Crazy(?) Mileage after New Spark Plugs

[ChefJRD]

A few weeks ago I had my 525i aligned and had a (second) new windshield installed, plus I replaced the spark plugs, put in a new water pump (German, Hepu I think, metal impeller), thermostat & alu housing, fog light bulbs and replaced the oil. So I was feeling good after finally getting a lot of things done.

The engine (or is it a motor??) runs smoother with the new plugs and I've noticed MUCH better highway mileage lately. On a few longer trips, I got approx. 28.5 and then 29.5 mpg!!! I had only hit a high of 26 mpg before. In town, however, it looks like I'm still getting 22 or so (and this tankfull seems to be really bad....).

Does this make any sense? Have I just been missing out on good highway mileage with old plugs? Is it normal for city driving to remain low? I forget the exact part number for the plugs, but they were the new version of the Bosch F7 LDCR, something like F7 LDCR+.

[632 Regal]

Im using the new + plugs too, the electrode seems to be a bit thicker. With old plugs it's very possible to have worse mileage.

City driving will always be lower than highway, so yes thats normal.

[ChefJRD]

I expected city driving to stay lower, but the thing is that city mileage hasn't changed, whereas highway mileage increased dramatically. I thought that city mileage would have increased too.

That could be normal, I'm just surprised!

[pyro]

A few weeks ago I had my 525i aligned and had a (second) new windshield installed, plus I replaced the spark plugs, put in a new water pump (German, Hepu I think, metal impeller), thermostat & alu housing, fog light bulbs and replaced the oil. So I was feeling good after finally getting a lot of things done.

The engine (or is it a motor??) runs smoother with the new plugs and I've noticed MUCH better highway mileage lately. On a few longer trips, I got approx. 28.5 and then 29.5 mpg!!! I had only hit a high of 26 mpg before. In town, however, it looks like I'm still getting 22 or so (and this tankfull seems to be really bad....).

Does this make any sense? Have I just been missing out on good highway mileage with old plugs? Is it normal for city driving to remain low? I forget the exact part number for the plugs, but they were the new version of the Bosch F7 LDCR, something like F7 LDCR+.

I would be hella happy with 22mpg city i only get 15-16... But then again i Have m50 non vanos

[Akhil]

here in Houston I can get as low as 11 MPG :( to as high as 20 MPG :) depenging on what time of the day it is. Man this Houston traffic really sucks. :(

[SRR2]

Take good fuel usage data over the next ten fillups, THEN get excited about any increase in mileage. I have detailed data on cars I've owned for the last 20+ years. One thing that I can count on is unexplained temporary short term increases/decreases in usage. One tank (or five) does not make a trend.

In order of significance, the biggest identifiable contributors to statistically significant mileage changes are:
1. Major change in usage pattern i.e. more/less highway miles
2. Ambient temperature
3. Change from heavy dino oil to synthetic -- this was in older cars
4. Change to low rolling resistance tires (Mich Energy MXV4)

[Scott C]

A few weeks ago I had my 525i aligned and had a (second) new windshield installed, plus I replaced the spark plugs, put in a new water pump (German, Hepu I think, metal impeller), thermostat & alu housing, fog light bulbs and replaced the oil. So I was feeling good after finally getting a lot of things done.

The engine (or is it a motor??) runs smoother with the new plugs and I've noticed MUCH better highway mileage lately. On a few longer trips, I got approx. 28.5 and then 29.5 mpg!!! I had only hit a high of 26 mpg before. In town, however, it looks like I'm still getting 22 or so (and this tankfull seems to be really bad....).

Does this make any sense? Have I just been missing out on good highway mileage with old plugs? Is it normal for city driving to remain low? I forget the exact part number for the plugs, but they were the new version of the Bosch F7 LDCR, something like F7 LDCR+.

I don't think your range is that different from what I see - 21 (mixed city/hwy) and 30 +/- on flat highway at 70MPH indicated, which really means about 65 MPH.

Scott

[Akhil]

Take good fuel usage data over the next ten fillups, THEN get excited about any increase in mileage. I have detailed data on cars I've owned for the last 20+ years. One thing that I can count on is unexplained temporary short term increases/decreases in usage. One tank (or five) does not make a trend.

In order of significance, the biggest identifiable contributors to statistically significant mileage changes are:
1. Major change in usage pattern i.e. more/less highway miles
2. Ambient temperature
3. Change from heavy dino oil to synthetic -- this was in older cars
4. Change to low rolling resistance tires (Mich Energy MXV4)

SRR2,

Does low rolling tires tires increase or decrease milage? My car had 225/50R16 tire since I bought it. I also had a vaccum leak from not-return value until 2 weeks ago, which I fix. That added up about 3 MPG to milage.

Thanks
Akhil

[SRR2]

Ideally you'd want the least rolling resistance possible. The problem is finding out how to buy such a tire. The manufacturers refuse to disclose the data. The best data I know of came from Consumer Reports who did a comprehensive test on it a few years ago. The differences in losses was pretty dramatic. Rolling losses add up to increased heat, wear, and fuel consumption.

[Akhil]

Ideally you'd want the least rolling resistance possible. The problem is finding out how to buy such a tire. The manufacturers refuse to disclose the data. The best data I know of came from Consumer Reports who did a comprehensive test on it a few years ago. The differences in losses was pretty dramatic. Rolling losses add up to increased heat, wear, and fuel consumption.

has any rule of thumb that increase of some numbers (from 2 numbers in tire size) will increase or decrease the it rolling resistance?

thanks
Akhil

[SRR2]

No, I don't think so. Rolling resistance is more a product of the design, materials, and manufacturing process, though there probably is some second or third order relationship to size. You might want to visit Michelin's web site and look at their information on the "Energy" series of tires. They claimed that the design of the 'bands' caused less tread squirm etc. etc. etc. Pretty interesting and I've been happy enough with the tires that I'm on my fourth or fifth set of them. Not on the 540 though. 8-)

[Akhil]

No, I don't think so. Rolling resistance is more a product of the design, materials, and manufacturing process, though there probably is some second or third order relationship to size. You might want to visit Michelin's web site and look at their information on the "Energy" series of tires. They claimed that the design of the 'bands' caused less tread squirm etc. etc. etc. Pretty interesting and I've been happy enough with the tires that I'm on my fourth or fifth set of them. Not on the 540 though. 8-)

flater and wider tires are, higher its resistance, means lower the milage. Was I wrong?

[onewhippedpuppy]

I would think the larger the contact patch, the larger the rolling resistance/ kinetic friction. It could of course vary between tires, but I would think as a general statement this would be true. Ever look at the tires on a Prius or electric car? Very narrow with a small contact patch. If I remember right, most are supposed to run at 40 or 50 psi to minimize it.

Oh yeah, and I had an old F150 with a 5.8, changing the plugs in it gained me 4-5 MPG. Of course it had 80k on it, and they were original copper.

[SRR2]

Ah, I see your point. I really have no idea about the effect a wider tire would have on net rolling resistance. It's a good question.

Unfortunately the manufacturers, with only a few exceptions, don't tell you much about rolling resistance. If I were buying for your 525 I'd consider those Mich Energy series or the Pilot Sports. Do the PSs roll as easily as the Energy series tires? Who knows. The only thing I can say with certainty is that when I put the Energy MXV4s on the 325, there was a statistically significant step upward in mileage of about 1-1.5 mpg. Also, their performance otherwise has been very good and I wouldn't hesitate to buy them again.

[Akhil]

I will keep all this in mind when I will be shopping for new tire next time.

Thanks
Akhil

[SRR2]

Contact patch? Don't think it has anything to do with it. Reason is that the contact patch is dependent on 1) weight on the wheel and 2) tire pressure. Nothing more, nothing less.

Think about it... The pressure the tire exerts on the road is exactly equal to the pressure inside the tire. The weight on the wheel is what it is, and has nothing to do with the tire itself. The size of the contact patch is therefore weight / tire pressure .

x lbs / (y lbs/in^2) = (x/y)in^2

[Akhil]

The pressure the tire exerts on the road is exactly equal to the pressure inside the tire. (Newton 3rd law) But remember pressure is lb per sq inch, right? So wider tire has waider area means more sq inches. Does is make any sence?

[F4Phantom]

I think the answer lies in the co-efficient of friciton. The CE is the figure given to the rubber on a particular surface. Strictly speaking the CE is the same (for a given weight) irrelevant of the tyres width, pressure etc. As the pressure of the tyre drops, the contact area increases, but the friction on each contact unit decreases. So the total amount of fricition is the same all the time. F1 tyres are so wide for to give the tyres a longer wearing time not for more friction. Anyway we are talking rolling resistance which means the tyres are subjected to static friciton. This means a hard tyre pressure and a hard compund rubber is best for consumption but will likley be bad for just about everything else. Also SRR2 "The pressure the tire exerts on the road is exactly equal to the pressure inside the tire" is not totally correct as the rubber itself will level itself according to tyre pressure and the rubber compound hardness. So a contact area of a very soft rubber with hard pressure may actually equal a lower pressure of a hard compound. Then there is all the profile stuff to worry about, which I really dont know about, except the lower the profile, the less movement per rev and less heat build up, less pressure build up so that actually could mean more wear! I hope there is a tyre expert that can help to clarrify.