For the past several months I have been pondering the purchase of new tires for my Triumph TR6. While this shouldn’t be a big deal, there were several considerations I had to bear in mind before making a decision. I sought the advice of experts and read several articles on the subject. Some of the advice I picked up may be of interest to anyone else looking for replacement tires for their British sports car.
Originally all US-bound TR6s came equipped with 185R15 size tires (Redline Michelins or Goodyears). These are 70 or 80 series profile tires, which is to say that the sidewalls are roughly 80% as high as the tire is wide. They look very good on the TR6 and provide a smooth, compliant ride. Being so tall and narrow these tires offer little as a performance tire. In addition, new 185R15s can cost upwards of $200.00 per tire— if you can find them!
A popular replacement for the 185R15 is a 205/70R15 size tire. These are readily available and will fit the 5.5” rim original to the TR6. The 205/70 tire has an overall diameter of 26.3” so in comparison to the 185R15s the tire diameter is virtually unchanged from the original’s 26.6”. This means that speedometer and odometer readings should not change, an important consideration.
Another important consideration is how the car will be used. Back in 1987 when I was using the TR6 daily, I purchased a set of used 195/75R15 tires to get me around town. Like 205/70R15s, they are almost equal on overall diameter to the original tires, but as I started to use the 6 as a weekend cruiser and occasional autocrosser, cornering performance became more and more of a consideration.
As I began my search for new tires I also had to be aware of how they would affect the overall ground clearance, gearing and aesthetics of the TR. Fifty profile tires would be the ultimate cornering tires, but weekend driving would be hazardous as speed bumps, driveways and potholes would become damaging hazards. Fifty profile tires would also radically alter the final gear ratio of the car making highway cruising difficult. On the other hand, acceleration would be greatly improved.
The compromise I chose between performance and the other considerations was to go with 205/65R15 size tires. The overall diameter of these tires is 25.5” about 1” shorter than stock. Ground clearance is not a problem and the tires appropriately fill-out the wheel wells for an updated, but not bad look.
As far as driveability, the overall gear ratio did change slightly. To figure out the new overall gear ratios and speedometer readings I used the following formula found in Grassroots Motorsport.
To find my new highway speed at 3500 rpm, the formula is: overall tire diameter times engine rpm divided by the differential gear ratio times selected gear ratio times 346.
So, at 3500 rpm in fourth gear (no overdrive on my car) my speed is (25.5 X 3500) divided by (3.71 X 1.0 X 346) = 69.52 mph. My old speed at 3500 rpm was 72 mph or thereabouts. So as far as driveability on the road there is not a great deal of difference with my new tire choice.
There is a great deal of difference out on the autocross course however! The tires I chose are made from a softer compound and carry a “V” speed rating. Even though 65 profile is still on the high side, they’ve made a great improvement in handling over my old high-mileage ‘truck’ tires. The car is now much more responsive than before, quicker, and so much more fun to drive.
The trade-off for the improved performance has been a harsher ride. The old 195/75R15s were very forgiving of minor bumps and road irregularities. Though lowering the tire pressures has helped somewhat, the new tires will never be as ‘cushy’ as the old ones, but this is a compromise I can learn to live with because of how I wish to use the car.
If you’re considering the purchase of new tires for your British classic I hope my experience can be of some use to you. The best advice I can offer is to talk to a qualified tire technician who will listen to what your specific needs are and recommend accordingly.
By Kevin D. O’Connor