MPG experiment

[B-52]

I wanted to see what benefit speed / driving style could have on MPG. A business trip over the past 2 days gave me the ideal chance to experiment. I was shocked enough at the results that I felt olbiged to share them here.

First thing yesterday morning, I filled to the top of the neck and headed out to visit a customer in south GA, about 380 Interstate miles away. On this stage of the trip, I drove "normally"; 77-85 mph, aggressive passing, uphill accelleration, whatever. One stop for food right off the Interstate. Got to where I was going and refilled to the top of the neck. 385. 4 miles / 24. 6 gallons = 15. 66 mpg.

On the return leg of the trip, I limited my speeds to 66-68 mph and concentrated on driving as smoothly as possible. Mostly hung out in the right lane with the semi's, passing only when needed. Tried to pre-plan passes to get them done on the downhills. No uphill acceleration. Stayed overnight in Atlanta and visited another customer this morning. As I got back home today, I refilled to the brim once again - 385 miles / 20. 1 gallons = 19. 15 mpg. 3. 5 mpg or 18. 2% better - same route, same weather, same traffic conditions. The only difference was me.

A couple of thoughts and observations:

First, I'm not posting this to "brag about" an MPG number. I'm posting it to help my fellow enthusiasts take some dollars out of the oil companies pockets just like I am trying to do - the numbers are only data points.

As I was planning this exercise, I was dreading the "slow, boring" part of the experiment. Ironically, I found it to be more a interesting drive because of the concentration required to drive smoothly. At the same time it was much less stressful because I wasn't constantly cursing someone for blocking the left lane.

Beyond the MPG difference, I did some further number crunching. If I applied the better MPG to the entire trip I would have saved roughly $25 in fuel cost.

Although as noted in my sig I have a Quad XZT, it was disconnected for this entire experiment. Next time I have a similar trip to make, I'll run the same experiment with the XZT coinnected and record the results.

Cheers,
Mike

 

[BNH4221]

Mike,



I can corroborate those numbers.



I drive about 100 miles round trip to work. Used to drive between 80-85, left lane all the way and averaged perhaps 15 MPG.

Then I slowed to a steady 70 using cruise control, right lane, minding my own business. Now average a steady 18-19+ MPG.



For me, the biggest benefit was the reduction in stress. Driving fast takes a TON of concentration. Drive slower, relax and enjoy the scenery a little. The extra MPG was an added benefit. Make my commute all the easier.



I think it cost me an extra 10 minutes each way. Big deal.



Of course, every once in a while I have to blast out of the toll booths to clean those injectors out :-laf



Best -

Steve

 

[GFritsch]

Speed kills mileage.



On a recent trip with our big overhead camper through Utah(flat) we played with the overhead gauge while changing speed and reseting the overhead. (not reliable I know).



Going by memory, so ...



At 50 mph around 19mpg.



At 55 mph around 17 mpg



At 60 mph around 16 mpg



At 65 mph around 15 mpg



At 70 mph around 12 mpg



At 75 mph around 10 mpg



We learned that just slowing down made a big difference. I usually set the cruise at 60 and stick with the trucks in the right hand lane.

 

[TowPro]

on your way home from Atlanta it was down hill (its on the map, you live "below" Atlanta ), that's why you got better MPG.



I drive 90% highway, and keep it at 55-60, that's why I get such good MPG.

 

[CPG]

There is no doubt that ones right foot governs your economy. Also yes, the driveing is more enjoyable with less stress. On our reg cab 2WD we consistently get over 20 mpg if kept to 65-68 MPG. Wind is the biggest enemy of economy also.



Chuck

 

[pwr2tow]

When I drive 65 mph I get 18 mpg.

When I drive 55 mph I get 22 mpg.



Towing I see about 3 mpg increase driving 55 mph verses towing at 65 mph.

 

[PSchwering]

B-52, Intersting experiment. Do you have any estimate of the time difference going each way? Just curious, P.

 

[pwr2tow]

I got in a hurry and forgot to add.



The puker factor is alot less when driving slower and you would be amazed at the things you missed on the same route when you were driving faster. Slow down, save lives, see the country, live longer, be less stressed, save fuel & money, help the inviroment.

 

[Rocketdoc]

B-52,



No surprise, because aerodynamic drag forces are proportional to the square of the speed. Although 80 mph is only 33% faster than 60 mph, aerodynamic drag is 78% greater (of course there are increased rolling resistances and mechanical losses as well). My truck consistently gets 23ish unloaded at 55-60 mph, but only 19-20 at 75-80.



You didn't mention one other major factor -- tire pressure. I usually don't run max pressure (unless heavily loaded) in order to get a smoother ride, but it's well documented that truly under-inflated tires can rob several miles per gallon. On a low-drag vehicle, underinflated tires can especially eat into mileage. My Civic coupe with wide low-profile tires would do 36-37 mpg with 35-40 psi. If I dropped to 30 I'd lose 4 to 5 mpg. On a high-drag truck pulling a large frontal-area trailer, low tire pressure may not be as big a contributor to mpg loss, but if you want to max the mpg, keeping the tires on the high side helps.



As a side note, Even though the overhead console computers aren't too great for delivering absolute accuracy as compared to calculated mpg, they are still pretty decent for indicating relative mpg, and so are useful for comparing mpg at different speeds, as long as you do an average over some reasonable distance.

 

[KBennett]

Good experiment B-52.

I have seen the best mileage, 21 MPG-hand calculated, by keeping the tach at 2000 RPM and using my foot instead of the cruise control to control the engine.

 

[RonCar]

I did the same type of mileage comparisons in both my truck and my little commmuter car. The truck would get an honest 16 at around 60-62, with very little in town driving,at 70 it dropped to 14. 6, all hand calculated and revs per mile set on truck. My little car (95 Civic) would get 31 on the dot every week doing the same commute as truck, 34 miles one way. If I drive 60ish it jumps to 36!

Could the tires on the truck really be taking of 1-3 miles per gallon away?Thats what some people say, truck in signature. Thanks...

 

[JThiessen]

Stop and go routes-vs-highway, or least obstructed routes will also yield significant differences. The difference for me going to work one route vs another is almost 4. 5 mpg. (and its virtually the same distance travled)

 

[Cattletrkr]

Driving my truck loaded at 70mph I can get it down to 9MPG, sometimes less. If I keep it around 60mph I get closer to 11mpg. I do have a problem though. I can't pull my trailer at 60mph in 6th unless the ground is flat.



I did a short EMPTY mileage check a couple weeks ago. Aprox 40 miles one way with combined conditions. 10 miles of freeways, the rest up and down, turn here, turn there, you get the idea. I kept it at 65mph--1975rpm on the freeway and just under 60mph--1825rpm on the rest. I filled up at home before I left and when I got back. 18. 3 mpg. I've done so much screwing around with the Juice lately I don't remember what setting that was. That's the best mileage # I've ever gotten for this 8900# monster.



For those of you trying to figure out my speed/rpm #'s, I've got 4. 10's and 245/19. 5's so the speed #'s are gonna look a little goofy to you.

 

[Vaughn MacKenzie]

My '96 Ram is a mystery to me sometimes. A lot of times I'd keep it around 70 hoping to get decent mileage (going slower isn't realistic when you're driving through remote territory on the freeway) and I often get mid 19s with it. Well a couple of times I have driven it hard where I was in a hurry and driving upper 70s to around 80 for a long distance (at least 1/2 to 2/3 of the tank) and when I fill up again and calculate by hand I still got 19 On that particular tank I was westbound, so no tail wind (it was calm that day).



The two times my '96 has broken 20 I was going around 72-73 mph. Truck seems to get better mileage there than at 68-70 for some reason.



Vaughn

 

[B-52]

Thanks to all for the good input.

B-52, Intersting experiment. Do you have any estimate of the time difference going each way? Just curious, P.

Yep - the trip there took about 5. 5 hours, while the return trip was approx 6. 25 hours. So about 45 minutes longer on the return trip.

Rocketdoc - Thanks to lots of Kevin Cameron articles over the years, I'm somewhat acquainted with the relationship of speed vs. aerodynamic drag vs. road horsepower. I've seen the ad's in TDR mag for AirTabs. I understand the principle behind them and have seen them on RVs and semi trailers. Been wondering if there might be some small value to using them on a pickup to clean up airflow at the rear. Anyone have any opinions about whether this is plausible and what placement might work - rear window, tailgate? Or does the shape of the truck not lend itself to such improvements?

Cheers,
Mike

 

[betterthanstock]

Anyone have any opinions about whether this is plausible and what placement might work - rear window, tailgate? Or does the shape of the truck not lend itself to such improvements?

I think if you don't have a shell, a bedcover is the best thing at speed. It will get you about 1/2 mpg at 70mph.

 

[DieselFiend]

I did something similiar. Recently I went from souther idaho to west texas. Averaged 18. 5 at around 80 the whole way. Here's what I learned. Use your cruise control. It will give you the most effecient use of your engine no matter what speed you choose to drive. I was fairly conservative on the peddle but let the cruise do the work. Only one variable in there though. On the interstate, going acrossed wyoming, the wind was at my back. This helped out a lot. Had cross winds going south. It should be interesting to see how much running against the wind effects my MPG on the way back. Post more later.

 

[JThiessen]

Based on what I've read, air tabs dont do much on pickups. A far as a shell or bed cover, I think mine is actually a little worse with the cover closed.

 

[Rocketdoc]

Thanks to all for the good input.

Rocketdoc - Thanks to lots of Kevin Cameron articles over the years, I'm somewhat acquainted with the relationship of speed vs. aerodynamic drag vs. road horsepower. I've seen the ad's in TDR mag for AirTabs. I understand the principle behind them and have seen them on RVs and semi trailers. Been wondering if there might be some small value to using them on a pickup to clean up airflow at the rear. Anyone have any opinions about whether this is plausible and what placement might work - rear window, tailgate? Or does the shape of the truck not lend itself to such improvements?



Cheers,

Mike

Mike,



I've read the articles on the AirTabs, and in principle their application sounds logical engineering-wise, and they claim to have objective test data indicating they work. HOWEVER, even though (or maybe because) I'm an engineer I always look for some sort of sanity check, so I would ask myself this: Given that high fuel costs take away a lot of profit in the commercial trucking industry, why don't we see AirTabs on the back of each and every semi-trailer owned by everyone who wants to keep more $ and thus have an edge over his competitors? Answer: 1) either they don't work, or 2) they haven't been properly marketed to transport companies. I wouldn't think their initial cost would be much of an issue, especially in bulk quantity, so I can only assume that the folks who spend hundreds of thousands or millions of dollars a year on fuel haven't been convinced that they can save them money. I'm definitely not saying that they don't work, but why don't we see them everywhere if they do???

 

[Cattletrkr]

Mike,



why don't we see AirTabs on the back of each and every semi-trailer ... Answer: 1) either they don't work, or 2) they haven't been properly marketed to transport companies.

3. They'll break off every time I back up to a loading dock.



They go on the bottom, not the top right?