Puzzling issue in gas mileage improvement

[Ketch22]

OK, so I need to bring this one to the forum.

I keep somewhat fanatical records on my vehicles. 69,450 miles ago there was a front end alignment done and the toe-in was set to +3/16" on the front end. My truck is a Toyota 4x4 which sees frequent off-road trips. Just over 1k miles ago I found some looseness in the inner tie rod ends, they were both showing ± 0.15". I opted to replace both inner and both outer rod ends (what the heck, they are cheap and I beat the truck a bit). I adjusted the toe-in to the same +3/16".

Starting immediately after this I observed almost a 2mpg improvement in Gas Mileage. I was kind of dumbstruck. I have been through all the calcs I can work out and I can find no connection between the resistance to wheel misalignment and that much more fuel consumption.

What have I missed? I have even been through a consideration of a difference between "winter fuel" and "summer Fuel" in my area.

 

[berkeman]

Tire pressure fixed as part of the work? Less offroad miles?

 

[Tom.G]

City vs highway driving? Additionally, the front wheels were either wobbling some with the loose tie rod ends or toed into take up the slack, thereby scuffing of the tires. It is unclear if that ±0.15" was the slop in the tie rods or the amount of toe variation. If that was the toe variation, that's roughly twice the toe specification, if that was tie rod slop, then much more. In either case it's not a cause I would attempt to mathematically 'prove'!

Perhaps the automotive guru @Ranger Mike can shed more light on this.

 

[russ_watters]

Summer vs winter outside air temperatures?

 

[OldYat47]

EVERYTHING can make a difference that small, including which shoes you wear (really, I've seen this on more than one occasion). How closely have you followed your mileage? I follow my mileage closely, and a 2 mpg difference is common, even though my driving patterns are pretty regular. And I've owned a lot of different cars.

(Aside: Back in the days of points, condensers and carburetors I followed my mileage to detect when a tuneup might be needed. I've been driving since 1962, a variety of cars and pickup trucks from dead stock to heavily modified hot rods).

 

[NTL2009]

EVERYTHING can make a difference that small, ... I follow my mileage closely, and a 2 mpg difference is common, even though my driving patterns are pretty regular. ...

I agree. It's very hard to pinpoint an "almost 2 mpg" delta. There are so many variables to driving. And also, "almost 2 mpg" out of how many?

... I can find no connection between the resistance to wheel misalignment and that much more fuel consumption. ...

Because there isn't one (or it is minor)?

What have I missed?

Maybe lots of things? Maybe nothing? This isn't a controlled experiment, I don't think it is answerable. If you put the car on a track, and drove lots of laps under precise driving profile, and repeated this several times, using graduated cylinders for a gas tank, and a fifth wheel to measure distance, you might figure it out.

There's an old joke about some guy that kept meticulous records on his mpg, and bragged to his friends how efficient his car was. They started adding a few gallons to his tank when he wasn't looking, and then he started reporting really great mpg, and boasting even more. So then they started siphoning gas out of his tank, so his mpg went to heck, and couldn't figure out what happened, but couldn't brag anymore either. Lots of teasing before and after the friends 'fessed up to the trickery.

 

[Ketch22]

Thanks all for the input. Mostly things that I have considered but some good ideas.

berkeman- Excellent question. Tire pressures are the same. I have aggressive offroad tires that are swapped on as needed. The onroad tires are quiet highway tires that are run at consistent pressures. I did just check them are they are still the same. No offroad miles at all in the observation period.

Tom G.- sorry about the ambiguity the numbers were the slop in the tie rod ends. You are exactly right I think the front wheels were wobbling from severe toe into light toe out. It always drove fine but when I checked it was all over the board.

Russ Waters- I had thought of air temps however the temps during the time observed were relatively consistent from low 60's to mid 70's.

OldYat47- excellent points, this was all with the same shoes and pressures. I monitor pretty close for the same basic reason as you mention. That and habit as I have been doing this for a while. Normally my mpg runs between 19.3 and 20.4 when I spend a lot of time in the mountains it drops to as low as 17.2. I did the repairs close after filling up. At the next fill up my mpg was 21.9 and each of the next 4 tanks was 22.2 to 22.6. I figure averaging to adjust for variations due to traffic and slight changes in driving patterns this is still notable. Kinda bugs me what happened, This particular truck I have maintained since new it is a 1990 4x4 with 389,750 miles on it. It is still in great condition but I know it pretty well,lol.

NTL2009- Good points, there may indeed be no connection. A coincidental occurrence is definitely possible. Still looking for it, Thanks for the joke however I don't brag on my truck too much.

 

[Tom.G]

A few random thoughts on possibilities:

Change in gasoline brand, also not all stations always get their gas from the same refinery line; sometimes it's whatever is available.
Different motor oil.
Thermostatic fan clutch failing. Is it running hotter?
You say a truck. Tailgate closed vs open/removed, cover over the bed? I hear that's an easy 2mpg. No experience though.

Oh well, at least you aren't driving that new Dodge Charger... full throttle it guzzles a TANK of fuel in 11 minutes (rated at 797HP)

 

[Ranger Mike]

Thanks Tom g. but guru goota know something to get that handle.


Suspension steering settings are determined by factory engineering data to give the best compromise between smooth driving / comfort and fuel mileage. Tire wear comes into it as well.

Toe In refers to the parallelism of the front tires in static state relative to the vehicle suspension center line. It is rated in degrees or fractionally such as 1/8” toe in.

Imagine each tire has a center line scribed on the tread surface. With the steering wheel locked at the perfectly centered position.

A vehicle having 1/8’” toe in would have the front of each tire 1/16” closer to the vehicle suspension centerline than the back of each tire. The total toe in is 1/8”.

When you have zero toe in or toe out the steering wheel feels floaty or will wander. You want small degree of drag or feedback to give you the proper feel.

At high vehicle road speed ( 60 mph) the front end toe In will change. The common grocery getter car has rubber (synthetic material these days) bushings sandwiched between the wheel control are (A-Arm) and the chassis mounting bolt to isolate vibration and noise.

These rubber bushings dampen vibration of suspension movement for a quieter, more comfortable ride. They also have a lot of give and are squishy.

At speed the toe in is reduced as the rubber bushing compress as the tire forces its way to a toe out position. You still end up with a little toe in just to kill off the floaty feel.



Race cars replace these rubber bushings with metal bushing with go “give” and run 1/8” toe OUT as the driver could care less about feel on the straights and want quick turn in at the corner entry. Toe out builds Ackermann which helps during cornering.



A vehicle having 1/8’” toe in would have the front of each tire 1/16” closer to the vehicle suspension centerline than the back of each tire. The total toe in is 1/8”.

What happened when you hit many pot holes or go off roading o the weekend with your 4 x 4 truck?



All vehicles today use things like ball joints and tie rod ends on steering suspensions. Race cars use rod ends but all of these components have the same thing in common. Both have a spherical ball or ball stud bolted inserted in a spherical cup mount. This is what connects various steering components together. Imagine your knee socket. Same thing. Ball stud and a knee cup to hold it. Ball joints and tie rods have these. So If you hit enough pot holes and jam the ball stud into the cup enough times you will wear away material of the cup and the whole connection will get sloppy.



Let’s say your right front wheel hits a lot of pot holes and the left front has a smooth ride over 30,000 miles. What happens is the left front is still in toe in specification but the right front now has 1/8” wear or slop in the tie rod end and is now toe out 1/16” scrubbing the inside of the tire at speed.

Now what happens if both front tires have the same pot hole wear? Both fronts go to 1/8” toe out setting and we have ¼” total toe out and huge tire scrub. If you have 4 x 4 tires up front that is a lot of scrubbing on the rinds.

It is hard to pick up this fact when checking the toe in of a vehicle at static position. Even worse (and its tuff to do) you can bend the tie rod link itself and put way more toe out in the front end.

But you should know you have a bad front end condition from the steering wheel feedback and looking at your gas gauge. Your gas mileage will drop big time with that much tire scrub.



One last thing on alignment. You absolutely must know the center lien of the vehicle suspension to do a good alignment. Do not go off vehicle centerline or even engine center line as both can be offset. You have to mark the ball joint to ball joint center line and use it.

 

[russ_watters]

I agree. It's very hard to pinpoint an "almost 2 mpg" delta. There are so many variables to driving. And also, "almost 2 mpg" out of how many?

I tracked my fuel economy with a spreadsheet for the first year or so that I owned my current car. It varies between about 22mpg and 30mpg depending on all of the multitude of factors listed.

 

[anorlunda]

I tracked my fuel economy with a spreadsheet for the first year or so that I owned my current car.

That's cool. I ought to do that. A sudden jump in mpg indicates that something changed; especially if the jump happens immediately after a trip to the repair shop.

I'm curious, how much does winter/summer change your numbers?

 

[russ_watters]

That's cool. I ought to do that. A sudden jump in mpg indicates that something changed; especially if the jump happens immediately after a trip to the repair shop.

I'm curious, how much does winter/summer change your numbers?

So I looked back at the data and it is actually closer to 3 years. Here it is:

Notes:

• I used odometer miles and fill-up gallons. I have an ODB II scan tool permanently mounted on the steering column, which gives me "current trip" (resets when you start the car) and "tank" data including mpg and ave speed. The mpg is from the odometer and gas fill-ups.
• Very low signal to noise ratio for trying to spot trends, except one:
• Strong correlation between mpg and average speed.
• The linear trend showed I gained 1mpg over the three years, from an 25-26 mpg ave.
• There is a distinct seasonal variation in the moving average, of about 3-4 mpg peak to trough.
• There is no winter drop-off in 2016, likely due to me getting a project 180 miles away and traveling there a handful of times that winter.
• I feel like there is a difference in mpg between "warm" and "hot" weather, but can't prove it from this data.
• A confounding factor on the seasonal average is two 30 mpg peaks adjacent to each other in June of 2013 (and repeating after that) must be the first year I drove 7 hours for my summer vacation.
• I get between 400 and 550 miles per tank; It's an 18 gal tank and I run it near dry before filling.

 

[jrmichler]

Here's the fuel economy chart for my truck for the last 8 years:

It clearly shows the difference between summer and winter. In fact, a ten degree F temperature change is almost exactly 1 MPG. Winter 2017-2018 mileage was worse than normal because almost all driving was in colder than normal temperatures with headwinds.

 

[CWatters]

69,450 miles ago there was a front end alignment done and the toe-in was set to +3/16" on the front end.

Just over 1k miles ago I found some looseness in the inner tie rod ends, they were both showing ± 0.15".

Did you check the toe-in again before replacing the worn ends? Perhaps it was way off.

 

[anorlunda]

So I looked back at the data and it is actually closer to 3 years. Here it is:

Thanks Russ. I think you inspired me to do a similar project. I enjoy post facto data analysis seeing what information I can extract.

 

[NTL2009]

If the car in question got ~ 20 mpg, and actually experienced a 2 mpg drop due to an alignment issue, wouldn't there be some clearly noticeable side effects? That's ~ 10% of the power of the car - a significant amount of HP. That much power being wasted would create heat, or vibration or something - it can't just go away.

Could that much added heat on the tires be dissipated through the air and road? I would think a dragging brake caliper that dropped 2 mpg would be smoking after a few miles at highway speed.

 

[ChemAir]

wouldn't there be some clearly noticeable side effects?

Yes. The most noticed effects would probably be in tire wear. Misaligned tires will wear far more quickly than tires running true. There are different patterns of treadwear associated with different types of alignment/maintenance issues that are fairly predictable. Also, engine temperature, transmission temperature will increase as well, but I don't think the gauge resolution is such that you would see this, though.

That's ~ 10% of the power of the car - a significant amount of HP.

Not necessarily 10% of the car's power, just 10% of the average power used over the tank full. Most/much engine power is unused except at maximum acceleration (at peak HP RPM). I only mention this because 10% of average power used is a smaller number than 10% of peak horsepower, and the heat dissipation issue is a little more realistic to imagine/calculate.

 

[NTL2009]

... Not necessarily 10% of the car's power, just 10% of the average power used over the tank full. Most/much engine power is unused except at maximum acceleration (at peak HP RPM). I only mention this because 10% of average power used is a smaller number than 10% of peak horsepower, and the heat dissipation issue is a little more realistic to imagine/calculate.

Right, it would be based on average power, which might be 20-30 HP? So 2-3 HP @ 10%, or 1500-2000 watts. My 2-slice toaster is 750 watts, so one of those on each wheel, continuously is a fair amount of heat. But I suppose at 30 mph and above, air could pull much of that away.

But wait - alignment isn't going to waste average power, it's zero while standing still, and minimal at low speeds. So to make up an average of 10%, it would have to be even higher at high speeds. So again, it just seems like it would be noticeable.

I will fall back on Occam's Razor (ouch!), and the 2 mpg is just noise. Perhaps a portion is due to alignment, but I really doubt that most of it is.

 

[Ranger Mike]

Empirical data is needed here!

This study was done in 1977 at Ohio Transportation center. Car was Chevy Vega ( quit laughing). And on bias ply and radial tires.

Slip angle is the plane of the tire relative to the direction of travel of the vehicle. Toe in and toe out are slip angles. The Vega had a total toe spec of ¼” which was typical back then with all the rubber squishy front end components.

For every 1 ° slip angle added, this equals 25% roll resistance.

For each 10% change in roll resistance figure on 1 to 2 % change in fuel consumption.

Here is the link



https://nepis.epa.gov/Exe/ZyNET.exe...ge&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL

 

[NTL2009]

...
For every 1 ° slip angle added, this equals 25% roll resistance.

For each 10% change in roll resistance figure on 1 to 2 % change in fuel consumption.
...

Well, I'm still laughing about the Chevy Vega, but is there any way to relate % slip angle added to the OP's measurements?

 

[Ranger Mike]

the only way to figure what you have on the front end is to find the suspension center line and measure the tire centerline relative to it. Or take it to a good alignment shop with the proper BEAR alignment machine. Just straightening the steering wheel and using a toe stick won't tell you the true orientation of the tires. The study i linked to has the data to support the fact that improper alignment means rolling resistance and hence, effects fuel consumption.

my sister blew the guts out of that Chevy Vega in down town Cincinnati and she sure was NOT a hot rodder. Maybe it wa s the banzi starts i did. i think the engine had aluminum die cast block that could not be re-bored and iron plated pistons. Banzi!

 

[Ketch22]

Thanks again for all the good input. It looks like we mostly centered on the same point as was of concern to me. That being that 2mpg was a 10% change in average energy. Thus the look for a cause.
Ranger Mike thanks for that link . I was not aware of that study good info.

As of today I found the issue and "Winner, Winner, Chicken Dinner is: NTL2009.
[QUOTE="NTL2009, There's an old joke about some guy that kept meticulous records on his mpg, and bragged to his friends how efficient his car was. They started adding a few gallons to his tank when he wasn't looking, and then he started reporting really great mpg, and boasting even more. So then they started siphoning gas out of his tank, so his mpg went to heck, and couldn't figure out what happened, but couldn't brag anymore either. Lots of teasing before and after the friends 'fessed up to the trickery.[/QUOTE]

I found out today that a friend (who borrows the truck about once a month and has for over a year) had a change of mind. He decided to throw in a few bucks when he borrowed it. The change coincided with the repairs and the last 5 tanks of fuel. He was not aware of my logs in the truck.

Guess now I am the joke. What was that bit about insufficient control of the experiment?

 

[Ranger Mike]

Ketch i remember that story. Was about the time cow magnets we the hot set up to gain more MPG. Aluminum foil on fuel lines. Spray the front end of the car with PAM to aid aero. 60 psi in the tires. but gas was 25 cent a gallon when i was pumping it! then the oil embargo. Gas was 75 cent a gallon say what!
i think grumpy jenkins put a big block chevy in a vega about that time.
who iron plates pistons?