tire question

Discussion in 'General' started by nervousmini, May 9, 2009.

  1. nervousmini

    nervousmini Well-Known Member

    I am planning on getting new tire for my Blazer and have found the brand and model I want, however I have several choices within my size and want to make sure I get the right one.

    My question is this: should I get the tire with the higher or lower revolutions per mile? One tire is rated at 720 and the other is 670 - quite a difference for the same size tire (235/75r15) but I can account for the difference in tread depth, even though they are labeled the same size.

    I have been trying to figure it out in my head but I'm not sure that I am getting it right. I think I want the lower rev tire (670) - right?
     
  2. JusBringIt

    JusBringIt Be Inspired

    Lower rev would be correct. the higher tread depth actually poses a higher rolling resistance due to the rubber flexing quite a bit more. Think of it as having softer tires. Also, with the higher revs, that throws your speedometer off quite a bit.
     
  3. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    According to TireRack, the OEM tire for a '97 Blazer 4dr 4wd was the Goodyear Wrangler RT/S, with the revs per mile listed as 716. So a tire rated to turn 670 revs per mile will cause the speedometer to read about 6% low, or 55mph indicated when travelling at 58.8mph. That assumes that the speedometer was calibrated perfectly for a tire that rolls exactly that many revs per mile.

    The advantage is that the odometer runs slow compared to miles covered. Comparing 670 to 716, the odometer would read 10,000 for every 10,686.6 miles driven. The disadvantage (if you believe the site I read that covers the good and bad of oversized tires) is that there is extra strain on brake components when using tires with a diameter that is more than 3% larger than stock. My solution to that stated issue is DWB as you can't strain something that you don't use. :D

    That 670 sounded a little low to me so I checked various tires in that size: 235-75-15 on tire rack. What I found is the all terrain tires seem to be "taller" and roll slower than the all season tires. The Kuhmo SAT-KL61, for example, shows 666 for the revs/mile (perfect for "driving like hell", I guess), and that tire is listed as off road all terrain. If the "low rev" tire is in fact an all-terrain on/off road tire, you may be better off with the tire that "spins faster" but offers a smoother tread with fewer voids and less tread depth. Off road tires are not designed for low rolling resistance and the losses due to the tread design and tire construction will offset any gains from the slower revs per mile.
     
  4. nervousmini

    nervousmini Well-Known Member

    Thanks,

    I did the calculation for speedo error and came up with the same 6%, but I can easily live with that - and I'm pretty sure the speedo isn't too terribly accurate from GM out of the box. The GY wrangler RT/S is horrible - especially in the wet/snow and worthless on ice - of course 2 of the 4 tires on the blazer are RT/S.

    I believe the extra strain the are talking about on brake components assumes a significant increase in unsprung weight from the tire - but the RT/S weighs 31 pounds same as the Kumho KL61. But my brakes will be upgraded probably this summer so I am not worried about 'added strain' lol.

    The Kumho SL61 is actually the tire that I am probably going to pick. The all-terrain on/off road designation vs getting more of a passenger style is just the tradeoff that I have to make, but it seems like the best choice of the type. I am sure that I am giving up some rolling resistance in my choice, but I have a child with some medical issues that make a 4x4 truck a necessity (I have to be able to get him where he needs to be, quickly in any weather condition - no excuses). If you look at the tread design and the utqg wear rating it looks like a pretty good roller, certainly better than something like a BFG all terrain or somethng.
     

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