Just picked up my new '15 today I was disappointed that my dealer didn't even set the date on the radio display AND the tire monitor showed them all over 40lbs! I fixed it myself by letting air out. My pocket gauge said 32/33 but when I turned the car on, it was saying 30/31. Is it supposed to be different, not a true reading?

Also about tires....Are the Mustangs made with air or nitrogen in them or is that at the dealer's discretion? My salesman checked with service and was told that they don't deal with nitrogen, I'd have to go to a tire store. So there are NO Ford models with nitrogen tires? If someone bought one here the dealer is not able to refill them... that sounds kind of odd. But we looked at the other GTs on the lot and none of them have green valve caps so I am thinking they're all air.

And I did get the one with the spare tire.

 

I can't answer the first part, but Mustangs do not come from the factory with Nitrogen. My dealer put nitrogen in for me after purchase.

 

Is it worth changing from air to nitrogen?

 

I'm not a good judge of this. I've been told that the fluctuation in pressure that happens between cooler/hotter weather is non-existent (or at least reduced). Others may have other input.

(This is the first car I've had with Nitrogen, and it's only been in for a day).

 

You sure the label on the door jam says 32 psi?

 

I think I would trust the monitor pressure reading before the hand gauge.

I have a pretty good pressure gauge and for every 3 readings one is different if it is not held at the proper angle when pushing it on the valve stem.

 

Not really, the biggest benefit to pumping nitrogen in the tire just comes from being a dry gas. If you live in an area that has little to no humidity that benefit is relegated as well.

Nitrogen does leak out slower but from what I understand its almost no different from regular air.

If its free, adding nitrogen couldn't hurt but if your paying for it then pass on it.

The worst offender I've seen in the nitrogen rip-off market is a local Honda stealership. They add 200 bucks to the invoice of the vehicle from filling the tires with nitrogen.

What makes this particularly offensive is that they are most certainly using a nitrogen generator which has no moving parts other than valves and a few filters to protect the membrane that strips the nitrogen out of the air.

Its just a passive positive pressure system and IIRC the generator costs around 6k with the holding tank and you plumb it into your air supply and costs pennies a day to operate, especially after the tank is initially filled and the rest of the time its just being topped off based on demand.

 

Trust TPMS over hang gauge. Those things can be off by a few PSI easily. TPMS is calibrated for sea level, and accurate to 0.1PSI

 

The tires were probably filled to 40 psi by Ford and left by the dealer so they wouldn't go flat while the car sat unsold or not test driven.

The issue with gauges and sensors can be a difference in calibration not only due to the reference device (hopefully accurate at time of manufacture) but age in combination with wear and tear as well as temperature. Also mechanical gauges tend to lose accuracy more often compared with a digital gauge (the worst offenders are those pencil gauges).

Its also worth mentioning that the placard on the drivers side door sill is a cold inflation pressure for a typical load and pressure should be checked when the tires are cold (vehicle has sat undisturbed at least 8 hours or overnight) or the vehicle has sat for a few hours.

As for checking the pressure with warm or hot tires (depending on the season and time of day and driving conditions the tire could be considered warm or hot in just a mile or two) its hard to say. I will typically add 2 psi in the winter or 4 psi on a particularly hot day if I've checked the pressure after driving my car.

Never adjust or let somebody adjust the pressure down to the cold inflation pressure on the door sill if the vehicle has been driven.

That's obvious but a lot of people don't realize that and down right amazing when people supposedly trained as tire technicians do it as well.

 

I use an 'Oil Filled Tire Pressure Gauge' that I purchased many years ago from Speedway Motors. It was the gauge, at the time, they were using for their race teams/cars.

Tires, IMO, are the most important thing on a car (next to brakes) to keep you, your family & the public safe...Why go cheap on a Tire Gauge-->

 

Congrats on the new Mustang and I think you made the right choice going with the spare.

 

The pencil gauge actually samples the air via the valve but how does the car's TPMS calculate it? Interpolates pressure based on what measurement? Is that really more accurate than a pencil gauge applied to the valve? If it is anything like the "miles to empty" reading then its not exactly accurate.

My car had sat overnight, and we test drove it about 4 miles, then it sat for about an hour maybe more being detailed before I checked it and let out some air. The bad thing here is that you have to pay for air at gas station pumps so I'd have to put some in, then check the dash. A much more involved process to get the dash reading to be 32 all around.

Joeywhat - I did look at the door label. It says 32.

 

TMPS uses 4x pressure senosors mounted in each wheel. This is called Direct TPMS. The sensors don't interpolate pressure, but actually measure it using a Wheatstone Bridge method, where the resistance of the measuring circuit changes with pressure, and based on the remaining constant thevenin equivalence it calculates pressure. Very accurate, and calibrated to sea level. The information then is sent via RF to RKE module (remote keyless entry). Than it makes it onto a CAN bus and can be displayed in the Cluster.

Indirect TPMS uses a change in wheel speed causued by deflation of tire, and reduction of rolling diameter. This method is not allowed in the US, hence direct sensing is used.

Tire pressure monitoring was my senior design project when I was finishing my B.S.E. in Computer Engineering. I spent the following 2 years working for a TPMS company.

Miles to empty is a simple average of MPG/remaining fuel estimate. That's why it is not very accurate at all.

 

Ahhhhh that's very interesting. Was there a time when indirect TPMS was used, like in the early days, especially on some GM models? I think I recall my mom's old Alero had a "low tire" light and that it was calculated from rotation speed just like ABS.

I noticed that the pressure does change quite a bit, up and down maybe 1 or 2psi as I drive. I assume this is normal? Also, does Ford's system accurately display when the car is parked? On one tire it did but the other tire it didn't change until I actually drove. Is there any way to reset it?

 

Absolutely true, you cant do anything without tires and tires can definitely make or break a car.

 

Yes, you are correct. Before direct TPMS was mandated in ~2005 there were some indirect systems out there.

Also, as the temperature of the tire goes up, the pressure will go up as well. That's described by Boyle's Law PV=nRT. So if volume stays constant, gas constant and R are constant as well, then increase in pressure is directly proportional to increase in temperature. I see my temps go from 35psi to 42psi after a 20-30 min road course session.

When the car is moving you'll get updates every 30s or so. When the car is stationary, the updates come once very 30min or an hour. Not much you can do about that. Sensors go to sleep to conserve battery at standstill, and transmit much more frequently when moving. But fear not they will wake up on a large change in pressure, and transmit. If the pressure changes by more than 4~5 PSI the sensor will transmit. These limits are in the sensors ASIC, and there is no way to change that.

 

Haven't heard whetstone bridge term in pr

 

So if you want to fill your tires to a specific indicated pressure, what's the best way to take the reading? You have to fill when standing still obviously, but the reading might not reflect putting air in and the more you move the car the more likely the number will increase on its own due to expansion.

 

My post obviously got cut short... Good info on the TPMS, I haven't heard whetstone bridge since college! Little throwback there for me.... Interesting read though!

 

I've also noticed the psi fluctuates. Is it normal for the tires to have different numbers. My left tires read between 38 and 40 and the right between 37 and 39.