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Harbor Fright
2005 XT
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363 Posts
Discussion Starter #1
Considering several tires that are on average 25.5" in diameter- does this throw the speedo/odo way out of wack, or is it livable?
 

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Registered
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1,130 Posts
Im 1 inch smaller than stock

google search for that tire calculator it shows you how far off the speedo will be. If i remember correct, its not a big difference for me
 

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Harbor Fright
2005 XT
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363 Posts
Discussion Starter #3
We all grok the calculations- Q really is this: is it livable day-to-day?
 

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K.I.T.T. meet A.L.I.C.E.
2008 Forester XT 4EAT
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837 Posts
Install a GPS that shows speed on the screen and it's a moot point :lol: :roll:
 

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Premium Member
2004 Forester XT MT
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1,433 Posts
Running 225/45/17 and the diameter is 25". At 1.1" smaller diameter than stock, the speedo is off but not an issue. Do the quick math in head for most speed limits by adding 4.5" to speedo. The gain in torque is such that it eclipses the speedo inconvenience.
 

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Recovering Modaholic
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1,306 Posts
Northwest Tan said:
Running 225/45/17 and the diameter is 25". At 1.1" smaller diameter than stock, the speedo is off but not an issue. Do the quick math in head for most speed limits by adding 4.5" to speedo. The gain in torque is such that it eclipses the speedo inconvenience.
the gain on the ODO doesnt help though ;)
 

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Harbor Fright
2005 XT
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363 Posts
Discussion Starter #7
Northwest Tan said:
Running 225/45/17 and the diameter is 25". At 1.1" smaller diameter than stock, the speedo is off but not an issue. Do the quick math in head for most speed limits by adding 4.5" to speedo. The gain in torque is such that it eclipses the speedo inconvenience.
Thanks Martin- that's the kind of feedback I was looking for!

DOHCammin said:
the gain on the ODO doesnt help though ;)
So is this- I am also not crazy about racking up "the extra miles"...But it will probably be worth it.
 

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Ze flying Russian
2007 STI
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2,008 Posts
Long time ago I found a good tire calculator. I liked it very much and since it was in Russian I translated it. Check it out, you might find it quite helpful too.

Click here
 

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coming from a 4x4 and off road background I can tell you that .5" is no big deal.

most of the jeep cherokee guys I know will go up 4 or 5 whole inches in tire diameter without making any changes to gearing or speedo sender.

I went up 2 whole inches on my truck with no such changes, and find it perfectly livable.
 

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Premium Member
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1,113 Posts
DOHCammin said:
the gain on the ODO doesnt help though ;)
^^^X2^^^^ Adding mileage blows! Oil changes come quicker, MAJOR SERVICE comes quicker. Your drive train is indeed driving farther than your XT!! I'm suffering from this now and can't wait to go to the proper height tire.
 

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226 Posts
NahdaDamDyke said:
...the engine will top out before redline...
Huh? I'm confused. Does this imply the tach is run off the drive train somewhere past the diffs and not the engine (or crankshaft)?

(I'm not quibbling with the conclusion of staying close to OEM tire diameter; I'm just confused by this.) HPH
 

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Premium Member
'09 STI
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4,129 Posts
I guess I should have said running to large of a tire will keep you from reaching redline in fourth gear or whatever gear is a 1:1.
 

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you will reach redline in any gear. you will just be traveling at a slower mph at any given rpm compared to the stock outer diameter. you will also reach redline faster in any gear with a shorter tire diameter.

i wouldn't worry about service intervals coming faster either. the forester doesn't know what size tires you have on the car, thats why we get speedo error. if you travel 1000 miles (according to the odometer on your dash) on the stock tire size, you actually traveled 1000 real miles. if you did the same 1000 miles on a tire that was 3% shorter overall, you'd only have actually traveled 970 real miles.

even your mpg will be off if you go by the stock odometer, and you will have to add 3% to whatever figure you come up with. but you will end up with lower than stock mpg because at any speed, your engine will be turning higher rpms.
 
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