The Merkur Encyclopedia

Tire & Wheel sizing

The chart below shows the tire and wheel sizing that fits XR's with stock fenders.

Tire And Wheel Spec Chart


Neil's comments and summary of what wheels work on an XR4Ti is right
Thanks Neil for pulling all this experience together. I'll just add
some elaboration on my experience.

> right offset. Maybe. TSW says, "says" that their 16x7.5 with a 40mm
> offset fits an Xr. Ask Ryan Donavan. He runs these and reports that a
> very small shim must be run between the wheel and the hub or the wheel, yes
> the wheel will rub the strut tube.

Agreed. Same experience here. Fortunately, my dealer agreed to a
test fit and exchange if there were any problems, so I didn't get
stuck with a misfit. For street I'm running TSW 16x7.5 with a +35mm
offset, and Yoko AVS-I 205/50-16.

> Now somehow Roland got the TSW in 16x7.5 in a 35mm offset. Special order I
> guess. Now that is a better fit. No inner clearence problems but he does
> have some slight rubbing cutting of the tires from the wheel trim. But his
> car is a little lower than Ryans'.

My Hockenheim R were off the shelf as I recall. The castings are
made for both 4 and 5 hole application, so it's just a matter of
how they're drilled. Had them for two years and so far, so good.
They aren't real race wheels, but are adequate for the street and
track event usage I got them for. With all the wheel changing at
autocross, they're getting pretty worn, so this spring I plan to have
Bill Cobb at Racing Wheel Service install steel inserts in them.

Yes, I did get some slicing of the outside shoulder of the tread up
front. The tire interferes with the flange inside the wheel lip at
full bump. The amount of interfernece is not much and could be
corrected by grinding back a portion of the wheel lip flange along a
length of about 4 inches. Other tire design may not rub at all.

BTW, this would apply whether your car is lowered or not. Full bump
is full bump, and the wheel will end up in the same place, regardless
of where it started it's travel.

Those folks running stock ride height with larger tires and claim they
don't get any interfernce up front just aren't using the full travel
of their suspensions. C'mon folks, drive that thing! :-)

> Roland may be running 205-50-16s all around, I don't remember.

Correct. IMHO, big rear tires may look nice, but it's the front
that's really friction impaired when it comes to handling. Until you
can stick the front like you want, adding stick to the rear can just
make matters less manageable. Oh, BTW, recall I have a Quaiffe rear.
Now that I think of it, folks with an open diff probably could make
good use of big rear rubber.

(FWIW, I like to rotate my flat spots anyway :)


If you follow the basic rules of plus sizing, and don't go crazy on width, there is, by definition no problem with 17's. The primary issue is the overall height and width of the wheel-tire combination. Tires are always wider than the wheel, so any overall dimension that works with 14's, 15's, 16's should work with 17's, because, as long as you don't increase overall tire-wheel dimensions, increasing wheel size only increases clearance inside the wheel.

To go into detail, plus sizing is the method whereby you increase wheel diameter without increasing the overall tire diameter. Overall tire diameter must be calculated, since it isn't explicitly in the tire size - a 195/60-14 standard XR4 tire is 195mm wide and has a sidewall height of 60% of 195 = 117mm, or, when divided by 25.4 mm/in., 4.606 inches. Multiply this by 2 and add to the 14" wheel diameter and you get an overall diameter of 23.213 inches. For 17 inch wheels that means you need a sidewall height of (23.213 - 17) / 2 = 3.106 inches or 79 mm. You can have a mm or two slack either way with a minor speedometer error, so for instance 205/40-17's would work since 40% of 205 is ~80. If you have Microsoft Excel or some other spreadsheet package, it's really easy to whip up a quick little chart to figure suitable tire sizes and speedometer errors.

The other issue is width. I'm not sure what the max width is for Merkurs, and as you increase width, you'll need to decrease offset by AT LEAST half the width increase, but no more than all of the increase. Keep in mind that increasing TIRE width will improve handling TO A POINT, but beyond a certain point, you fall victim to the law of diminishing returns, and radically exceeding the design width will actually degrade handling and risk damage to suspension components. Also, you don't necessarily need wider wheels. The stock wheels are 5.5 inches wide; this will handle up to 205 tread width tires. 6 inch wheels will take up to 215.s, I believe, and I think 6.5's will handle 225's (tire stores have a book with a table showing the range of tread widths for a given wheel width.) There is little or no benefit to exceeding 225mm tread width on an XR4 - that means 6.5 inch wheels should meet all your needs, and keep in mind that if your intended tire size will work with a narrower wheel, you should go with the narrower wheel because less metal means lower unsprung weight. Since 6.5 inches is actually less than the stock 195mm tread width, there should be no clearance problems. Any tread width that works with smaller wheels would work on 17's if the overall dimensions stay within the envelope. The primary advantage of going to larger wheels is that less of the diameter is soft tire sidewall and more of it is rigid wheel, making the overall combination less subject to flexing under lateral forces and causing sideslip. Of course, the price is a harsher ride, but there are no free lunches.