The original rims and tires fitted to the 1978 Jaguar XJ-S were Kent wheels mounted with 215x70x15 tires. Offset was 28.5mm. This was my starting point.
According to Tire rack's website, Jaguar upgraded the wheel/tire combination through the model lifespan with 225x60x16, 235x60x15, 225x50x16, 225x55x16 while maintaining the 28.5mm offset. I believe the TRW modified XJ-S came with 16x8" wheels mounted with 225x50 tires in front, and 245x55 in the rear, however the wheel wells might have been modified. The current Jaguar XK8 17 inch rims, with 33mm offset, have been used, however they require the front offset to be modified (via a spacer) to approx 20mm in order to clear the tie rod ends.
I was convinced that a 17x8inch rim was ideal for my wheel upgrade. The problem was determining what size tire to use. The original 215x70x15 tire has a diameter of 26.85 inches. Any upgrade would have to be in the 26 to 27 inch range to preserve some semblance of speedometer accuracy. Note that the XJ-S did not change significantly throughout its model production. The conclusions reached here should apply to any Pre or Post HE model XJ-S.
Wheel offset is the distance that the mounting hub is in relationship to the center of the wheel as the following diagram illustrates.
Offset values must change when upsizing a wheel. Offset will decrease when going from a 15x6 inch original wheel, to a 17x8 inch optional wheel. That additional 2 inch width has to be split, both inboard and outboard on the wheel. In addition it must fit properly with the newer, larger sized tires, must clear the inboard suspension geometry, and must not rub on the outside of the fender.
There is not an overabundance of aftermarket wheel manufactures making wheels for the XJ-S. The bolt pattern (5x4.75) is not uncommon. Many GM vehicles use this bolt pattern, however the diameter of the XJ-S's front hub is larger, with a hubcentric fitment. A hubcentric fitment is one where the centerbore of the backside of the wheel is an exact fit to the hub. The weight bearing of the car on the wheel is done on the hub when it is mated to the wheel. This has to be an exact fit. The lug nuts simply attach the wheel and hold the wheel to the car. It has no effect on load bearing. Because of the hubcentric fitment, I don't recommend modifying a GM wheel without specialized equipment and knowledge of the rim strength.
Note that all offset discussed are "negative" (see diagram above). Increasing negative offset will move the rim inward towards the suspension. Decreasing the negative offset will move the rim outward towards the fender.
I ordered 17x8 rims from Ronal, with offsets of 20mm front and rear. Note that Ronal's sales department does not list wheels for the Pre-HE XJ-S. No manufacturer will have fitment guides for a 25 year old car. Ronal wheels are manufactured with a "hubcentric adaptor system". This is essentially a hubcentric spacer that determines offset. The standard wheel has a 35mm offset. A 15mm spacer determines the final offset of 20mm. This "spacer" system also means I could change offsets without purchasing new wheels.
Initially I wanted the 245x45x17 tire, however I decided the 235x50x17 tire was a better fit, It was almost the same width as the 245 tire, and more importantly, close to the same diameter as the original tire.
The following compares the original and new wheel tire combinations
The original 215x70x15 wheel/tire combination has the following specs.
Diameter of 26.85 inches.
Sidewall 5.92 inches.
Circumference 84.35 inches.
Revolutions per mile 774.6.
The optional 17x8 wheel with 235x50x17 tire has the following specs.
Diameter of 26.25 inches.
Sidewall 4.62 inches.
Circumference 82.46 inches.
Revolutions per mile 792.3.
Speedometer difference is approximately 2%. I can live with that.
The 235x50x17 tire on 17x8 inch wheels with 20mm offsets were mounted on my 1978 XJ-S. The fronts did not seem to be a problem. The rears however, rubbed the fender wells under hard suspension travel. Removing the rear tires revealed the rub area as seen in the photograph below. (red arrow) Note that the fender lip is not the problem. Rolling the fender lip inward will not help. The problem is the inner fender well.
Looking at the wheel mounted with the 20mm offset, it's apparent that there is sufficient room to move the wheel inward by increasing the offset. An offset of approximately 30mm I believe is what's needed.
I removed the wheel and custom "spacer". The wheel offset is 35mm. The spacer is 15mm. This makes the offset of the wheel and spacer 20mm.
Pictured below are the spacers I acquired from Ronal. They are 15mm, 10mm and 7mm. This will allow me to vary the offset by 35mm, 28mm, 25mm and 20mm. Ronal was kind enough to agree to allow me to return the extra spacers when the project was completed.
First I mounted the rear wheel without a spacer. As the following photograph shows, the standard offset of 35mm clears the fender and suspension parts at the rear of the car. For this wheel this is the recommended offset.
I wanted to move the tire out as far as I could without rubbing the outer fender. I also wanted to minimize the offsets between the front and rear tires. A 7mm spacer was installed on the rim (to produce a 28mm offset) and the rear tire mounted. As expected the tire easily clears the rear suspension parts.
However, there is virtually no clearance at the fender lip as seen in the photograph below (blue arrow). Full suspension travel determined that this offset just barely clears the fender inner wheel well. Variations in production might make this offset unworkable in an identical XJ-S. For my application however, I decided to use this offset at the rear tires. This tire/rim offset is not for the faint hearted.
With the rear wheel offset determined, it was time to focus on the front wheels. The wheels currently have 20mm offset. In normal driving there is no problem, however under full lock the tires rub the front fender and rear wheel well opening.
Fortunately the tire rub is on the tire tread, not the edges. (blue arrows)
More negative offset might alleviate this problem, so I increased the offset from 20mm to 25mm by substituting the 15mm spacer for a 10mm. As the photograph below shows, this offset just barely clears the anti-roll bar.
However, the larger problem was the front hub. The early XJ-S Jaguars have a longer front hub than the later cars. With a 25mm offset the wheel cap could not be installed.
Removing the wheel cap shows how little clearance there is between the grease cap and the outer edge of the wheel.
The front offset was now determined. It had to be 20mm with the early XJ-S's. The later cars with the shorter front hub could fit a 25mm offset if they so desire. There would be some slight rubbing of the tire tread at full lock, but I could live with that.
The following are my personal conclusions only:
Any wheel/tire combination that originally came as standard equipment on the XJ-S will obviously work. If upgrading to a larger wheel/tire combination is desired, then be prepared to tell the wheel manufacturer what size and offset you want. Their fitment guides will probably not have information on cars older than 5 or 10 years.
If upgrading to a 17x8 inch wheel, then you must have 20mm offset in the front and 33mm to 35mm offset in the rear.
Tires larger that 245x45x17 will not fit under the rear fender wells. 245x45x17 tires on 17x8 rims with 33mm to 35mm offset are the largest tires that will fit. If the tire is increased to 245x50x17 it will not fit. Note that a 235x50x17 rear tire with an offset of 28mm should also work. If the tire is increased to 245 then the offset should be 33mm to 35mm.
For my car the final specifications will be.
17x8 wheels. Front offset 20mm. Rear offset 28mm. Tire 235x50x17.
The installation of these wheels and tires are covered under another link on my website.
Jaguar XJ-S Home Page
Bernard Embden's Home Page