One of the interesting aspects of a Prius is the effect of different tire sizes for performance tuning. Recently someone asked 'what is the largest tire' and 'what are the effects on mileage' so this is what I shared:
- * * I have experience with Sumitomo:
175/65R14 (919 revs/mile) - what Toyota service centers sell as replacement tires. I got 50,000 miles from first set and only an unrepairable puncture led to their retirement.GPS and mile markers all confirmed the actual was a little generous and the actual speed a little lower. But this was taken as the 'stock tire' from Toyota. This goes a long way to explaining another reason why Prius may be perceived as being slow ... their speedometers are showing a faster than true speed.
175/70R14 (886 revs/mile) - this first pair tested revealed a slight stability improvement on a 750 mile trip to Madison WI and back. I drove up with the smaller 175/65R14 on front and 175/70R14 on the back. At Madison, I swapped front and back so the larger 175/70R14 was on the front and that was when I noticed the slightly improved, straight-line stability. GPS and mile markers show the speed and MPG are less than 1% from true. I really think these are the best tire for the car.195/70R14 (849 revs/mile) - a pair are now my front tires and the
175/70R14 are the rear. Again, my perception is more stability and certainly it is not so flighty. However, it indicates ~6% slower speed than actual (47 mph indicated is 50 mph true) and lower MPG. However, when I correct for the true MPG, there has been no loss. Best of all, the true hybrid transition speed has gone from 42 mph to just under 45 mph. This means I keep up with traffic and the engine continues to shutdown when it can.Here is an image of the spreadsheet showing what I was looking for:
- * * Now I have to be careful and use my GPS speedometer or always subtract
The Prius has a threshold speed, 42 mph for the NHW11, that above that speed, the engine runs all the time. Below that speed, the engine can shutdown. With the larger tires, I can travel 40 mph with traffic and the engine will shutdown whenever it can giving better mileage at this speed. But above 45 mph true, drag rules and the car only gets 52 MPG at 65 mph which still beats the snot out of gassers and diesels: