How much of a hit can I expect to get from ultra high performance summer tires?

Discussion in 'Gen 2 Prius Fuel Economy' started by sabbosab3354, Jul 10, 2011.

  1. sabbosab3354

    sabbosab3354 Junior Member

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    - What fuel economy are you getting and how are you determining fuel economy? - Trip Computer - 38mpg average over 96 miles, manual calculation - 25mpg :eek:

    - What fuel economy are you expecting and why? 40-45 doesn't seem too unrealistic.

    - What are the approximate outside air temps? 85-90 Degrees, very high humidity

    - How long are your trips? 10-15 minutes

    - How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving? 100% City driving, delivering pizzas, speeds from 20-45 MPH

    - What's the terrain like of your drives? I live on the Florida coast...we have no hills, lol

    - Is your oil overfilled? No

    How old is your 12v battery? - Unknown

    Have you had your alignment checked? Any pulling or abnormal tire wear? - No, tracks perfectly

    - Are you using the factory tires and wheels? No - Firestone Firehawk Wide Oval AS - 255-50/16

    - What are your tire pressures? 35 PSI front, 32 PSI back

    - Make, model, year, engine and transmission of previous car? 2005 Saab 9-2x Aero (Produced by Subaru while both brands were partially owned by GM, mechanically identical to the Subaru WRX) - Manual transmission, 18MPG

    - How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking? I've been trying to drive as judiciously as possible, lots of coasting, trying to get electric only as much as possible.

    - Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on? No.

    - Are you driving using D or B mode? D

    - HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to? A/C all the time, Auto, set to 75

    I just bought my Prius, it's got 105K miles. The dealer I bought it from slapped new tires on the car before I bought it - nice, but they put the wrong ones on. They are not LRR, but ultra high performance tires.....:(. I figured I would take a hit, but my manual calculations from driving tonight were dismal...roughtly 25MPG. Granted, I didn't use the same gas station or same pump, but I wouldn't expect such a HUGE discrepancy - I will measure at the same pump tomorrow.

    So what do you guys think? Can tires be responsible for such low MPG figures? If so, what would you recommend?
     
  2. cwerdna

    cwerdna Senior Member

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    Thanks for answering the questions!

    I can't speak to those tires off the top of my head, but your short city drives are hurting your mileage. Most fuel-efficient cars is what CR got in their testing of your gen of Prius.

    Do average out your manual mileage calculations over several tanks. You will see quite a divergence between MFD and manual calcs in both directions (one being higher than the other, then reversed) due to the bladder tank and "guess gauge".

    Do test your 12 volt via http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html. I'd suggest doing it after the car has sat being turned off overnight. If the car is still on the original 12 volt, it's likely toast now due to the mileage and heat of your area.

    Try to avoid accelerating so gently that you're trying to stay in EV. That's not the most efficient way to drive. Also, if you can avoid regen (but not brake hard to go to friction brakes), do it. You actually want to limit the battery use (both in and out). It's the so-called hybrid paradox. You have the HV battery but yet need to limit its use to get the best mileage.

    Your constant AC use is also hurting your mileage, but I don't think you have much choice.

    Regarding expectations, http://www.fueleconomy.gov/feg/sbs.htm after conversion to the current method shows 48 city/45 highway, 46 overall. See http://priuschat.com/forums/other-c...uth-about-epa-city-highway-mpg-estimates.html for details about the tests.
     
  3. F8L

    F8L Protecting Habitat & AG Lands

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    Your tire size and compound are not what you want for max mpg on a Prius. They are too wide and designed for LRR. Someday you should switch them out to the correct oem size of 185/65/15. Your current tire is slightly larger than OEM which means it turns less revolutions per mile and will show less MPG.

    Cwerdna covered the rest.
     
  4. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Based on 16 inch tires, I assume he has 16 inch wheels, this may be a 'touring' model.

    The correct tire for the touring model is 195/55-16

    38 MPG would be correct if you were doing rural mail delivery, I would think pizza delivery would not be as stop and go as the mail.
     
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  5. uart

    uart Senior Member

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    Hi rbloedow. Over such a short distance it's extremely difficult to make an accurate manual calculation due to uncertainly in the fuel volume. So at this point we'd have to take 38 mpg as the more reliable of the two figures.

    Yes your tires could easily be causing a loss of up to about 6 MPG. Add to that another 1 to 1.5 MPG in calibration error (an error for both MPG and odometer) due to their oversized rolling circumference and you could be down over 7 MPG on the tires alone.

    Add that 7 and you're back to 45 already. You could probably add another couple of MPG due to heavy use of A/C at low speeds. (Generally the A/C is pretty efficient, most people find that at reasonable speed in free flowing traffic that the loss is negligible, but at slow speed with lot's of stop/start conditions the AC does reduce the MPGs more noticeably).

    One last thing to look into is whether the engine is running all the time (or more often than necessary) under those slow speed conditions, which can knock down the MPG a bit. Do a search here for "hybrid stage 4" to find out more, but for a brief description :

    If the car is warmed up and you find that the engine is continuing to run (idle) even when the car should be "gliding" - then stop the car and wait (in "D" with on foot on the brake) for 7 to 10 seconds at which point the engine will shut down and the Prius will enter "stage 4" with your subsequent low speed driving much more efficient.
     
  6. F8L

    F8L Protecting Habitat & AG Lands

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    I made a mistake and input a 15" wheel instead of 16". If he has a touring modle then the diameter of tire is not too far off but the width is killing his mpg. If hih is a non- touring models then the tire diameter is way off and showing approx. 4-5 mph too slow.

    I would determine what model you own and buy the correct tire size. Preferably a LRR tire.
     
  7. uart

    uart Senior Member

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    Yes it's the width and non LRR construction that's killing the MPG. But the rolling circumference is still about 2.3% over spec, which is enough to account for a drop of at least one MPG indicated.
     
  8. sabbosab3354

    sabbosab3354 Junior Member

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    Thanks you guys - you have been very helpful. I was unaware of the bladder tank...or should i say, unaware of the possible discrepancies with manually calculating MPG from fill-ups.

    That said - this sucks, because I have a great set of sticky, well handling tires....something I would have PRAISED a dealer for in the past...lol.

    Thank you :)
     
  9. F8L

    F8L Protecting Habitat & AG Lands

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    I agree but unless I input the tire size wrong again, it looks like the news tires are 6.11% oversized. Eek
     
  10. F8L

    F8L Protecting Habitat & AG Lands

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    Sticky ultra high performance tires also do not last long so you're taking a huge economic hit. If you like the performance aspect then you may want to look into lowering springs and or a rear antisway bar upgrade. Then replace those tires with something better. With such a big hit in mpg and the likely hood they won't last long it may be better to replace them sooner rather than later.
     
  11. uart

    uart Senior Member

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    I calculated it as follows F8L.

    Extra sidewall height = 255*0.5 - 185*0.65 = 7.25 mm

    The above is essentially the increase in rolling radius, so the increase in rolling circumference is 2*Pi that amount. 2Pi*7.25 = 45.55 mm (4.555 cm).

    Putting this in relation to the nominal rolling circumference of 188cm we get,

    4.555/188 = 0.024 = 2.4% :)

    A change of 2.4 percent would normally amount to around 1.25 MPG on a Prius.
     
  12. F8L

    F8L Protecting Habitat & AG Lands

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    I was using the calculator at 101 Tires dot com. I input 195/55/16 and 255/50/16. I think the difference is you are calculating circumference and I was using diameter.
     
  13. uart

    uart Senior Member

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    Arh yes I missed the fact that it's the touring model with 16" tires. You're correct F8L, it's a whooping 6+ percent oversized!

    Extra sidewall height = 255*0.5 - 195*0.55 = 20.25 mm (2.025 cm)

    Extra rolling circumference = 2.025*2*pi = 12.7cm

    In relation to the nominal rolling circ of 188cm that's about 6.5%. This is enough to account for about a 3.5 MPG error on a Prius.
     
  14. F8L

    F8L Protecting Habitat & AG Lands

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    Don't feel bad, uart. I did the same thing in my first post. :)
     
  15. sabbosab3354

    sabbosab3354 Junior Member

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    Alright - tonight I drove 84 miles, my gas meter didn't move at all. Trip computer reflected 37MPG average, manual calculations were at 44MPG (84/1.8gl).
     
  16. cwerdna

    cwerdna Senior Member

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    I would suggest against short fills as that really increases the margin of error (on your manual calculations) in either direction. Wait until you're down to 3 pips or less.
     
  17. SageBrush

    SageBrush Senior Member

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    I have two years of data following MPG each time I filled up. Most fill-ups were ~ 9 gallons, and I would *still* see big swings in calculated MPG that would even out over two tank calculations. The MFD is going to be MUCH closer to correct than the manual calcs OP is doing on 100 mile runs.

    I suggest heeding the advice given earlier --
    Do not try to drive in EV;
    Head down to the dealer pronto and change out the tyres to the correct size and a formulation for good fuel economy.
     
  18. cwerdna

    cwerdna Senior Member

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    I agree about still seeing big swings. My largest fill was 9.975 gallons but mine seemed to average almost 7 gallons.

    I really think that doing such short fills and runs will cause REALLY big swings, percentage-wise.
     
  19. tonyrenier

    tonyrenier I grew up, but it's still red!

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    If the dealer won't exchange the tires at least inflate them to Max Pressure on the sidewalls, you'll gain a lot no matter what. Buckle your seatbelt the ride will be a lot rougher.
    Good Luck
     
  20. F8L

    F8L Protecting Habitat & AG Lands

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    Another idea is to leave your car on when making frequent stops. This may reduce the amount of time spent in warm up mode which occurs everytime you shut the car off.
     
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