TR6 TR6 Tire Sizes - Modern Equivalent to 185 SR 15?

[Ted19428]

I know this issue has come up countless times in the past and I have read all the posts. I spoke to one of the tech reps at Tire Rack and they are confident that 205/70/15 tires will fit a TR6 with standard wheels. The redlines need to go and I cannot afford new redlines. Therefore, I want to purchase new, modern tires, but I want to make sure I don't completely mess up my car in the process.

A final question. The redlines require tire pressure of 20/24 psi. Modern tires have much higher psi. Do I need to do anything else to the car if I place modern tires on it, in light of the increased tire pressure?

As always, I appreciate everyone's help.

 

[Guest]

I found this very useful for size comparison purposes.

https://www.miata.net/garage/tirecalcold.html

AFAIK you need do nothing else apart from getting the wheels balanced, but I'm happy to be corrected...

 

[Adrio]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

Regarding the tire pressure. I am no expert but I thought it was the car design and in particular the suspention. As such I thought one was suppose to use the tire pressure the car manufacturer specifies, not the tire pressure. The tire pressure is just the MAX that the tire can handle. Again I am happy to be corrected.

As to the tire size, I was able to get the local tire store to order in (it took a week or so) tires that were of the old size spec. In my case for the TR4 and TR3 he got me Kelly Metric 165 15. They were not that much either. You could ask about getting the actual tires, the price will surprise you (as long as you stay away from redlines)

 

[Eric]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

Yes, the tire pressure on the sidewall is the max allowable, not the recommended pressure. The recommended pressure is specified by the car manufacturer, and will vary from vehicle to vehicle even with the same tires.

That said, many modern tires benefit from more pressure than the old ones would. For something like the 6, I'd start around 32 psi all round and fiddle from there.

 

[Simon TR4a]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

I agree with Adrio and Eric.
The air holds up the weight of the car so the pressure needed is determined by the volume of air in the tyre and the weight of the car.
If you changed to a much smaller or larger tyre, or loaded the car heavily you would need to adjust the pressure.
A mid-size family car is probably in the 3,000 lb range so if it has similar size tyres to your 2500lb. TR6 it would need more pressure.
I don't think you need as much as 32psi, but 20/24 seems a bit soft, and will give a comfortable ride but will waste gas, squeal on corners and make the steering feel heavy.
The differential between front and rear will ensure the front of the car will push wide in a corner before the back breaks loose, I agree withj Eric that even pressure front to back improves the handling balance.
Simon.

 

[Dave Russell]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

I agree with Eric.
We are talking two different tire designs. The old bias ply tires had a stiff sidewall & a flexible tread area. Over inflation would cause the tread to hump up in the center & reduce tire contact patch with the pavement.

Belted bias ply tires still had the stiff sidewalls but a somewhat stiffer tread which didn't hump in the center so much with higher inflation pressures. The stiff sidewalls kept the tire beads seated to the rims under side pressures.

Radial tires have a very flexible sidewall & a stiff tread which does not change the tread contact patch much with variations in pressure. For this reason, radials with LOW inflation pressures stand a good chance of having the bead separate from the rim during hard cornering. They also feel very squirrely due to the excess sidewall flex. Inflation pressures should be a minimum of 28 & 32 to 36 might be optimum for handling & safety.

The amount of sidewall flex over road obstacles will be about the same with a bias ply at 24 & a radial at 32. So comfort Is not much different.

With a radial, usually the more pressure, the better the tire cornering traction. Front & rear pressures are often different to give the best overall handling performance. Lower aspect ratio radial tires such as 65's (shorter sidewalls) will also usually require more pressure than the older 80 or 85's.
D

 

[Geo Hahn]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

FWIW -- using 165x15s on the TR3 & 4 I put the rear tires at max (36) and the front a couple of pounds less than that.

I expect the rear end to break loose before the front runs out of road. Well, of course I don't want either to happen but oversteer is what I think these cars were built with.

As the saying goes: Understeer scares the driver, oversteer scares the navigator.

 

[Guest]

Ted,
I run 205/70 R15 (Michelin Hydroedge) tires on my TR6 at about 34lbs and love the performance. Now, to clarify, I have Panasports and I know for a fact that they are slightly wider than the OE wheels that came on the car. Perchance, you should consider running a slightly narrower tire because of rim width? I know they will fit but are they gonna bulge a bit? Now, getting a "performance" tire to fit a 15" wheel is getting a bit tough. The Michelins can take a lot more punishment than I can dish out with my car and they are not considered performance tires. You just want something that has a nice grip, corners well and looks good. At a reasonable price. Any good minivan tire will do. Just don't overdo the width thing.

Bill

 

[wingsandwheels]

Take a look at Vredestein tires. I put a set of 185/15 Vredestein Sprint Classics on my '74 TR6 2 years ago and am very happy with them. They are a modern tire in the stock size and have a very period look. They ride and handle well and are pretty quiet but are a little soft and I wouldn't expect great treadwear. I think they were in the $110 range from Universal Vintage Tire.

 

[cyaker]

I have seen recommendations for both 205/70-15 and 215/70-15s with differnt opinions as to correct size for keeping speedo as accurate as possible. I see on Coker Tire site their redline for TR-6 is 27" total height, a 205/70 normal runs only 26.3", 215/70 runs closer to 27".
What about the 205/75-15 maintains less width while adding height(26.8")and I think a more original appearance. Michelin X-DT at Sams is $71 and rates pretty well in Michelin's eyes on their site. Tire is special to Sams, BJ's and Costco. My wheels are out for sandblasting and I am tempted to try these. Any problems anyone might anticipate?

 

[71tr]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

I have BFGoodrich Touring T/A's (205/70/15) on my six and think they look great. They are mounted to the original steel rims and the BFG design is not so modern that it looks out of place on the car.

A useful formula for calculating the outside diameter of a tire is below. Using this formula I found the 205/70/15 tires to be very close to original in outside diameter, within 1-2%. The 215's are even closer but run the risk of rubbing.

Inputs: width, aspect ratio and inside diameter
Formula: ((width*aspect)*2/25.4)+ inside diameter
Example: ((205*70%)*2/25.4)+15 = 26.30 inches OD

 

[Mark_Gibson]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

I elected to go with the Diamondback Redlines for the TR-6. It's a modern Firestone radial with the look of the old Michelin Redlines. The car handles great and I run 35 psi in them. They are a bit more pricey, $155.00 apiece.

 

[angelfj1]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

Mark: Is that a photo of your TR-6 on the Diamondback web site?

https://www.dbtires.com/

 

[bricktop]

Re: TR6 Tire Sizes - Modern Equivalent to 185 SR 1

What tire size did you go with on the firestones for your 6?

Carter