Bad mileage myth?

[DocVijay]

So, just to see what my Prius could do, I drove very conservatively today to see how my mileage really has been reduced by moving up to 17" wheels. If you check my link in my sig, you will see that the wheels tires I'm using are 10 mpounds heavier each, so there's 40 extra pounds of unsprung rotating weight.

Before the swap I was averaging 52 mpg on my daily 120 mile commute. After, I dropped down to aroud 42 mpg, but that was entirely due to my much higher average speeds (80 mph+) and aggressive driving style.

Now first let me qualify my conservative driving by saying it's nothing close to the way other members drive. It would still be considered very unefficient by many. However, this morning I was able to average 51.3 mpg on the first half of my drive.

So basically I only lost about 0.7 mpg.

When you consider that and that I could NOT get below 40 mpg by driving at extremely high speeds and always at full throttle acceleration, I don't understand how people are getting dismal mileage.

Anyway, that's just my experience.

[DaveSheremata]

Originally posted by DocVijay@Mar 13 2006, 09:02 AM
So, just to see what my Prius could do, I drove very conservatively today to see how my mileage really has been reduced by moving up to 17"

...
So basically I only lost about 0.7 mpg.
...
When you consider that and that I could NOT get below 40 mpg by driving at extremely high speeds and always at full throttle acceleration, I don't understand how people are getting dismal mileage.

Anyway, that's just my experience.

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Wow - I'd love to see how the 17"s look - got pics? (I'll check for your earlier posts...)

As for general mileage - you are in Tampa, right? Warm temps and mostly flat? That's very different than the low 30 temps and hilly terrain I've been struggling with to get up into the 50 mpgs here...
it was really nice this weekend to get into the 70s and not have to work so hard to get better economy. I was was getting 48 mpg without trying, mixed highway and city, but lots of hills.

Cheers!
Dave

[ceric]

I figure that your commute is mostly long distance highway. The heavier wheels would not affect your MPG as much as local city commute (due to repetitive accelleration - spin up heavier wheels). Yes, the re-gen braking may recapture some, but that is far below 100% efficient (you know that).

[Allannde]

Originally posted by DocVijay@Mar 13 2006, 06:02 AM
So, just to see what my Prius could do, I drove very conservatively today to see how my mileage really has been reduced by moving up to 17" wheels. <snip>
Anyway, that's just my experience.

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How are calculating your mileage? If your new wheel/tire combination has a different circumference, your odometer is giving you a different distance measurement. You will get a number but it is not necessarily correct. Do you have a way to check the accuracy of your odometer? WA state has mile posts along the highway.

Allan de

[EricGo]

Originally posted by Allannde@Mar 13 2006, 12:26 PM
How are calculating your mileage? If your new wheel/tire combination has a different circumference, your odometer is giving you a different distance measurement. You will get a number but it is not necessarily correct. Do you have a way to check the accuracy of your odometer? WA state has mile posts along the highway.

Allan de

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Quite right.

If the "17 inch" refers to the tyre radius and stock is 15", then there is an (17/15) underestimate by the MFD of FE = to 13%.

Addendum ... I think it is under, and not over ... but this is confusing my sleepy brain. AFAIK, the Prius computes distance by axle rotation * a presumed wheel Circ, and then divides by directly measured fuel injection. So the same real distance with a larger tyre will take a smaller angular rotation, and be falsely calculated as a shorter distance than actually traveled.

[Marlin]

Deleted not quite relevant post.

(Missed the 17 inch wheels part)

[gschoen]

Make your trips short in cold weather, and 30s MPG isn't hard to acheive, if that's most of your driving.

[jbarnhart]

I find it easy to break the 40 mpg barrier and get worse milage! Just take your Prius on lots of short trips. Take my commute -- one mile, over an overpass (above a train track). Poor little Prius isn't even warmed up by the time I arrive.

But is this bad milage? Probably not. My other car gets 13 mpg average over this same route!

The "correct" answer is to walk or ride my bike to work. The weather has been too bad lately, but nicer weather is coming. Then my Prius fuel economy will magically jump!

[Tom_06]

Originally posted by EricGo@Mar 13 2006, 11:39 AM
Quite right.

If the "17 inch" refers to the tyre radius and stock is 15", then there is an (17/15) underestimate by the MFD of FE = to 13%.

Addendum ... I think it is under, and not over ... but this is confusing my sleepy brain. AFAIK, the Prius computes distance by axle rotation * a presumed wheel Circ, and then divides by directly measured fuel injection. So the same real distance with a larger tyre will take a smaller angular rotation, and be falsely calculated as a shorter distance than actually traveled.

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The 15" or 17" would be rim diameter. Normally you use a different (lower) profile tire when upsizing the rims. Generally you can get within a couple of % of the original outside diameter. This is in the ballpark of tire pressure changes, different "same" size tires, or tire wear changes. Sites like Tirerack have calculators that will show the % gain/loss.

The main problem in plus sizing is that you are more likely to damage the rims and tires on potholes with the lower profile tires. Running higher pressures helps alleviate this. The handling is definitely improved (less side flex in the tires on cornering) and most people like the appearance (although I personally think it starts to look stupid when taken to extremes).

Most handheld GPS units give a very accurate speed and distance readout if you get a decent number of satellites locked on. This is great for evaluating how accurate your speedometer and odometer is before and after any tire change.

- Tom

[andreaswin]

nice look .-)

and love Your mod tread!!


ive been thinking of changing weels aswell when summer starts

ive been thinking of the O.Z racing rimms , the superturismo GT

this are suposed to be superduper light :-)

friend of mine had older model on his alfa romeo , and they were amazingly light


http://www.ozracing.co.uk/Superturismo_gt.htm


just have to get the spec on them and se if they fit and exact weight



cheers

Andreas

Originally posted by DocVijay@Mar 13 2006, 04:02 PM
So, just to see what my Prius could do, I drove very conservatively today to see how my mileage really has been reduced by moving up to 17" wheels. If you check my link in my sig, you will see that the wheels tires I'm using are 10 mpounds heavier each, so there's 40 extra pounds of unsprung rotating weight.

Before the swap I was averaging 52 mpg on my daily 120 mile commute. After, I dropped down to aroud 42 mpg, but that was entirely due to my much higher average speeds (80 mph+) and aggressive driving style.

Now first let me qualify my conservative driving by saying it's nothing close to the way other members drive. It would still be considered very unefficient by many. However, this morning I was able to average 51.3 mpg on the first half of my drive.

So basically I only lost about 0.7 mpg.

When you consider that and that I could NOT get below 40 mpg by driving at extremely high speeds and always at full throttle acceleration, I don't understand how people are getting dismal mileage.

Anyway, that's just my experience.

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