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40/38 vs. 42/40 - Tire pressure

Discussion in 'Prius c Fuel Economy' started by UnSurreal, Apr 28, 2012.

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  1. UnSurreal

    UnSurreal Junior Member

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    Location:
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    When I drove the car the first week and half - it was at 35/33. (max 44)

    After upping to 40/38 - the results were great. Thinking about trying 42/40 for the second tank.

    Just wondering what everyone else is doing - 40/38, 42/40 or even higher?
    Pros and Cons?

    Thanks.
  2. jdcollins5

    jdcollins5 Senior Member

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    I went from the dealer 35/33 to the 42/40 and saw immediate mpg increase of 1-2 mpg. After a while the ride was just too harsh on uneven pavement. I lowered mine down to 40/38 and the ride was much better and I could not tell any mpg change.
  3. DC ebikes

    DC ebikes New Member

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    Hi! Just drove my PC II home from the dealer...pretty awesome car- I didn't adjust pressures yet- will do it tomorrow. 27.9 miles 64.4 mpg!
  4. priusCpilot

    priusCpilot Member

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    I am now running 43/40 because I wanted to keep the factory rear 3 psi bias. Wow I love the road feel now and yes it is more bumpy but nothing to much for me to change. I even have the 16" so its harsher but still not that harsh at all for me. Let see how this tank comes out?
  5. GraeSack

    GraeSack New Member

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    Just picked up my Pc3 yesterday. Set tires to 40/38. See how it goes.
  6. kkim

    kkim Active Member

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    after trying a bit of different pressures, we've settled on 38f/35r as a happy medium between comfort and mileage. It all depends what you are willing to sacrifice in the holy grail of MPG.

    Be aware that over/under inflated tires can lead to abnormal tire wear and tire grip is lessened after you hit the sweet spot. What that sweet spot is depends on your road composition, surface conditions and many other factors. In other words, you need to experiment to find what works best for you and your locale.
  7. HaveNoCents

    HaveNoCents Conservative Tree Hugger

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    I went form recommended tire pressure to 40/38 and immediately saw a 4 mpg improvement. I am currently trying 42/40 and so far I am at an additional 3 mpg. 42/40 is the most I am going to go on a 44 max psi tire. With the heat here in Texas I don't want to push it.
  8. F8L

    F8L Protecting Habitat & AG Lands

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    Temperatures are warming up all around the northern hemisphere so be sure that you take weather conditions into account when calculating changes in mpg....
  9. UnSurreal

    UnSurreal Junior Member

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    ^ so the car performs better when it's warmer out?
  10. F8L

    F8L Protecting Habitat & AG Lands

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    Indeed!

    Warmer temps:
    Rolling resistance is reduced considerably
    Air resistance (drag) is reduced
    HV battery efficiency goes up
    Fuel burn tends to be more efficient or at least summer fuel blends are used
    Engine warm up times are reduced
    Less water/snow on roadways so rolling resistance is reduced
  11. j_benj

    j_benj Prius C Fan

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    I'm riding 40/38 now for a good balance of comfort and MPG. I'm in Jersey and lately it's been cold mornings and warm afternoons. I notice a bit of a reduction in MPG on my morning commute (53-55mpg) to my evening commute (58-63mpg). Another factor in the morning disparity is that I hit the freeway while ICE is still warming up. When I leave work I have about 8-9 miles of back roads before hitting the freeway. Makes a bit of a difference.
  12. XRinger

    XRinger Member

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    I'm trying 40/38 on the c2 and it feels pretty good.
    Did a ~50 mile road trip this morning and got 62.35 average MPG.
    My wife drove out (59.6 MPG), I drove on the return trip(65.1 MPG).
    After I get some more records, I might try a higher pressure,
    if the comfort level stays the same.. (Tires are Goodyear Assurance FuelMax).
  13. Indy John

    Indy John Member

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    Max PSI is not safety-related - it is the pressure at which the tire's ratings were established. "Max" means only that no further improvement in load rating is achieved above that level. A thread many moons ago from a radial tire design engineer provided that insight.
  14. pubby

    pubby Junior Member

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    How is it so far? Any differences running it 3 compared to 2 bias?
  15. HaveNoCents

    HaveNoCents Conservative Tree Hugger

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    Did you see his credentials? Will he pay for all accidents and injuries caused by over inflating tires. Although I love the people on this board they are not the know all, and be all in tire safety.

    First off, over-inflation makes it more likely for tires to be cut,
    punctured, or broken by sudden impact. Does this not have anything to do with safety? All of you that own cars with no spare please be aware of this.

    Your handling characteristics with over-inflated tires during wet weather, or on road irregularities are much more dangerous than with properly inflated tires. Lowering your ability to handle your car is not a safety issue?

    I think I will stay with 42/40, thank you.



    There
  16. zebelkhan

    zebelkhan Member in good standing

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    Obviously if you over inflate anything you increase its chance of failure. I think what Indy was saying was that the max pressure rating on the tire is there because that is where all the ratings were done, not because that is the maximum pressure you can safely have.
  17. walter Lee

    walter Lee Hypermiling Padawan

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    Even a brand new tire can blowout if you hit a road hazard. I have overinflated my 2010 Toyota Prius III stock Yokohama Avid S33D tires to 50 psi front/48 psi rear for over a year and they are not wearing out faster or unevenly after +23,000 miles - the change form 44psi front/42psi rear to 50psi front/48psi rear has help pushed my best hypermiling tank fuel efficiency from 66 mpg to 70 mpg. Note that my onboard computer display average speed is about 18 mph (my normal top speed is about 35 mph) and I'm driving on smooth asphalt roads most of the time in mainly in city-stop n go and suburb slow n go traffic. A tire can wears faster under the heat generated by driving at superhighway for long periods of time. In addition, I accelerate slowly and I'm using regen braking mainly and using the hydraulic brakes only lightly (under 15mph). My experiences is that above the maximum sidewall tire pressure, there is some loss of tire traction under special conditions: 1) when the road is wet and the vehicles is going over 35 mph downhill then there is some loss in stopping power so you'll need more distance to stop. 2) when the road is dry but the vehicle is going over 65 mph and there is strong crosswind then you'll need to do some slight steering corrections. For those wishing to be extra safe, at 38 psi front 35 psi rear for Yokohama Avid S33D with a 44psi max tire sidewall pressure I found no loss in tire traction/grip/braking performance even at speeds of 82 mph. Given everything I know sofar, I believe that given the same driver and route conditions a Prius c should beat a standard Prius hatchback in fuel efficiency .
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