Prius Tire Tuning

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:

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and mile marker testing showed the 175/70TR14 brings theindicated and measured MPG to within 1% of true) I wanted to find the largest possible tire that could fit on an NHW11 Prius. Of the candidates, the 195/70TR14 was the largest so I bought one and mounted it to see if it would fit in the rear wheel well ... it did. I then drove around looking for speed bumps and dips to make sure it didn't rub ... it worked fine. So I ordered a second one and put both on the front.

  • * * Now I have to be careful and use my GPS speedometer or always subtract
6% from the indicated to know my true speed. I also have to adjust my mileage records by 6%. But so far, there has not been a loss of vehicle MPG other than I catch myself driving a little faster now.

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:

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Bob Wilson

Reply to
bwilson4web
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The Prius is a gasser too, Bob--or have you forgotten that already?

Reply to
Elmo P. Shagnasty

hehehehe So Bob *does* lie and make things up in order to try to make himself look better to the ignorant others in the world.

Reply to
Elmo P. Shagnasty

What's that I said, Bob?

Reply to
Elmo P. Shagnasty

Ummmm....no. They use instrumented data to measure braking performance.

It's a simple thing to do, really--and yet you can't/won't do it?

Reply to
Elmo P. Shagnasty

It has nothing to do with damaging the ICE, it has everything to do with damaging the transaxle, though. The lubricating oil/fluid pump for the transaxle on NHW11 and NHW20 is driven by a shaft that extends from the planet gear carrier of the PSD, through the center of MG2, to the pump which is in the MG2 side cover just inside the driver's side wheel well (on LH drive cars) Since the PSD planet carrier is driven only by the ICE, any time it is off the trans is essentially unlubricated and relies on the residual oil film in the bearings to keep it from self destructing.

Also for NHW11 and NHW20 there is no such thing as a speed where it transitions from just electric to electric and gas, because the speed for the transition cam be anywhere between 0 and the top speed of 105MPH depending on throttle pedal position. If you hammer the throttle from a dead stop it starts immediately, if you were to put it in drive and roll down a mountain road without touching the pedal it might never burn a single drop.

What does always happen is that at 42MPH indicated the engine crank begins to turn, even if it isn't burning any fuel, AKA the car transitions from "stealth" mode to "warp stealth" mode.

Reply to
Daniel who wants to know

SURE, bob. SURE.

So far so good.

So what does any of that have to do with braking distance?

You were asked, quite directly: "did you ever measure how the tire sizes might effect the braking distance?" And your answer was, as quoted above: "Not yet since I'm trusting the ABS to moderate the braking force."

You have yet to provide any connection between your willingness or ability to measure braking distance and the fact that ABS is functioning.

Of course, there's a reason for that: there's no connection between the two. You just spouted random words that didn't answer the question at all.

Why was that, Bob? Was it because you don't want to answer the question?

Reply to
Elmo P. Shagnasty

REFERENCE: J.W. Daws, ?Technical Considerations for Plus Sizing,? submitted for presentation at the 2008 International Tire Exhibition and Conference, September 16-18, 2008, Akron, OH

"General Motors Corporation stated in a recent publication [?Plus-sized Problem??, Tire Business, July 19, 2004] that wheels used for their upgrade packages have the ?same mass, same offset, same width, same mounting flange, same tire pressure monitoring requirements, same brake clearance, [and] same dimensional tolerances? as the original equipment wheels. Implicit in this tight specification are the impacts of the plus-sized fitment on anti-lock brake systems, electronic stability control systems, and so on. Obviously, a wheel with the same mass at a larger OD has a larger rotational inertia than the OE wheel. Generally, as wheel diameter increases, the mass of the tire and wheel for equal load capacity generally increases, as does the inertia of the rotating system. This change in unsprung weight may affect the response of the suspension system. The change in rotational inertia may affect the response of systems like antilock braking and electronic stability control. Tire pressure monitoring based on antilock brake sensors may also be affected by changes in tire size and vertical stiffness. At this time, there is a significant lack of public domain data available to assess these effects."

Reply to
News

So your idea of "testing" involves nothing more than your own example of "yes it stopped".

Oh, bullshit. You have no data to back you up. Again, you're making things up.

It puts all of your pretty charts and graphs into perspective.

I don't have to have data. You claim that braking distance is unaffected, and I asked you for the data to back up that claim. You refuse to provide such data, to the point where you say that all you need is the seat of your pants.

If you want to make claims you can't back up, expect to be called on that in public.

Reply to
Elmo P. Shagnasty

There's a lot of potential liability attached to them, hence their seriousness.

Reply to
News

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