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02 OB sport, 2.5, 5MT, WRX seats/catback/rear bar, Hellas, Home Despot CAI and roof rack
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Discussion Starter #1
anyone got any idea of how far low speedo readings in these things run? i'm toying with idea of larger diameter tires for ground clearance and want to be able to calculate what % of speedo error i'll cause if i do.

also appreciate any input on strut, wheelwell, etc clearance for larger dia tires from anyone who's tried this.

helpful info and derisive jeers welcomed, as ever.

thanks!
bw
 

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'02 Outback Limited, '00 GT
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25 Posts
Speedometer error

I have noticed a difference of 3 to 5 mph difference between my speedometer and un-manned radar traps (those little radar trailers that simply post your speed as a warning). My speedometer always reads higher than the radar. This may be a function of poor calibration of the radar though.
 

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soon, an OBS
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you need to consider the effects on the anti-lock braking system as well as the effect on the transmission and differentials.

the SHO site has a good primer on this subject. speedo chart at the bottom will give you an idea of what to expect.

http://www.shotimes.com/SHO3tires.html

hope this helps.

-steve
 

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02 OB sport, 2.5, 5MT, WRX seats/catback/rear bar, Hellas, Home Despot CAI and roof rack
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1,572 Posts
Discussion Starter #4
thanks for reference. not sure how it might affect trans, diff, or ABS if all 4 tires are identical, tho...i suspect where you'll get into problems is if all 4 tires aren't same diameter. ABS isn't measuring actual road speed but relative speed of each wheel to the other 3 (and/or the VSS, which will also be affected by same amount). with a Sub, this isn't an issue since different diameter tires will fry the viscous center coupling on MT models anyway, or so i've read.

thanks a mil for link to chart. :)

bw
 

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'04 Subaru Impreza Outback Sport H-4 4EAT Platinum Silver Metallic
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195 Posts
This web page looked good, about possible Impreza wheel sizes:
http://wac.addr.com/auto/obs/wheels.html

I checked my speedometer on the freeway (70 mph mark), stock 16" aluminum wheels and the 205|55 Potenza tires, by using a GPS to track speed. Found it to be approx. 0.5 mile fast but didn't realize at the time my tire pressures were far below the recommended spec. of 32 psi front and 29 psi rear.
Front-left was at 24.5 psi, front-right 22.5, both rears were 25.5 psi. Now I'll need to recheck with "proper inflation".
 

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Formerly 04 Outback 3.0R VDC, now 2011 Mitsubishi Pajero GLS DiD
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I've checked mine against a GPS several times and at highway speed the speedo reads about 5% high. At lower speeds I think it is more like 10% high. I agree there's a lot for you to consider in trying to use higher profile tires to gain ground clearance, I'd probably consider a suspension mod using spacers before doing that.
 

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02 OB sport, 2.5, 5MT, WRX seats/catback/rear bar, Hellas, Home Despot CAI and roof rack
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1,572 Posts
Discussion Starter #7
I agree there's a lot for you to consider in trying to use higher profile tires to gain ground clearance, I'd probably consider a suspension mod using spacers before doing that.



your point is well taken. i like the tire size idea just because it increases ground clearance for EVERYTHING, incl hubs, and shd help mpg slightly. but there are probably too many compromises to get more than abt 3/8-1/2" clearance (3/4-1" wheel diameter increase) this way, and even that poses some issues. on to the spacers....

also planning on trying to weld up a brushguard eventually. i'll post fotos and info if/when i do.
 

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2002 Outback 2.5L
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26 Posts
my old Toyota Tacoma is calibrated with 29in tires and iwas running 31's and i know that i prob had between at least 3 to 5 mph error.
 

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Formerly 04 Outback 3.0R VDC, now 2011 Mitsubishi Pajero GLS DiD
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I've noticed a non-linearity in the speedo error now that I have the navigation GPS reading the real speed constantly. At 60km/h the error is virtually zero, same at 70, at 80 there's a 2km/h error, at 100 there's a 4km/h error and at 110 the error is 6km/h at 120 the error is around 8km/h. I would love to test this at higher speeds, but with 110 being the max limit anywhere in the state I'll have to be careful. Anyone else notice this?
 

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2004 Outback Wagon, Mystic Blue Pearl
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I have been amazed at the consistency of the Speedo errors in my different cars, I have noticed that almost all of the cars I have checked that had stock wheels and tires typically read 2 mph faster than the car is actually going, and this is consistent with different brands of cars, and different brands of GPSs.
 

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The EQ

This isn't too hard to figure.

Your Actual Speed = TheSpeedoSpeed * (NewDiameter/OriginalDiameter)


It's not a set mph difference, it's a percentage difference. So taking stock tires with a diameter of 21in (is this right? I'm pretty sure...) and new tires with a diameter of 23. This is a speed increase of 9.5% over what the speedometer lists.

So if the spd'o'meter reads 70mph, you're really going between 76 & 77 mph.
 

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2003 OB 2.5L
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The other issue with larger diameter tires is the change in effective gear ratio. You will be operating lower on the power/rpm curve. The car will be a bit more sluggish off the line. This will be a dissadvantage when towing or hauling alot of stuff on the roof.
 

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08 OBW, 03 TS
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I have put larger than OEM tires on both Subarus we own. Our TS came with 195/60/15 and we put WRX wheels with 205/55/16 tires on... that's about 3% bigger. Now the speedo is very accurate per the roadside radar displays, so I'd assume it was reading about 3% too high.

On my 97 OB I put 205/75/15 tires on to replace the stock 205/70/15... again around 3% bigger. Now my speedo is dead on per the roadside radar things. So, again, it was probably reading around 3% too high.

I gained 1/2 inch of clearance with the OB but mainly got beefier tires out of the deal, that was my goal more than the clearance. And I can't fit anything bigger than that, it'll rub. On newer Subies tires alone won't get you much because there's just not a lot of room. Next time a truck or SUV goes by with stock tires note how much more space they typically have to fit oversize tires... much more than us.

Steve
 
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