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How do 13"s affect your MPG?; Bigger tires less MPG?
Topic Started: Feb 6 2011, 08:01 PM (1,732 Views)
climbinghalfdome
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MPG OCD

Im sure that this topic has been hashed out before but I cant seem to find it in the search engine.
I have a 1993 Metro that has the stock 12s. I know that tires for these little guys are hard to find and tire for the 13s are more available.

Question 1 How would upgrading to 13s effect my MPG?

Question 2 What wheel / tire combo will yield the highest MPG?

Question 3 Do you know of any links or posts that deal with these issues?
Thanks
Kevin in the Grand Canyon
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mwebb
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FOG

..."
Question 2 What wheel / tire combo will yield the highest MPG?
"...


used 15" alloy wheels from a
Acura legend or one of the many other cars that have 4 x 114.3 pattern that were born with
15" alloy wheels

see the pinned thread that shows which wheels will fit your car

with
Low rolling resistance tires like
Bridgestone RE92 or Michelin Harmony or Goodyear ASSurance FE
the size should be
185 - 60 - 15 or 195 - 55 - 15

that is
15 not 13
compared to a 15" wheel tire combination anything in 13" will provide lesser fuel economy
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mcmancuso
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13's are better than the 12's it gives you the equvalent of the xfi transmission swap, maybe 2-5mpg depending on driving conditions better on the highway. Mwebb is correct though, if you go with honda or acura 15" you can get LRR tires easily and further drop rpms on highway driving. The xfi cam or 3tech econo cam and +6 gear are a very nice compliment to the larger diameter wheels and will give more power at low rpm, allowing lower shift points and even better mpg
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bogs
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Duct tape heals all wounds

climbinghalfdome
Feb 6 2011, 08:01 PM
Question 3 Do you know of any links or posts that deal with these issues?
Thanks
Kevin in the Grand Canyon
http://geometroforum.com/single/?p=299186&t=3548347
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Coche Blanco
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Troll Certified

See sig?
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Jim-Bob
Junkyard Engineer

Running a taller tire will cut your effective final drive ratio and in most cases will give you more MPG. This is true no matter what diameter the rim is. There are other mitigating factors though such as rolling resistance and aerodynamic drag. Rolling resistance can be reduced by designing the tire a certain way as well as by increasing tire pressure. Drag increases with width and rolling resistance can increase with width once you pass the point where the extra width is needed to properly carry the load. You also want to be careful of how wide a tire you use due to the increased resistance of the front wheels to change direction. This may not be a concern in a car with power steering but most Metros do not have PS and so a wider tire can make low speed steering very difficult. I personally wouldn't go wider than a 175 for my needs but then I deliver pizza with my car and so I need to make a lot of low speed turns. If you are on the highway for most of your driving then your needs may differ.
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GA Geo


Jim-Bob
Feb 7 2011, 09:29 PM
Running a taller tire will cut your effective final drive ratio and in most cases will give you more MPG. This is true no matter what diameter the rim is. There are other mitigating factors though such as rolling resistance and aerodynamic drag. Rolling resistance can be reduced by designing the tire a certain way as well as by increasing tire pressure. Drag increases with width and rolling resistance can increase with width once you pass the point where the extra width is needed to properly carry the load. You also want to be careful of how wide a tire you use due to the increased resistance of the front wheels to change direction. This may not be a concern in a car with power steering but most Metros do not have PS and so a wider tire can make low speed steering very difficult. I personally wouldn't go wider than a 175 for my needs but then I deliver pizza with my car and so I need to make a lot of low speed turns. If you are on the highway for most of your driving then your needs may differ.
:gp
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nerys
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Grr

it depends. larger tires does multiple things.

it increases your final drive effective gear ratio.

it increases rolling and rotating mass

it increase the "load" on your engine in each gear during acceleration.

if you live in the CITY this is a really bad idea. you want the tinniest tires you can get to maximize acceleration efficiency. (I mean CITY CITY like major metro philadephia etc.. city blocks lights at each block etc..)

anywhere else with proper driving it will usually result in HIGHER fuel economy. the reason is most of your driving will be "cruise" where acceleration loses are irrelevant.

with a lower gear ratio your will spin lower rpm's with a more open throttle. IE better fuel economy.

if you live in the mountains with LOTS of massive ups and down IE constant acceleration same as city you want SMALLER wheels not larger and lighter mass.

so it depends on what your typical commute is. City or rural?

do you drive like a bat out of hell? or do you drive like your grandma? if the later larger tires will help if the prior larger tires will kill your power and ruin your fuel economy.

also don't forget once you go "off stock" your changing your gear ratio WITHOUT telling your car your changing it. this means your odometer and speedometer will no longer be correct. they will now "under report" both your speed and difference.

you will have to figure our your multiplication factor to get your correct mileage to figure out your MPG's
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Good bye


I upgraded my 91 to rear disc brakes from a swift gt. I plan to install some 98 front brake/struts that I have and this gives me 12 mm studs. The factory 13" wheels including the mini spare fit over these brakes. There are so many choices for 13" tires, including low rolling resistance models from several manufacturers. Nice narrow tires are 155/80r13s and a little bit wider but close to the same height are 175/70r13s. I even have a pair of 185/60r14s on swift gt wheels on the car right now to run out some nice tires. These are also close to the same height, something like 22.6" tall. To make the speedo read correctly get a driven gear from a 95+ car that came with 13" wheels. Hope this helps
Andy
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mjspiess
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Yes this topic has been discussed before and is a fairly popular "upgrade" or "mod" done on the older '89-'94 Metros with 12" tires/rims mainly for the reason you described...12" tires are getting more difficult to find and/or more expensive and the bonus of gaining some hwy MPGs.

Question 1: How would upgrading to 13s effect my MPG?
Improve MPGs on your long 60 mph flat road commutes and possibly decrease your city mpgs, although if shifting at lower RPMs and getting into 5th gear ASAP, MPGs in the city will be just fine and possibly better. The XFi camshaft can help this process.

Question 2: What wheel/tire combo will yield the highest MPG?
Tallest/Skinniest tires that will fit in the wheel well assuming all you're concerned with is highway mileage.

Question 3: Do you know of any links or posts that deal with these issues?
These have already been noted.
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