Question:
>".... I have a '62 Mark II. I have bias-ply Dunlop gold seal (4 ply) size 15-6.00. ...."

Original tires were called Gold Seal, but I think that was a manufacturer's trade name in those days, not a model name, so those were probably not Dunlop tires. They were also not tubless tires, and if you ever come across a set in really good condition some show type originality oriented person will pay big bucks for them.

Original tire size was 5.60-15, so your current tires are about 4/10 inch wider. These widths are about like metric sizes 145 and 155 respectively, but with a taller profile, so larger in diameter. The old bias ply tires were basically 100 series, or about as tall as they were wide (or nearly round in cross section when you think about it). The most common metric radials for these cars are 80 series profile, sidewall 80% as tall as the tire is wide. Unless otherwise labeled on the tire, radial tire profile will be 80 series by default (and never taller then 80%), and the speed rating will be "S" (106 mph 10-minute run) by default. So when you see 165R15 on a tire it means the same thing as 165-80SR15.

If you want your odometer to run true, the nearest metric tire side to match the diameter of the originel 5.60 tires would be 165R15. These tires are about two sizes wider than the original tires, about equivalent to a 6.50 tire in width. Also with a more squat profile they have a wider tread. A 155R15 tire has the appearance of being much closer to the original tire size, but is still a bit wider, and smaller in diameter so it will read out more miles and higher speed on your speedometer. The 165R15 will get your speedometer and odometer nearly spot on. Also, the 165 size is about the widest tire recommended for our 4" wide rims.

The 155R15 and 165R15 tire sizes were common on VW Beetles, and so are still commonly available today, although with not a wide selection to choose from. The 165's may be easier to find and lower priced because of the volume of sales, and will make your car handle better as well. 165's are very common an MGAs these days, although some show car types still prefer the 155 size for appearance sake, even though it doesn't look anything like the original 5.60 tires. In years past I have taken a distinct liking to Dunlop SP4 tires, which were superceeded by Dunlop SP20a/s 165R15. Much better grip and handling than the more expensive Michelin equvalent, longer tread life than the more expensive Pirrellie equivalent, a very nice price, and generally good availability. You may have to ask for them and wait for delivery, because they are not always stocked locally. Apparently the SP20 tires were discontinued after the Dunlop buyout by Goodyear.

My current favorite is a new tire in 2002, the Kumho Euro Metric 771. The treadwear rating is a bit lower that the Dunlop tires, but adhesion to the road is better, and since I'm an enthusiastic driver, I like these tires even better than the Dunlops. Last I checked these were available through Tire Rack for a very attractive introductory price of $29. Check here. Availability changes with time, and tire models eventually get discontinued. Whatever tires you buy, settle for nothing less than "A" Traction rating rating on the sidewalls. Don't worry too much about the Treadware rating numbers, as they are not comparable between different manufacturers. Also, tires with the highest Treadwear ratings usually don't stick to the pavement very well.

Addendum Aug 27, 2004:
Apparently the Kumho "Euro Metric 771" has been discontinued, at least in the 165-15 size.. But today I find Kumho "Power Star 758" in 165-80-15 size, priced at $27 from Tire Rack. This one has UTQOG rating "400 A B". With the high treadwear rating I don't know how good the grip will be, but at least it has an "A" traction rating. Since I have had good experience with Kumko tires, I think I would give this one a try. If anyone buys a set of these I would appreciate a user report.

>"The tire says tubless, but I suppose it's wise to guess there are tubes as they're mounted on 72 spoke Dayton chromed wires."

Generally good assumption, although Dayton has been known to offer a tubless option for their wire wheels. They do this by adding a thick coat of silicone sealant over the spoke nipples inside the rim. Also, original tires were not tubless, because these cars predate the general availability of tubless tires. So the tires you have would be at least a little newer than the car. Now as an addendum (and exception) to this last comment, in later years of production there was a tubless tire option for the MGA with steel disk wheels, but I think it was still a bias ply tire.

>"Max PSI is 32,"

Nearly all bias ply tires have a max rating of 32 psi for max load capacity. Also, nearly all car tires are sized to have some significant reserve load capacity, and so will generally perform best at less than the max spec'd pressure (except in the most severe competition use).

>"and the PO has it at 22psi front and 20 rear, but that may be from neglect and ...."

Original tire pressure was spec'd at 17 front and 20 rear. For fast motoring add 4 psi all around. For competition work and sustained high speed motoring, add another 2 psi all around. But that means absolutely nothing unless you happen to use tires of the original size and design (not very likely), and your suspension is still dead stock.

It is significant to notice however that it was spec'd for more pressure in the rear than in the front. The stock rear suspension of the MGA is quite stiff with 7 leafs in the rear springs. The stock front suspension is much softer by comparison. Even though the front carries about the same weight as the rear, the front springs have a much lower spring rate, so more vertical travel at the wheels to pick up the load. The end result is that in just moderately hard cornering the car has a high degree of body roll, and the front tires pretty much stay planted on the ground while the car will lift the inside rear tire off the surface. This puts a higher temporary load on the outside rear tire during cornering, so that tire tends to mush over and squirm more and loses some of its grip. This causes the rear to get loose and causes oversteer where it wants to fish tail or take the turns sideways. If you lift off the throttle in a high speed turn you may find yourself traveling tail first down the road. To avoid this somewhat wild and hard to drive handling characteristic, they spec'd more pressure in the rear tires to carry that higher momentary load.

If you install a front anti-sway bar to increase the front roll stiffness and reduce the body roll, the rear pressure can be much closer to the front pressure. This will also improve steering response because with less body roll it takes less time for the suspension to take up the load in a quick turn. This makes the handling more responsive to steering input, gives the car a lighter and more lively feeling, more fun to drive, and incidentally somewhat quicker getting around an autcross track.

At any rate, newer tires have different presure requirements. Even the tubles bias ply tires you have now will like a little higher pressure than the original tube type tires.

For a short time I had 5.60-15 2-ply nylon tires on the car, and it was absolute mush to drive. The tires would look half flat and handling was commensurate with appearances. I had to use 30 psi just to keep those tires from running on the sidewalls in hard cornering, and then they had very little grip, rather like driving on sand or pea gravel. I promptly pitched those tires, bought a set of Michelin X radials (1988), and have been wearing bigger grins ever since. But then my car likes spirited driving and is generally allergic to auto shows. And I have used many other types of tires since.

If you want to change to wider tires for appearance or better handling, try to keep the overall diameter about the same so it doesn't mess up the speedometer and odometer too much. In general, for each 10 points increase in width you should go about 5 points less in profile. Following that formula, if the following sizes were available they would be a fairly close match to the original daimeter: 175-75-15, 185-70-15, or 195-65-15.

A friend of mine uses 185-65R15 (nearly 1" too small). This over-revs the odometer by neary 4%, and makes the speedometer read about 2 mph high at highway speed. This size is also a little too wide for the stock rim width, so unless you also install wider wheels you may notice strange wear patterns on the tires, especially scrubbing off of the outside shoulder.

For my racing tires I use 195-60R15 which is about 1/4" smaller diameter yet, but this is the narrowest tire size available in race rubber compound for a 15" wheel. And if you're concerned about clearance for these larger tires, you can stop worrying. The MGA has lots of open space inside the fenders to accomodate wider tires. However, front tires of larger diameter may touch the frame on full steering lock (or the anti-sway bar if your car has one).

Also, do not be surprised if you start breaking wheels when using real race tires, because real race tires are really sticky. For more information about my race tires and special wheels, check out Mag's and Stickys.

The next page, Tire Tech 102, tells you how to go about adjusting your tire pressure for street cruising.

Barney Gaylord