Aerodynamicists and efficiency experts, tire advice needed.

Discussion in 'Gen 2 Prius Accessories & Modifications' started by Bunce, Jan 10, 2021.

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  1. Bunce

    Bunce Active Member

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    Firstly I am looking to make my Prius sort of like the Subaru Outback or Volvo XC90 by getting more clearance and more grip so I have less worries driving the amazing forest and desert dirt roads of Oregon. (Obviously I still only have 2wd.) I know my MPG will decrease and I'm ok with that. It'll still beat my VW Vanagons 19mpg best. I'll still keep the old wheels and tires for long on road trips.
    I have lifted the Prius 1.5" already. I have just bought some used OEM wheels as they are the lightest for a good price at 14.2lbs. I now have to decide which tires to mount.

    I'm currently tied between the 207/70/15 and 215/70/15 Yokohama Geolandar AT GO15.
    GEOLANDAR® A/T G015 | Yokohama Tire

    There's not much difference, but if one gives more mpg, that's the one I'll go with. But, how does one factor in the different variances of tire width, tire weight and vehicle height?

    The 205/70/15 has a diameter of 26.3" and weighs 27.3lbs
    The 215/70/15 has a diameter of 26.9" and weighs 26.7lbs

    Which would you choose and why?
     
    #1 Bunce, Jan 10, 2021
    Last edited: Jan 10, 2021
  2. TMR-JWAP

    TMR-JWAP Senior Member

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    the stock 185/65-15 calculates to a 24.5" diameter, (185 x 0.65 x 2)/25.4 +15) so either tire already lifts the car about 1 inch extra. Will the tire place work with you? Throw the biggest ones on the front and see if they clear from lock to lock? If they'll work with you on it, I'd try to get the larger ones.
     
  3. Bunce

    Bunce Active Member

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    I'm pretty sure both will clear, I'm just looking for the size with the least bad MPG effect and don't know how to weigh the variables. I'd guess the shorter, narrower tire, but weight is important. I have snow tires with steel wheels on at the moment and the extra weight can definitely be felt. The new setup should weigh about the same but the weight will be pushed further towards the circumference due to the heavier A/T tire on an alloy rim as opposed to a heavy steel wheel with OEM size winter tire.
     
  4. TMR-JWAP

    TMR-JWAP Senior Member

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    Those tires look like beasts, lol. I think they're going to work great for off-paved road driving. I would probably go with the 205, since it is the maximum recommended for a 6" wide rim.

    Equivalency table
    Rim width Minimum tire width Maximum tire width
    5,5 Inches 165 mm 195 mm
    6,0 Inches 175 mm 205 mm
    6,5 Inches 185 mm 215 mm
    7,0 Inches 195 mm 225 mm
     
  5. Bunce

    Bunce Active Member

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    Well it looks like the decision has been made for me, that makes it easier. (Even though Miata guys run much wider tires on the same width rims and drive way harder than I'll be driving the Prius.)
    Now to just wait for them to go on sale, hopefully Presidents day.
     
  6. Just Peg

    Just Peg New Member

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    Hi Bunce,

    I read a Prius chat today of various owners buying different rims and tires and showing them off in pictures. The one that got the most praise was the lightest weight (forged) rim wheel and tire that only weighed 8 pounds. Everyone was jealous of the light weight wheels. Based on their reaction, I think they would say get the lightest weight set. They thought the lightest weight set would get the best MPG. I agree. Everytime you accelerate and brake, you are fighting the inertia of the weight. To accelerate, more weight means using more gas. To decelerate, more weight means more wear and tear and heat on your brakes. Heat on your brakes is lost energy.

    Other than that, the wider tires will work better in sand and mud. The larger surface area makes it less likely to sink into semi-fluid mediums. Wider tires are more likely to hydro-plane on wet roads, increasing the chances of skidding.

    The skinnier tires will cut deeper into the snow. (Skinnier tires with smaller surface areas are more likely to sink into semi-fluid mediums.) If skinny tires cut deep enough into the snow, they might reach the asphalt and then you will get better traction. They will also get better traction on wet roads.

    If the diameter is larger than the factory standard, it will distort your cars calculation of mileage driven: distance traveled per revolution = 2 * pi * radius of wheel. Change the radius and you make the odometer reading proportionately wrong. Larger radius = odometer reading is smaller than actual distance traveled. Smaller diameter = odometer reading larger than the actual miles traveled (affecting resale value, and warranty.)

    If the diameter is WAY TOO BIG, then you risk scrapping the wheel wells, especially during sharp turns where the wheel sticks out the sides of the wheel well.

    I just read that the other reviewers said the 215 was too wide for your OEM wheel.

    If you go to Tire Rack they list a UTQG rating for the tires they sell. It is an industry rating of treadwear, traction and temperature range that the tire can endure. I use this plus the customer review ratings to choose my tires. I looked up your tire. Customers rate it 8+ out of 10. That's good. The UTG is 600 A B. That's good too. This is the ONLY All-Terrain, on and off road tire, that Tire Rack recommends in this size. GREAT CHOICE! That's exactly what I would have chosen, if I could put that size tire on my vehicle.

    Please tell me how to get my car raised up. My car is way too low to drive over the snow in the winter, and over the rocks in the road. I would sacrifice a few mpgs to raise my car up too. Not all Prius owners live in the city. I live in the forest, down a steep winding hill, with fallen rocks, a speed bump, and lumps in the driveway.

    - Peg
    (An Aerospace Engineer) MSAE
     
    #6 Just Peg, Feb 27, 2021
    Last edited: Feb 27, 2021
  7. TMR-JWAP

    TMR-JWAP Senior Member

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  8. Bunce

    Bunce Active Member

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    Thanks for the detailed response. I've been wanting to chat with an Aerospace engineer for a while.

    Here's the easiest way to lift your Prius. Priusoffroad - Prius Offroad, Lift Kits

    I think I might try a different tyre. I've found all weather tires, not to be confused with all season tyres. The Toyo Celsius 205/70/15 with a 26.3"diameter, weighing in at 22 lbs per tyre, looks like a good option for increased traction without as big of a weight penalty. Not as heavy duty but with a full diameter spare ( I would have to track down a suitable replacement, perhaps from Subaru) it's a risk I'm willing to take. I prefer a taller, narrower tyre as it keeps more of the tyre in the wheel well and only exposes the extra height/2 under the bumper so minimal frontal area difference. Also, in low traction situations letting out air makes a longer footprint for more forward traction.

    Now, I'm thinking about a rooftop tent. I read somewhere that installed a roof top cargo box backwards is much more aerodynamic and I'm assuming that's the case with a roof top tent also. Could you lend some insight into rooftop box aerodynamics? And whether blocking wind from flowing under it is better or creates a vacuum and is worse?
     
  9. Dylan Decker

    Dylan Decker New Member

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    Hi Bunce,

    This thread has been extremely helpful (thank you) I am currently looking into the Prius Off-road lift kit 1.5”. Can you share what MPG you’re currently getting with the lift kit + the larger tires?
     
  10. Bunce

    Bunce Active Member

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    I don't have larger tires at present, I have snow tires on which are near enough stock size. But the steel wheels and snow tires are noticeably heavier. With things the way they are we haven't been anywhere to get a good guage on mpg yet. I haven't even had it aligned but will remedy that soon. I can report back in a few weeks.
     
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