reconciling auto P5 mpg claims

[pasadena_commut]

After reading a post by -pixy- (and a followup PM from her) it finally dawned on me: when people are citing City/Highway usage they are stating time ratios but the relevant measure is distance. This makes perfect sense since we can easily recall how long we sat in the car during different parts of a trip, whereas we'd generally have to look at the odometer to see how far the car went during those times.

Example based on -pixy-'s commute:
mpg city = 22
mpg highway =32
time on highway = 30 min
time in city = 30 min
distance on highway = 30 miles
distance in city = 5 miles
Claimed driving mix: "50%/50%" (this is by time).
Claimed average mpg 30.

Hard to see how to average 22 and 32 with equal weighting to get 30. The
thing is, that's not how to do it, this is how to do it.

gallons in city = (miles in city)/(mpg in city) = 5/22
gallons on highway = (miles on highway)/(mpg on highway) = 30/32
total gallons = (gallons in city) + (gallons on highway) = (5/22) + (30/32) = 1.165 gallons
total miles = (miles in city) + (miles on highway) = 5 + 30 = 35
total mpg = (total miles)/(total gallons) = 35/1.165 = 30.0 mpg

And that corresponds very closely to the claimed mpg. This is because her driving mix by distance was actually 14%/86%. This calculation is still an approximation (since there aren't really just two speeds and corresponding mpg values), but the formula fits her commute data well. It also fits mine.

Anybody else want to plug in their numbers?

EDIT: uncolored -pixy-'s name by request.
EDIT2: oops, the first edit mangled the 1st line of this post.

 

[i12drivemyMP5]

We would need to calculate driving habits & mod lists & elevation & ethanol content & octane rating of the fuel used & so on & so forth to get it any more precise. I'm sure the car has it's overall capabilities but the human interaction with it will cause wild variations due to many different factors. The only place you could get the same would be on completely identical cars driven in identical fashion. Anything else is riddled with variants.

 

[-pixy-]

Example based on -pixy-'s commute...numbers?

#1. I don't like being used as an example.

#2. I dont like how my name looks in red, can you change that?

#3. You have some major MPG issues, I think you need to invest in a Prius or a pogo stick.

 

[Robot Romeo]

gallons in city = (miles in city)/(mpg in city) = 5/22
gallons on highway = (miles on highway)/(mpg on highway) = 30/32
total gallons = (gallons in city) + (gallons on highway) = (5/22) + (30/32) = 1.165 gallons
total miles = (miles in city) + (miles on highway) = 5 + 30 = 35
total mpg = (total miles)/(total gallons) = 35/1.165 = 30.0 mpg

wow. (uhm)

 

[Salmon_Rob]

#1. I don't like being used as an example.

#2. I dont like how my name looks in red, can you change that?

#3. You have some major MPG issues, I think you need to invest in a Prius or a pogo stick.

(rofl)

 

[xvdarkyvx]

seems right to me. i get like 22 on the city and like 30+ on the freeway with my auto.

 

[jred321]

you also have to remember fuel burned at startup until switching to closed loop (or is it open? i never get that right) which is especially bad on a cold day. so if your commute is 7 miles vs. someone else's 50 mile commute, you're going to get worse gas mileage than that other person because you will go through the warmup cycle more often but your miles will be less, so you'll have crappier gas mileage.

also a big factor in mileage is your right foot. one person's normal city driving is another person's racing through the streets.

 

[pfive]

you also have to remember fuel burned at startup until switching to closed loop (or is it open? i never get that right) which is especially bad on a cold day. so if your commute is 7 miles vs. someone else's 50 mile commute, you're going to get worse gas mileage than that other person because you will go through the warmup cycle more often but your miles will be less, so you'll have crappier gas mileage.

also a big factor in mileage is your right foot. one person's normal city driving is another person's racing through the streets.

weather is a HUGE factor. my pfive does better when its warmer out as would all cars. my old jeep got 16.5 avg during the winter, but 19.5 in the summer....

allowing cars to idle more kill MPG also, this is why i never idle my car for more then 3 minutes unless driving.

 

[pasadena_commut]

#1. I don't like being used as an example.

Apologies - nothing negative was intended.

#3. You have some major MPG issues, I think you need to invest in a Prius or a pogo stick.

Kind of hard to pick up the kids from school or bring home groceries on a pogo stick.

 

[pasadena_commut]

weather is a HUGE factor.

Not so much in S. California though - we claim to have seasons but people from other parts of the country don't agree.

 

[djp5]

just think of all the fun you would have calculating the mpg of your pogo stick.

seriously, why does everyone make such a big deal about gas mileage? provided you already drive economically, you can't do anything about it. it's like counting every hair on your head...it will just frustrate you when the number goes down.

 

[pasadena_commut]

just think of all the fun you would have calculating the mpg of your pogo stick.

http://www.bpmlegal.com/wpogo.html

seriously, why does everyone make such a big deal about gas mileage? provided you already drive economically, you can't do anything about it. it's like counting every hair on your head...it will just frustrate you when the number goes down.

The main reason to keep tabs on a car's mpg is so that you can tell when something is wrong with it. If your mpg goes down, and your driving hasn't changed, then something needs service.

The tricky part with a "new" used car is that you don't know what the baseline is supposed to be. For instance, many moons ago I bought a used manual 323 and it gave a consistant 25 mpg on my daily drive. That seemed reasonable to me for a car of that size so I assumed that was what it should be. A couple of years later my mechanic decided that a car of that age (125k miles at the time) should have its injectors cleaned. After that it went up to 27 mpg and stayed there. Lots more power too.

That demonstrates the problem with a "new" used car - you hardly ever know what sort of mpg (and power) the car was getting when new. The best you can do is find out what other people are seeing, and if you're significantly below that, then there's probably something wrong with the car. This has been particularly challenging on the auto P5 because the reported mpg has been so incredibly variable.

The auto gives up at least 3-5 mpg with respect to the manual in city driving (which is a lot more than in a typical auto vs. manual comparison) but is pretty close in highway driving. I used 22 and 32 mpg in the example that started this thread and I think those numbers are pretty close for an auto P5 at least at normal temps and running on oxygenated gas (5-10% ethanol). And yes, the mpg for the first 2-3 minutes will be less when it's running open loop, or when climbing hills, or with more people in the car, or etc.

 

[Robot Romeo]

I think I get the average atomatic mileage too: 22/30. My mixed driving always seems to be around 25mpg.

 

[pfive]

im sure my mpg will improve more when i get the injectors clean and my egr valve. MPG is also important when u have a 70 mile commute. To some it doesn't matter, esp. if mommy and daddy pay for gas, but aside from knowing if something is wrong...crappier mileage from either of my cars can mean a cut back elsewhere in a budget.

 

[Protege52003]

The thing is, that's not how to do it, this is how to do it.

gallons in city = (miles in city)/(mpg in city) = 5/22
gallons on highway = (miles on highway)/(mpg on highway) = 30/32
total gallons = (gallons in city) + (gallons on highway) = (5/22) + (30/32) = 1.165 gallons
total miles = (miles in city) + (miles on highway) = 5 + 30 = 35
total mpg = (total miles)/(total gallons) = 35/1.165 = 30.0 mpg

drugs are bad.........

i just take the number of miles and divide my how many gallons my poor tank takes......i have a general idea of the % city/highway driving and can see how it compares with my normal driving.........

-R

 

[madcanvis]

28.7 at 33.6mph

Around 355 miles/fuel up +-5
Add 12.5 gallons/ fuel up +-0.25
Commute time approx 1.25 hours +- a schoolbus (you all know what I mean)
Commute distance 42 miles 1 way 5 Days a week

Beats the 94 Yota 4Runner coming in at 17.5 mpg!

Enjoy

 

[pasadena_commut]

28.7 at 33.6mph

Around 355 miles/fuel up +-5
Add 12.5 gallons/ fuel up +-0.25
Commute time approx 1.25 hours +- a schoolbus (you all know what I mean)
Commute distance 42 miles 1 way 5 Days a week

The 5spd has been reported to give much better mpg than the auto below highway, or at least expressway, speeds. There is apparently a much bigger difference than the "1-2 mpg" one sees quoted as the loss for a typical auto vs. manual.

Does your commute break down neatly enough to estimate % [distance, not time] at "city" and "highway" speeds? My impression is that if a 5spd drove the same "city" route as an auto it would return 25-27mpg where the auto would turn in 21-22mpg, and that they would be really close at highway speeds.

 

[solkjare]

My MPG is really bad. I got average around 15-17 MGP!!!! I got my P5 auto tranny 3 months ago. It had 64900 Miles on it. I put less than a thousand miles in 3 months mixing in city and highway (70:30) What i did is changed air filter, check tire pressure, add fuel injection cleaner and reset the ECU. No luck, nothing improve. What should I do? Help Please!!!

 

[Matthew1785]

i get bad gas milage end of story there said and done, theres been more threads about gas milage then there have been about turbo's

 

[Kansei]

My MPG is really bad. I got average around 15-17 MGP!!!! I got my P5 auto tranny 3 months ago. It had 64900 Miles on it. I put less than a thousand miles in 3 months mixing in city and highway (70:30) What i did is changed air filter, check tire pressure, add fuel injection cleaner and reset the ECU. No luck, nothing improve. What should I do? Help Please!!!

spark plugs (NGK ZFR5F-11) maybe spark plugs wires (OEM or NGK), and perhaps seafoam to steam clean the head and such.