everything im more confused then i was before happier for it.


"As Intelligence goes up, Happiness goes down. See, I made a graph. I make a lot of graphs."

 

[JoBo]

The only thing in the article I tend to disagree with is the cruise control, though this may also depend on the vehicle. I think for one thing that it's possible to drive efficiently by varying your speed a good deal depending on circumstances. In hilly country, for example, if there's nobody behind you, there's really no need to sustain your speed up every rise, and no need to hold it on the way down. I don't have cruise, but my wife experimented with her Honda and found that her mileage went down with cruise, because it was always struggling to maintain speed, and since it couldn't see ahead, it would be surprised by changes in slope and have to do so abruptly. At least on that little low-powered car, in hilly country, cruise was not the best choise.

In the days before EFI, carbs dumped gas in like a shot from a squirt gun everytime you pushed the gas peddle. This could be where the got their idea about the cruise control from.

 

[Vertisce]

I drive about 50 miles to work each day. This morning I kept an eye on my meters.

At 60mph my RPMs are at exactly 2000. At 65 it rests just under the 2250 mark. At 70 it rests just above the 2250 mark. Finally at 75 it jumped to 2500 and started getting much much worse from there with each 5mph increment. I would say that my sweet spot would be 60mph but I usually run between 65-70 on the freeway.

I am sure that it varies from one Jeep to another but those were my figures as rough as they are. Does that help anybody?

 

[Bronco]

Its whatever speed you are going in your highest (numerical) gear (i.e. 4/5) at the lowest throttle position while maintaining velocity. As mentioned wind resistance increases with the square of velocity (while doubling your speed will increase your wind resistance by 4, 5x the speed = 25x the wind resistance... OUCH!), however, at the same time, you want to go as far as you can per each engine revolution.

Whoever said 0 (or parked) got it wrong also (only due to a deficiency in mathmatics though . Although you are saving gas, you aren't getting anywhere either. It's the old, numerator vs. denominator debate when both approach 0... is it infinity or is it 0....

... ok... Nerd... out....

...and the nerds have it!

 

[Eagle]

I drive about 50 miles to work each day. This morning I kept an eye on my meters.

At 60mph my RPMs are at exactly 2000. At 65 it rests just under the 2250 mark. At 70 it rests just above the 2250 mark. Finally at 75 it jumped to 2500 and started getting much much worse from there with each 5mph increment. I would say that my sweet spot would be 60mph but I usually run between 65-70 on the freeway.

I am sure that it varies from one Jeep to another but those were my figures as rough as they are. Does that help anybody?

Nope.

The figures don't add up.

Start with 60 mph = 2000 rpm. From that we can calculate that 30 mph = 1000 rpm, 15 mph = 500 rpm, and 5 mph = 166.5 rpm.

If (and I do mean "if," because factory speedos typically read about 5% higher than actual speed) your initial 60 mph number is correct, then at 65 mph your tach should be reading 2167 rpm, and at 70 mph it should be reading 2330. At 75 it should read 2500, but it would hardly be a "jump" -- it's a strictly linear progression.

 

[Vertisce]

Nope.

The figures don't add up.

Start with 60 mph = 2000 rpm. From that we can calculate that 30 mph = 1000 rpm, 15 mph = 500 rpm, and 5 mph = 166.5 rpm.

If (and I do mean "if," because factory speedos typically read about 5% higher than actual speed) your initial 60 mph number is correct, then at 65 mph your tach should be reading 2167 rpm, and at 70 mph it should be reading 2330. At 75 it should read 2500, but it would hardly be a "jump" -- it's a strictly linear progression.

Your math doesnt work. Or I should say vehicles dont work like that. As you gain momentum it takes less and less effort to KEEP the momentum. But...then the engine shifts gears and the math changes again. Im telling you what my meters read...and they ARE right.

 

[MaXJohnson]

Your math doesnt work. Or I should say vehicles dont work like that. As you gain momentum it takes less and less effort to KEEP the momentum. But...then the engine shifts gears and the math changes again. Im telling you what my meters read...and they ARE right.

now that you've put Eagle "in his place", tell us more about your amazing "gear shifting" engine with the exclusive "Momemtum Sensing" technology.

 

[Vertisce]

now that you've put Eagle "in his place", tell us more about your amazing "gear shifting" engine with the exclusive "Momemtum Sensing" technology.

Say what? Get a clue...when your engine shifts down your RPMs go UP and vice versa. Linear progression has nothing to do with RPMs vs MPH. I didnt even mention that wind resistance and other elements can factor in. He is the one that came and told me that MY numbers were wrong when MY numbers came straight from my meters.

 

[Jeep839]

i have noticed that as long as i keep the rpm's under 2500 then i don;t go threw gas like it's going out of style, i have 4.0 aw4 242 3.55 gears with 31x10.5 bfg muds 3in skyjacker just my 2cents

 

[jrgilchrist]

Still no idea how fast I should go to get the best MPG Im going to do some research and post the results