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Car Forum / Ford / Ford Mustang / December 2007Tire Pressure
Hi all, I have a 2004 Mustang with the V6 that came with the 16 inch wheels. Along the way someone changed out the wheels for 17 inch. The tire size is now 245/45 ZR 17/99W. Does anyone have any suggestions on what would be a good tire pressure to use with this tire / wheel combination ? Thanks for reading my post. Big Duke > Hi all, > [quoted text clipped - 5 lines] > > Big Duke No matter who you talk to, proper tire pressure remains an elusive subject. There should be a sticker somewhere that lists proper pressure for your car and tire. I don't think changing rim size has much affect on tire pressure. Tire pressure is more determined by what the car manufacturer recommends than the tire manufacturer. Tire manufacturers recommend what is best for longevity while car manufacturer recommends what is best for the handling characteristics of the car. On either the doors or door pillars there should be a sticker that recommends proper pressure. My tires say 32 lbs. while the door sticker says 28 lbs. Mine are set to 28 lbs. and I can still smoke the tires :) mike > My tires say 32 lbs. while the door sticker says 28 lbs. The pressure listed on the tire's sidewall is the maximum pressure, not the recommended pressure for a particular application. Ed >> My tires say 32 lbs. while the door sticker says 28 lbs. > > The pressure listed on the tire's sidewall is the maximum pressure, > not the recommended pressure for a particular application. The doorjamb sticker on my 6-cyl 2006 Mustang lists the original 16x7 wheels and 215 65 tires (max pressure 51 lbs), and specifies 35 lbs front and rear. I run GT 17x8 wheels and 235 55 tires (max pressure 45 lbs); the GT sticker specifies 32 lbs front and rear. I reckon the manufacturer is interested in economy for the 6-cyl (narrower, harder tires) and performance for the GT (wider, softer tires). All this determined on the basis of weight per tire, among other factors. I understand the 6-cyl weighs about 150 lbs less on the front wheels than the 8-cyl. They both understeer to a marked degree in their natural state. My suspension is neutral-steer with the (GT) fronts at 35 and the rears at 32. I like the little bit crisper turn-in and the little earlier rear wash-out. On the autocross track I run 44 front, 41 rear. I have the GT 35mm front sway bar, a 26mm H&R bar in the rear, Steeda Sport springs, Tokico D-Spec adjustable struts and shocks (five turns out from full firm front and rear on the street, two turns out from full firm in the rear on autocross track), a BMR strut tower brace. A real treat to drive. I have yet to install the Steeda adjustable front sway bar, reputed to increase front roll stiffness by ten percent on the stiffest hole. The air is a little too thick with smoke and ash to do any work today.  SignatureFrank ess The "Witch Fire" is more than fifteen miles away, so far Packed and ready The Mustang will stay home, if we go > Hi all, > [quoted text clipped - 5 lines] > > Big Duke You need enough tire pressure to provide a flat contact patch under normal operating conditions. "What does that mean", you ask? That means nobody can give you an exact number, only suggest how you might find that number. You're correct in assuming that changing rim size will change the pressure requirement. Also, changing the brand or model of tire will also change the proper pressure. The best way to tell is with a tire pyrometer. Drive it down the highway, then check tread temps. SHould be pretty even across the tread. What? You don't have a tire pyrometer? I thought everybody had one? Isn't it a standard toolbox item? ;) What you want to do is drive for about 15-20 minutes and get the tires warmed up. Preferably in a straight line, with very few curves. Like down the highway. Pull off at the next roadside rest. Cover the entire width of the tread with common blackboard chalk. Now drive foreword about 10 feet. Look at the chalk mark. It should be evenly worn away across the tread. If more is gone from the middle, you have too much pressure. If more is gone from the edges, you don't have enough. If more is gone from the outside edge or inside edge, you alignment is off. Signature.boB 2006 FXDI hot rod 2001 Dodge Dakota QC 5.9/4x4/3.92 1966 Mustang Coupe - Daily Driver 1965 FFR Cobra - 427W EFI, Damn Fast. >> Hi all, >> [quoted text clipped - 26 lines] > more is gone from the edges, you don't have enough. If more is gone > from the outside edge or inside edge, you alignment is off. What a clever way to make a test. Reading the problem, I recalled a similar scenario put to me in a Physics class. The weight supported by the tires was given as was the contact area. For us the question was how much to change the airpressure in the tire if the weight supported was changed by some X amount. If you remove 500 Lbs from the load, how much less air pressure is needed to maintain the same contact area. This problem is similar only what is changing is the contact area and the car weight remaining the same. I think that the factory stickers are the air pressure at max curb weight. Changing the tires changed the amount of contact surface area. It increased it. Figure out by what percentage and reduce the air pressure by that amount. May want to weigh the car and see how much lighter it is from its max curb weight. Then figure out how much to reduce the pressure from the sticker. You now have a base line, Now figure out how many Sq/in the larger tires are and reduce the psi by that percentage. Check it with the Pyrometer or the chalk. |
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