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Tires. Tires, Tires!

Discussion in 'Gen 2 Prius Fuel Economy' started by dormanm, Jul 29, 2006.

  1. dormanm

    dormanm New Member

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    Hi group,

    I wrote week before last regarding a 15,000 mile lifetime of 34-38 mpg milage on my Blue package #9 2005, and the relative ineffectiveness of dealer advice and the driving skills I had accumulated over 40 years of driving. The common thread of the many (thanks!) responses, centered on tire pressure. As I reported, I checked my Pirellis (17 inch) and found them to be inflated to a whopping 25 psi all the way around. I set them all at 40 and promised to report the effects of the higher pressure on fuel economy after our recent road trip from Orlando to Ocean Isle Beach (NC) and back. We're back, and here are the astounding results. Milage on the way up at 65 mph in cruise control with AC on auto at 74 degrees was (no s**t)! 48 and a fraction mpg!!!!! I know this is a 28% increase in fuel efficiency, which stunned me. The other data point that I found interesting was that the calculated milage and the onboard cumulative computer result were virtually identical. This is another piece of dealer misinformation, as I was assured when I first complained, that I "couldn't go by that." lastly, the ride was not noticeably stiffer than before.

    On the way back, I drove at my normal turnpike speed of 73-74 mph, (still in cruise and auto ac) and got 43 plus. After a refuel I drove the rest of the way right at the speed limit and hit my driveway at 45 mpg even. I would have flatly refused to believe tire pressure could make such a difference if I hadn't proven it to myself over 2 and 1/3 tanks of gas with me doing all the driving under controlled conditions. The question now is why the dealer let a car leave their lot with tires so woefully underinflated. (And by the way, on these 17 inch low profiles it's almost impossible to tell by visual inspection if the tire is underinflated!) I suspect it is to give the customer a smoother ride, but what a price to pay! I will have a serious heart to heart with my service manager tomorrow, rest assured. Again, thanks to all, since it's this forum where I found the answer, not my dipshit dealer!

    Mike
     
    #1 dormanm, Jul 29, 2006
  2. B Rad

    B Rad New Member

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    Were the wheels and tires a dealer installed option. If so it seems they would have told you not to expect as good of results as with the stock 15" set up. That being said, I think your trip results were great concidering 17" wheels, what brand/type are they ? I just made a similar trip at similar speeds and got 47.4 mpg. with stock tires set at 42/40 lbs.
     
    #2 B Rad, Jul 29, 2006
  3. dormanm

    dormanm New Member

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    <div class='quotetop'>QUOTE(B Rad @ Jul 29 2006, 07:03 PM) [snapback]294399[/snapback]</div>


    Yes, as usual, we took the car the way they shipped it. I certainly didn't want the 17 " $1200 wheels, lol!
    There was no discussion as to better or worse milage with the 17s (Pirelli tires, a much better tire than stock)
    What frosts me is that they apparently also failed to ,measure tire pressure and make sure the car was ready for delivery, as the tires should have been at 36 and 38 or thereabouts, and were as I said in reality at 25, which is sorta like trying to run in sand. The other problem is they don't really look much different at 40 than at 25, since they're low prpofile tires anyway. I'm seriously considering trading at my tire dealer who also sells wheels for a sporty set of 15s to replace the 17s and then using the stock 185s. Do you find that yt onboard cumulative computer mpg is accurate compared to the miles/gals calculator method? Mine is almost dead on.

    Mike
     
    #3 dormanm, Jul 29, 2006
  4. B Rad

    B Rad New Member

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    <div class='quotetop'>QUOTE(bubblehead1026 @ Jul 29 2006, 09:04 PM) [snapback]294445[/snapback]</div>
    I hate to tell you this, but your car was not shipped from Japan with 17" wheels, they were installed in the USA. most likely by your dealer !!! Shame on them for not telling you about the milage hit. If only we didn't live on different coasts, we could make a trade on our wheels. So if you really what new sporty 15's then I bet you could just post some pics of your wheels right here on Prius Chat and find a slew of buyers willing to trade a couple mpg for a spotier look !!!!!
     
    #4 B Rad, Jul 30, 2006
  5. dormanm

    dormanm New Member

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    <div class='quotetop'>QUOTE(B Rad @ Jul 30 2006, 04:13 PM) [snapback]294718[/snapback]</div>
    Already decided to do just that when I need new tires, but the pressure was the real issue, and of course I know the car wasn't built with Pirellis, but mine came to Orlando from a dealership in Miami, where the wheels (and probably the leather upholstery too) were installed.
     
    #5 dormanm, Jul 30, 2006
  6. theorist

    theorist Member

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    Thank you for the powerful illustration of the importance of not underinflating tires.

    For better or worse, I don't trust dealerships too much. I've taken delivery of new cars with tires inflated above the maximum sidewall denoted pressures, despite their handwritten recommended pressures on their pre-delivery inspection list.

    Over 15,000 miles, you could have easily lost some tire pressure. You'll probably want to check your tire pressure yourself monthly, using your own pressure gauge now.

    In the southern states, 17 wheels and other high markup options are installed at the port. These options are often specific to the South. Before moving back to 15" or 16" wheels, read reviews by others here who observed very modest increases in fuel consumption moving from the lightweight OEM 15" wheels to 17" wheels. I agree that you might easily find someone near you who is happy to trade their 15" wheels and tires for your 17" wheels and tires.

    Pirelli tires are often less fuel efficient that other tires of the same size and performance level. When it's time to replace them you might find this post or this rolling resistance data helpful in finding more fuel efficient tires in whatever size.
     
    #6 theorist, Jul 30, 2006
  7. tazman

    tazman Junior Member

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    Kailua, HI
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    2006 Prius
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    One
    Just picked up my Prius on Friday. Drove around 110 miles and finally got to check my tire pressure today. 25 psi all around!!!! These are the stock 15 inch rims and tires. Can this be the way the cars are shipped and then the dealer does not even check the tire pressure? Pumped'em up to 40/38 psi. Still on the first tank of gas, but even with the 25psi the computer mpg was low 40's. Hopefully this will help the mpg go higher. By the way, I love my new Prius!! :lol:
     
    #7 tazman, Jul 31, 2006
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