WHP difference between 15in and 16in?
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WHP difference between 15in and 16in?
I've just bought a MY04 2.5TS and it came with stock 15 inch wheels (195/60 HR15). How much would moving up to the 205/55 VR16s (that come stock with the 2.5RS and OBS) affect my wheel horsepower and torque? I saw a post once on some car forum that gave a formula for figuring this out. Also, would the above change affect my speedometer readings or are the tires the same circumference? Thanks in advance!
Registered User
Joined: Nov 2003
Posts: 28
From: Atlanta, GA
Car Info: '02 2.5RS/'01 Forester
The TS and OBS have the same gearing so your TS will run the same as the OBS. Expect it to accellerate a bit slower but get better fuel mileage with the 205/55-16s. On the other hand you could go for a 205/50-16 and have close to the same overall tire diameter as the 195/60-15s. I don't know how they handle the speedo calibration difference between the two models. It may be in the speedometer ciircuit board.
VIP Member
Joined: Feb 2003
Posts: 488
From: 12.9 / 105+
Car Info: black my03 5mt wrx s/w
heavier/lighter wheels/tires will affect the rate of change of velocity. in other words, acceleration. it will NOT affect total power output, ie top speed. if the dyno measures acceleration and derives power (as most do) then you will see a difference; if not, then the power level will remain the same regardless of rotating mass.
also know that the effect of rotating mass is compounded by the distance from the axis of rotation... a 300 lb rod is much easier to spin than a 300 lb disk 6 feet wide. thus the rotational inertia is what you're really looking at. in this respect, a heavier rim and lighter tire will accelerate (and decelerate) faster than a lighter rim and heavier tire, even if the total wheel weight is exactly the same, since the tire is further away from the axis.
hth
ken
also know that the effect of rotating mass is compounded by the distance from the axis of rotation... a 300 lb rod is much easier to spin than a 300 lb disk 6 feet wide. thus the rotational inertia is what you're really looking at. in this respect, a heavier rim and lighter tire will accelerate (and decelerate) faster than a lighter rim and heavier tire, even if the total wheel weight is exactly the same, since the tire is further away from the axis.
hth
ken
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