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Old 05-02-2010, 05:27 PM   #1 (permalink)
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Big mpg loss with summer wheels

Ok, maybe not that big. Two tanks ago, I swapped out my winter wheel set up, which is a set of stock Mazda 3 alloy rims in 17 x 6.5 size with 215-50r17 Yokohama iceGuard tires. I replaced them with the original Mazdaspeed3 wheels in 18x7 size, with the original Bridgestone Potenza RE050s. I figured that the upsize in wheels would have a negative effect on mpg, but I was hoping that would be somewhat offset by the lower rolling resistance of the shorter sidewalled summer compound tires. Bad assumption on my part! I understand that 2 tankfuls does not exactly make for a scientific study, but as my habits and commute haven't changed, then at least anecdotally I'm seeing almost a 9% drop in mpg, versus the previous 4 tanks. And considering the warmer, less dense air along with the presumably better (non-winter blend) gas, both of which should have helped improve the mpg, that 9% could arguably be low.

Ugh.

If I hadn't just put in a bid on a new house, I'd be placing my order for some lightweight 17 inch Enkei RPF1s tonight, purely for scientific reasons . I guess I'll just deal with the factory boat anchors for now. Hopefully, I'll be able to make use of my new ECU datalogging device and especially the advanced tuning capabilities it provides. I haven't seen any insights here on how to best take advantage, and plan my tuning accordingly, but hopefully the local recommended tuner shops can shed some light with their experience, and tuning expertise. Of course, that comes at a cost too. Well, I hope to be reporting some mpg improvement back soon. Back to the drawing board!


Last edited by Mifunego; 05-02-2010 at 05:33 PM..
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Old 05-02-2010, 06:18 PM   #2 (permalink)
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Doesn't surprise me that the 18's would cause you to lose MPG, not just because of weight (assuming you mostly do stop-and-go driving), but I'd be willing to wager the RE050's are more rolling resistant than the Yoko's.

If you do get those Enkei's tell us how the mileage goes with them! I'm curious to see how much effect the weight drop has on MPG's for city driving.
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Old 05-02-2010, 07:07 PM   #3 (permalink)
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Maybe you rotated them? But probably those tires are directional, so you didn't rotate?

Current wisdom is to never rotate side to side, and some say not even front to rear. They take a set and you drive a few thousand miles to reset them to the new rotation direction.

In any case - ya lost me on tire size. What size are the summer tires? Wide isn't good for FE. Narrow is better. For increased control, pump 'em up so they don't roll over sideways under G's.
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Old 05-04-2010, 12:12 AM   #4 (permalink)
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wider is better what is OD ?

as per the auto speed article
wider is better , for FE and every thing else as well
except drag

did you account for the change in OD of the tires ?
larger OD tire will cover more ground , but show the same value in the odometer , your FE numbers will be affected .
have you compared actual distance covered to measured distance covered ?

errors in the speedo can be huge and quickly seen by comparing speed on a GPS to speed on the speedometer
and
do not assume the odometer shows you actual distance covered

on your next long trip
compare your odometer values to the mileage markers on the side of the highway
======================
using your new scan tool
look at calculated load , never at absolute engine load
lower calculated load is better for fuel economy
most of the time

calculated load should get to real close to 100% at WOT up hill over 4k rpm
if not
suspect restriction to flow somewhere .
clogged cat or restricted air filter or air filter ducting
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Old 05-04-2010, 09:43 AM   #5 (permalink)
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Quote:
Originally Posted by mwebb View Post
as per the auto speed article
wider is better , for FE and every thing else as well
except drag
I'm pretty sure drag = lower FE.


The rims are heavier, due to being wider. Also, the weight is farther from the center, which has a CUBIC effect on drag. Ignoring the spokes and extra width, 17" rims have 20% more inertia resisting your accelerating force, compared to 16" rims.

I'd guess these are "sport" compound tires, good for grip but bad for FE. My brother's Mazda3 ran through the OEM tires in about 25,000 miles, which is a good sign of soft sticky rubber.
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Last edited by PaleMelanesian; 05-05-2010 at 09:32 AM..
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Old 05-06-2010, 09:53 PM   #6 (permalink)
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Quote:
Originally Posted by PaleMelanesian View Post
I'd guess these are "sport" compound tires, good for grip but bad for FE. My brother's Mazda3 ran through the OEM tires in about 25,000 miles, which is a good sign of soft sticky rubber.
While the 215-45r18 size stock tires are categorized as "Max Performance Summer" tires, they're not DOT racer-sticky, and they can't have nearly the rolling resistance of all the friction-generating edges in a well-siped winter tire. I've got plenty of tread left on my tires, although they are stored winters. They are stickier than those hockey-puck 70K mile tires though, that's for sure.

And measuring the correct size to correct speedo/odometer readings? Come on! That's just common sense. FYI, these particular winter tires measure at 99.052% of the stock tire size, based on odo vs. gps readings over long distances. And I have to say, Mazda did a heck of a job calibrating the speedo to the stock tire size. My GPS has confirmed amazing accuracy on long summer trips with the stockers.

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