225 to 235 .... Another "Tire size on a Soobie" question. (if therehasn't been enough already)

I recently replaced all 4 tires on my '00 Outback Limited. The original
Potenzas were toast; but 94K wasn't a bad run for those as OEM!

The OEM size by the manual is 225/60R-16 , as were the Potenzas. I upped
that to 235/60's, Falken ZE512's.

I had the chance to measure mounted OD on the 225 and 235 versions of
this as well as the (worn) Potenzas.

28.8" on the Potenzas (remember, they are down to "Lincoln's head" on
tread wear)

29.1" on a 225/60R-16 ZE512

29.4" on the (now installed) 235/60R-16 ZE512

The main reason for the larger size was gaining a little tire overlap
(rim size being slightly smaller than tread width) for a softer ride.

I felt it was a reasonable choice; as did the tire dealer. A few days
later I went to the local Soobie dealer to pick up some new floormats
and got my butt reamed by a mechanic for the tire upsize. He did say
that was standard procedure (as far as he knew) to always disuade even
minor size changes.

He did like the tires tho ;}

I could understand problems with major changes, but .3" ???

I've been upsizing tire size on vehicles since 1966, I know the pitfalls
and all of the usual fixes like new differentials if you go too far. I
realize the AWD system does put more into the "issues" but IIRC the
actual size of these 235 Falkens is in the range of 225's from other
manufacturers.


Did I screw up??

 

I don't think so. I think the mechanic is full of shit. As long as all
4 tires are of the same model/size
you should be fine, The possible problem is the clearance for the
chains and an possible issue
with the wider tire not clearing the wheel well liners at the extreme
positions of the steering
wheel.

I think the mechanic's gf/wife refused to put out that morning and that
he took it out on you.

 

When you change the tire size it effectively alters the gearing, and
also changes the speed sensor readings to the ECU and ABS controller.
Your speedometer will not be as accurate, ABS operation may be
effected, and the ECU will get inaccurate speed info.
That .3" difference in diameter translates to about a 1" difference in
rolling circumference which could be significant.
It is best when changing tire sizes to use a proper "plus" size that
will keep the same diameter and rolling circumference to within a very
small tolerance, usually 1% or less. There are tire size calculators
available online that can assist with this. Tire Rack has one on their
site. You can do a "plus zero" using the same wheel size, but you
can't arbitrarily go up to the next wider size tire.

 

BUT, even tires with the same nominal dimension are different diameters
between brands and models so rhetoric that this .3" difference is a problem
is pure bullshit. I got rid of the OEM Geolandars on my Forester and
replaced them with the same nominal size Goodyear Tripletreads and found my
speedometer was out by over 8%, I went back to Fountain Tire and they
installed the next size up to most closely match the original tires
diameter.

 

Good point, as I am considering going up in size to correct my speedometer
error. Tirerack shows the Forester stock Geolandar's tires as having a
diameter of 26.2". When I read 70 mph I am really going 67 mph. So if my
math is correct, I would need a tire of 27.4" for the correct reading.

I am also considering the Goodyear Tripletred. It shows 26.1" vs. 26.2" for
the Geolandar in the stock 215/60 HR16 size.

Which size did you put on the Forester?

Blair

 

215 60R16

 

That is the only size I see used for the 16" wheel on the Forester from the
start. I see for 2006 in Japan most Foresters come with 17" 215/55 R17 which
still has the similar diameter of 26.3"

 

BTW, you'll love the TripleTreads

 

In this case, I *did* do my research. Putting on a researched/measured
tire that just happens to be a 235 instead of a 225 turned out to be
better than putting on a 225 from a manufacturer that makes 'um bigger.

FWIW:
I hit a 10 mile 'Speedometer Test Area' today where it was reasonable to
slow down to an indicated 60 mph on cruise control and fairly tight
stopwatch action. Speedometer was 59.5 mph; odometer was 9.94 miles. I'd
say that was a pretty damned good match. It's possible the Soobie's
speedo was off before, but I haven't owned it long enough to tell about
the old tires.

Post FWIW:
Speedometer error is more common than most people think. I never
understood why my father never got a speeding ticket with his '83 Ford
Rander pckup. My brother later found out when we pulled off the stock
185/75-14's and put on 215/70-15's (about 1.7 " larger) and got almost a
perfect 60mph over 10 miles after the swap. Doing the math said he was
doing about 54 mph with an indicated 60. Now we know why.....

 

Take a look at the Miata Tire Size Calculator
( http://www.miata.net/garage/tirecalc.html )

This will calculate the difference for you.

 

The issue is not so much the radius of the tire as the cross section. If you
put a tire on a rim that's too narrow for it you'll distort the case of the
tire. This effects the handling of the vehicle and in extreme cases the
sidewalls move so much it feels like flat tires.

 

What would happen if you stick to the proper width but go down 1 size
in the aspect ratio,
say 195 50 15 instead of 195 55 15 ?