Statistics show, slowing down saves fuel (long)

[TboneMan]

OK to start off, I'm not a statistician, but I've had enough exposure to know a few things (maybe just enough to be dangerous). So, any REAL statisticians feel free to chime in if I'm off-base.



All of the talk about MPG's on this board made me think that it might be fun to run some numbers through SAS (as staticial analysis software package). I've tracked every ounce of fuel that has gone into my truck and every mile driven since Dec. 2000. I've also note when I've towed and when I've made modifications to the truck (only two that I tested).



Method: Using this data I loaded 6 fields into PROC GLM statistical model. The fields were:

MPG - this one is obvious

WINTER - I have two sets of tires, OEM 265/16's and BFG TAKO 285/16's I run in the winter. The values were Yes and No

TOW - indicates that the tank was filled during or following a trip where I towed a trailer. The values were Yes or No.

OIL - Back in 7/2003 I switch to Valvoline PB Extreme synthetic. The values were dino and synth.

EXHAUST - I added a 4" exharst (Rip's) in 4/2004. The values were oem and large.

SPEED - I qualified the speed traveled based upon driving habits on my daily commute. A speed rating of "slow" was assigned to tanks when I maintained 68 mph or less. All other were labeled "fast". All other things being equal, my gut feeling was that my attitude towards highway speeds had the greatest impact on MPG. I have made a point of slowing down to save fuel $$$ and it seems to have worked.



Now the results:

Total number of full-ups: 282

Towing: 258 no, 24 yes

Winter: 199 no, 83 yes

Speed: 195 fast, 87 slow

Oil: 191 dino, 91 synth

Exhaust: 55 large, 277 oem



Towing was an obvious factor in MPG averages:

yes: 12. 6638

no: 17. 5487

F value: 279. 86

Pr > F: <. 0001

SIGNIFICANT



Since towing was so significant I segmented the other results based on tow or no-tow. To keep this thread short (Ha-Ha), the analysis indicated that no other factors show significantly increased MPG when towing. All the remaining analysis listed is based on non-towing fill-ups.



We all know that speed plays an important role in fuel economy. Therefore, it was no surprise when I saw the following results:

Yes: 18. 4498

No: 17. 0903

F value: 79. 56

Pr > F: <. 0001

SIGNIFICANT



Next I looked at the relationship between MPG and the use of oversized tire and winter fuels. I was very surprised to discover that speed is a stronger influence on MPG than winter conditions alone.

Winter alone:

Yes: 17. 9070

No: 16. 7933

F value: 47. 09

Pr > F: <. 0001

SIGNIFICANT



Winter with FAST rating:

Yes: 17. 4831

No: 16. 5630

F value: 27. 84

Pr > F: <. 0001

SIGNIFICANT



Winter with SLOW rating

Yes: 18. 4747 (n=10)

No: 18. 4466 (n=77)

F value: 0. 01

Pr > F: <. 9362

NOT SIGNIFICANT



This appears to indicate that slowing down can offset some of the influence of winter conditions.



If you are still reading this thread, congratulations, I only have a few additional things to add.



The analysis I performed on both the oil and exhaust categories indicated that there was NO significant fuel savings when controlling for speed, towing, and winter driving. So the claims of better fuel economy are really just marketing hype, as when applied to real world situations, the gains are not significant enough for statistics to see.



To summarize, we only need to slow down to increase fuel economy. Since my daily commute includes only 17 highway miles, driving 67-68 mph versus 72 mph takes only one minute more to travel the distance, but I save a significant amount of fuel.

 

[EricBu12]

GREAT POST!!



Thats been my philosophy when trying to get better mileage.

I also track all the fuel MPG when I fill up. Amazing alot of people dont. For the small car commuter, nobody does itand nobody knows what MPG they are getting. If they only did, they would see how driving fast and zipping in and out of trafic is really costing them.



I saw an article in a big rig publication that states all friction and road resistance from weight is the determining factor up to 50-55 mph, above that speed, the aerodynamics or turbulence of the vehicle takes over as the most significant. I think what I read was like ... ... Every 10 mph above 55 reduces a % of mpg. I think it was like 5-10% but don't quote me.



I have seen this in my truck and my new heavy trailer I bought. I did a trip to Denver from Salt Lake City (solo) and I wanted to get there in a hurry so I went the speed limit (75) mileage was like 17-18 (can't remember exact. ) when I returned home, wanted to do a more sedate pace (around 65) and I got about 20. (also note. . winter time, snowing, etc) Including some around town driving here in SLC after I got home.



The test I did with my trailer, Went to So. Ca. desert... . i only went around 65 for safety, but the mileage was great. Mostly flat and some steep hills I got about 10-11. On the return trip, there was more hill climbing from sea-level, stair stepping up to 7000 ft in ut then up and down to 4500 ft at home..... I averaged about 9-9. 5 ... ... . If I had done a faster pace with this setup, (which is 23,000 combined) i wouldn't have gotten the mileage I did. Probably like 9 down to CA. and 7 or 8 back up. By the way, this was a 1400 mile trip.



Thanks for your statistics and testing.

Eric

 

[tgee]

I saw a good one at work this week. A customer came in for a tune up, ( set valves and injectors on his series 60 Detroit, W900 Kenworth, not the most aerodynamic vehicle) He was getting 7. 4 MPG> This truck averages maybe 5. 5 MPG. His deal was that he kept his speed low, also kept his RPMs low, kept the engine close to its rated max torque, where the engine is most effiecient.

 

[NIsaacs]

I agree that speed is a major factor in MPG. I pulled a 9,000 lbs load from AZ. to Montana several years ago with my "91". On the way up I traveled the short route, back road two lanes at 55-65 mph, also droped 3,000' in elevation overall. I got 14 mpg. On the return trip I traveled all freeway at 75mph, gained the 3,000' and got 10 mpg. The load was a 20' heavy duty car type trailer with a Ford 4x4 pickup, a medium wind resistant load.



"NICK"

 

[E7mack96]

I can attest to the big-rig statement. When I drove the 2000 mack, I could get nearly 8 mpgs. This was loaded to 80,000 one way, and empty back. The other drivers on the run couldn't believe it. This run was 3 loads a day over the same section of road. I let them watch me fuel up at the end of the days running for 3 consecutive days. Every day was within 1 gal of the other. (factoring traffic/wind,ect. that is pretty consistant)



Speed costs like 15-20% per 10 mph over 55 in a big rig.



For the record, when I drive this unit aggresively, it can get 4. 8 MPG :{

 

[bmoeller]

With the '95 Mack at work, I usually get 6. 1mpg with the bullrack and almost 7mpg with the grain hopper. This is 99. 9% empty on the return trip with the hopper and empty on the way to get the load with the bullrack.



I did make a run through Nebraska and S. Dakota several years ago w/ the same tractor. Round trip avg was 9. 5mpg on a 1250 mi. run. Bobtailed to Nebraska to pick a new trailer, went to SD to get a load, and came back grossing 77k. Had the cruise on 70-75mph on the way out. Maybe could've gotten better mileage, but had to stop several times to let the truck cooldown on the way back. The fan clutch was acting up.