2001-03 Prius tires, NHW11

This posting is about my tire experience with my NHW11 Prius and future plans. To replace the MasterCraft that were on the car when I bought it, I went with Sumitomo T4, 175/65TR14, because they were listed as a low rolling resistance tire in Consumer reports. To my surprise, they were affordable from my local Toyota parts store. This is a general purpose, all-weather tire but there are many other fine tires, too many to cover in this note.

The tread depth is now 7/32 from a start of 10/32 and ending at 3/32. This puts the wear at ~57% after ~25,000 miles. I'm well on the way to reaching the rated miles and probably two years away from replacing them. I've had no problem with handling in the mild, snow-free winters of North Alabama. There was no mileage change when they replaced the MasterCraft tires. But I've started looking at what tires might do for my mileage.

The following, CSV, tire table was assembled from Tire Rack and vendor web sites:
Make,Size,Max psi,veh. lbs,tire lbs,tread wid,rev / mi,% rev/mi,indicated 42 miles per hour=,indicated 65 miles per hour=,% cross area,Notes
Yokahama,205/50R15 AVID T4,35,1113,20,6.7,897,-0.99%,42.4,65.6,28.85%,too big for rear wheel
Yokahama,195/55R15 AVID T4,35,1102,20,6.5,887,-2.10%,42.9,66.4,25.00%,too big for rear wheel
Sumitomo,195/70TR14,51,1321,19,5.8,849,-6.29%,44.6,69.1,11.54%,
Sumitomo,185/70TR14,51,1201,17,5.5,867,-4.30%,43.8,67.8,5.77%,
Sumitomo,175/65TR14,51,1019,15,5.4,919,1.43%,41.4,64.1,3.85%,current RJW
Mastercraft,175/65R14,44,1047,15,5.3,912,0.66%,41.7,64.6,1.92%,former RJW
Bridgestone,175/65/SR14,44,1019,15,5.2,906,0.00%,42.0,65.0,0.00%,OEM
Sumitomo,175/70TR14,51,1102,15,5.2,886,-2.21%,42.9,66.4,0.00%,

My 175/65TR14 Sumitomo tires came out with the largest number of revolutions per mile so the actual speed should be less than the indicate speed. My first calibration tests when they were new showed about a 1% error from the mile markers. I've also included some Yokahama tires that the owner reported an 8-10 MPG mileage hit. He also reported they didn't fit in the rear wheel well and hitting bumps would impact the outer part of the well.

The 2001-03, NHW11 Prius has control laws governing MG1 rpm and limiting performance. There is a mileage dip at 42 miles per hour and a bigger hit between 65-75 miles per hour. In fact, it looks like there is a 'sweet spot' at 65 miles per hour that I'd like to shift up:


With tires it is desirable to reduce rolling drag and minimizing the tire flex, the smallest amount of rubber that has to bend. If you ever look at bicycle tires, the narrowest, highest pressure ones are built for speed and achieve it with the lowest rolling resistance.

Going back to the tire table, the leading candidates are:

• Sumitomo, 175/70TR14 - goes back to the OEM tread width with a modest reduction in revs per mile. The higher load rating is welcome as is the higher, maximum pressure. They are also lighter.
  
• Sumitomo, 195/70TR14 - looks interesting for the front tires because of the reduction in revs per mile. This could shift the sweet spot up 5 miles per hour but lead to an indicated miles per hour and MPG error. At an indicated speed of 65 mph, I would actually be traveling at 69 mph. I can correct for indicated errors but this tire may not fit in the rear wheel wells. Careful measurements will be needed.

I am thinking about buying a pair of 195/70TR14 and running them on the front wheels. I'll also check to see if they can work in the rear without hitting the wheel well. Back out is easy and I can quickly determine how well they work.

Comments?

Bob Wilson