[DeTomaso] Tire question

[Garth Rodericks]

Hey Chris,

>> I don't use my car on the track, but I have been known to drive it thousands of miles across the country to Fun Rallys, so I need tires that are Z rated since I might hypothetically drive rapidly.


How much time do you actually drive your car in excess of 149 MPH?  You may not actually need Z-rated (149+ MPH) tires, unless you plan to cruise at those elevated speeds for an hour or more.
For a tire to earn its speed rating it has to be able to run at sustained speed for an hour without breaking down or failure! 
Following is how an H-rated tire earns it's "up to 130 MPH" rating. therefore.
Initial Test Speed (ITS) = Tire's rated speed (130 MPH) minus 24.9 MPH, 6 speed steps, each 10 min in duration: 

Test speed and duration:
(1) 10 minutes at   0 to 105 MPH (2) 10 minutes at  105 MPH(3) 10 minutes at  111 MPH (105 MPH + 6.2 MPH) (4) 10 minutes at  117 MPH (105 MPH + 12.4 MPH) (5) 10 minutes at  124 MPH (105 MPH + 18.6 MPH) (6) 10 minutes at  130 MPH (105 MPH + 24.9 MPH) 

Therefore, if you had an H-rated tire, you could safely drive at elevated speeds for some time.  If you're cruising through long stretches of open highway at 100 MPH and then do a blast up to 130 MPH for a few minutes, tire failure will be the least of your concerns.  Road condition or debris, cross-winds, wild animals, other traffic, etc. (maybe even the local gendarmerie) pose a much greater threat to your safety and well being.
Not that I don't believe in having an adequate margin of safety in your equipment, but just saying your car has probably never seen sustained speeds above 105 MPH for more than a few minutes at a time.  Anyway, just something to think about.

Following is the NTHSA guidelines/test for a tire to earn it's speed rating. 

>From the NTHSA...
A. PASSENGER TIRES—HIGH SPEED TEST 
RMA 2000 concluded that  
The SAE test [J1561] conditions were found to be the most consistent discriminators required for completion of the rated speed within the customary one-hour duration.[17] Test inflation pressure had the greatest effect in determining completion of the rated speed. Maximum load was also shown to have an effect on performance, although not as great as inflation.  
RMA 2000 recommended that the agency revise the High Speed Performance test in FMVSS No. 109 to reflect the conditions found in SAE J1561:  
(1) Test speed and duration: (Initial Test Speed (ITS) = Tire's rated speed minus 40 km/h), 6 speed steps, each 10 min in duration: (1) 0 to ITS, (2) ITS, (3) ITS + 10 km/h, (4) ITS + 20 km/h, (5) ITS + 30 km/h, (6) ITS + 40 km/h.[18]  
(2) Inflation pressures (kPa): 240 for speed rating through N, 260 for P, Q, R, & S, 280 for T, U, & H, 300 for V & Z, 320 for W & Y.  
(3) Load and ambient temperature: 80 percent of maximum rated load, 38°C ± 3°C.

Source: https://www.federalregister.gov/documents/2002/03/05/02-5151/federal-motor-vehicle-safety-standards-tires
-------------- next part --------------
   Hey Chris,

   >> I don't use my car on the track, but I have been known to drive it
   thousands of miles across the country to Fun Rallys, so I need tires
   that are Z rated since I might hypothetically drive rapidly.

   How much time do you actually drive your car in excess of 149 MPH?  You
   may not actually need Z-rated (149+ MPH) tires, unless you plan to
   cruise at those elevated speeds for an hour or more.
   For a tire to earn its speed rating it has to be able to run at
   sustained speed for an hour without breaking down or failure!
   Following is how an H-rated tire earns it's "up to 130 MPH" rating.
   therefore.
   Initial Test Speed (ITS) = Tire's rated speed (130 MPH) minus 24.9 MPH,
   6 speed steps, each 10 min in duration:
   Test speed and duration:
   (1) 10 minutes at   0 to 105 MPH
   (2) 10 minutes at  105 MPH
   (3) 10 minutes at  111 MPH (105 MPH + 6.2 MPH)
   (4) 10 minutes at  117 MPH (105 MPH + 12.4 MPH)
   (5) 10 minutes at  124 MPH (105 MPH + 18.6 MPH)
   (6) 10 minutes at  130 MPH (105 MPH + 24.9 MPH)
   Therefore, if you had an H-rated tire, you could safely drive at
   elevated speeds for some time.  If you're cruising through long
   stretches of open highway at 100 MPH and then do a blast up to 130 MPH
   for a few minutes, tire failure will be the least of your concerns.
   Road condition or debris, cross-winds, wild animals, other traffic,
   etc. (maybe even the local gendarmerie) pose a much greater threat to
   your safety and well being.
   Not that I don't believe in having an adequate margin of safety in your
   equipment, but just saying your car has probably never seen sustained
   speeds above 105 MPH for more than a few minutes at a time.  Anyway,
   just something to think about.
   Following is the NTHSA guidelines/test for a tire to earn it's speed
   rating.
   From the NTHSA...
   A. PASSENGER TIRESaHIGH SPEED TEST
   RMA 2000 concluded that
   The SAE test [J1561] conditions were found to be the most consistent
   discriminators required for completion of the rated speed within the
   customary one-hour duration.[17] Test inflation pressure had the
   greatest effect in determining completion of the rated speed. Maximum
   load was also shown to have an effect on performance, although not as
   great as inflation.
   RMA 2000 recommended that the agency revise the High Speed Performance
   test in FMVSS No. 109 to reflect the conditions found in SAE J1561:
   (1) Test speed and duration: (Initial Test Speed (ITS) = Tire's rated
   speed minus 40 km/h), 6 speed steps, each 10 min in duration: (1) 0 to
   ITS, (2) ITS, (3) ITS + 10 km/h, (4) ITS + 20 km/h, (5) ITS + 30 km/h,
   (6) ITS + 40 km/h.[18]
   (2) Inflation pressures (kPa): 240 for speed rating through N, 260 for
   P, Q, R, & S, 280 for T, U, & H, 300 for V & Z, 320 for W & Y.
   (3) Load and ambient temperature: 80 percent of maximum rated load,
   38ADEGC A+- 3ADEGC.
   Source: [1]https://www.federalregister.gov/documents/2002/03/05/02-5151
   /federal-motor-vehicle-safety-standards-tires

References

   1. https://www.federalregister.gov/documents/2002/03/05/02-5151/federal-motor-vehicle-safety-standards-tires