Tire PSI & MPG's

FL335i · August 29, 2014

Well I've had my truck for about 2 weeks now and am driving it a lot for work. I drive 180 miles per day. Tampa to Orlando. I now have about 1,100 miles on the truck. The MPG's on the truck overall are good. I'm happy with them. I have a 5.3L, 3.42 gear, 4x4, CC/SB config with SRA tires. I set my tires pressure cold, as in early Saturday morning before the sun hits the truck and after it's been sitting all night, to 35psi on a known dial gauge that is very good. My TPMS read-out shows 33 cold, and up to 37 when running during the day down the interstate. I noticed in the mornings/at night pressures stay at 34-35 on the TPMS on the Hwy and mileage is not as good as it is in the afternoons when the sun is out and it's hotter and the PSI's are 36-37. That's the only variable I can see. Mileage at night/morning is 18-19mpg. Same road, same speed, etc at 5pm and I'm gettings 20 solid.

1- How accurate is the TPMS system?

2- Does the TPMS system show actual PSI, or correct for temp any? IE- I set them to 35 cold on my dial gauge, when I see 37 on the TPMS at 75mph, is that actual 37?

3- I notice the rear pressure is MUCH more stable. I'll be running 75mph and I normally run just me, empty. no load at all. Rear pressure is 33-34. Fronts are always 1-2psi higher. So they're running hotter and have more weight on them. Wondering if I should bump the fronts 1-2psi vs the rear. 35 rear, 37 front cold. Thoughts?

Thanks guys.

KARNUT · August 29, 2014

Ill take the smooth ride and 22 MPG at 32 PSI hot. 5.3-342- Hypertech tune.

scott19677 · August 29, 2014

Ill take the smooth ride and 22 MPG at 32 PSI hot. 5.3-342- Hypertech tune.

That is exactly what I do. Goodyears suck. Waiting on 'em to wear out. 32-33 -much better ride.

Spurshot · August 29, 2014

The tire pressure variations due to temperature is only one parameter that varies. Another is air density. As the air cools, the air molecules get close together. Your engine may be happier at night for making power(not necessarily mileage) but the truck is a virtual aerodynamic barn. Thicker air is more drag. Aerodynamic drag can be about 2-3 times tire drag at highway cruise speeds.

FL335i · August 29, 2014

^ didn't think of that. And on my work truck, sam '14 config but 2wd and 3.08 gear with the GY Eco tires 33psi cold was the golden ticket. Rode harsh above that. When I bought my personal '14 the dealer had them at like 40psi and it rode like a cement truck. I've only gotten down to 35 cold so far. rides a bit rough. I'm probably going to do 35 Front and 33 rear, cold. Using my dial gauge. Let the TPMS say what it wants.

Retired Bob · August 29, 2014

So, is the fix for a harsh ride to lower the tire pressure up to maybe 5psi (or more) 'below' what the recommended door pillar cold pressure states? Is that the harsh ride blame - certain brand tires, or, could it be something else (shocks, springs, loose components, etc)? I have the GY Assurance Fuel Max P265 - 65R18 with door pillar saying 35psi all around. I have tried 33 all around and now I'll try 32 cold all around as a test. BTW, the TPMS reading in my DIC seems to vary tire-to-tire with no consistency or reason so I use it only to see if any one tire is losing air and needs to be checked.

redwngr · August 29, 2014

Well I've had my truck for about 2 weeks now and am driving it a lot for work. I drive 180 miles per day. Tampa to Orlando. I now have about 1,100 miles on the truck. The MPG's on the truck overall are good. I'm happy with them. I have a 5.3L, 3.42 gear, 4x4, CC/SB config with SRA tires. I set my tires pressure cold, as in early Saturday morning before the sun hits the truck and after it's been sitting all night, to 35psi on a known dial gauge that is very good. My TPMS read-out shows 33 cold, and up to 37 when running during the day down the interstate. I noticed in the mornings/at night pressures stay at 34-35 on the TPMS on the Hwy and mileage is not as good as it is in the afternoons when the sun is out and it's hotter and the PSI's are 36-37. That's the only variable I can see. Mileage at night/morning is 18-19mpg. Same road, same speed, etc at 5pm and I'm gettings 20 solid.

1- How accurate is the TPMS system?

2- Does the TPMS system show actual PSI, or correct for temp any? IE- I set them to 35 cold on my dial gauge, when I see 37 on the TPMS at 75mph, is that actual 37?

3- I notice the rear pressure is MUCH more stable. I'll be running 75mph and I normally run just me, empty. no load at all. Rear pressure is 33-34. Fronts are always 1-2psi higher. So they're running hotter and have more weight on them. Wondering if I should bump the fronts 1-2psi vs the rear. 35 rear, 37 front cold. Thoughts?

Thanks guys.

Correct tire pressures should be on label on end of drivers door or on B pillar.

Pressures will be for a loaded truck, and may seem harsh (especially the rears) if truck is empty. Higher pressure will improve mileage and decrease ride comfort.

There is no such thing as temperature compensation for tire pressure. The tire pressure will go up as the tire temperature increases.

Rear seems more stable because the tire temp of the unloaded tire is not increasing as much as the tire temp of the more heavily loaded fronts. Load the back up to max and when driven the rear pressure (and tire temps) will go up also. (flexing the tire creates heat -- flexing it more due to load creates more heat)

FL335i · August 30, 2014

^ exactly. OK, I have 35psi cold on the front, 33psi cold on the rears. I noticed with 35psi in the rears, unloaded I the outer edges were not wearing. I also drove home last night in a rain storm on I-4 from Orlando to Tampa. If I dropped the hammer to pass a car at 60mph the traction light would come on!! So I'm going to try 35F/33R now. If I tow or load up I'll up them to 35. I may even drop the rears to 32. I'm going to put a paint marker line across the treads. I've done this on many cars and it gives you an idea if you have to much or not enough air in the tire after driving it for a few hundred miles. If you see 1/2" of paint on the outer edges you need to let some air out. In Autocross racing they use this method also. I can tell the fronts DEFFINITELY need 35. Heavy front end.

I set my spare to 36 or 37psi. Being I only check that thing about every other month or before a long (2hrs or more from home). From my experience on multiples vehicles the spare always loses some air even doing nothing. The tires on the car lose air too, but I check those 2x month.

brannnor · August 31, 2014

what about Nitrogen filled tires anybody tried this Norm

MikeNH · August 31, 2014

what about Nitrogen filled tires anybody tried this Norm

Won't hurt anything but paying extra for the service is a ripoff. Maybe if you're landing a space shuttle or using a race car but not road vehicles.

Spurshot · August 31, 2014

what about Nitrogen filled tires anybody tried this Norm

If you use a source with lots of moisture in the compressed air, dry nitrogen may help slightly to give more stable pressures vs temperatures. But don't bet on seeing any difference if you air source is normal.

jocassee · September 1, 2014

I have the Cooper Discoverer H/T plus on mine.The dealer gave me a 500.00 credit for the crap goodyears that came on it toward the coopers. The size is 275/55-20. I currently have 12031 miles on them and ive been keeping a record of the wear every 500 miles with a depth guage. Now i'm sure different tire companies get different results, but from what I can tell, mine are wearing dead even on the front with 38psi and even on the back at 35psi. I've tried the nitrogen route with my wife's Murano, but I still had to add air to them not two months after it was done. If youve never tried the cooper tires, I would suggest trying a set. The last set I had on my 2010 SIlverado were wore out at 46000 miles. Ive never got that off any other brand so far.

ToesonTheNose · September 1, 2014

Nice thread, I too live in Tampa, that drive to Orlando on I-4 bless you man, I make the trip maybe once a month when I go surf in cocoa beach. My tires are between 30-33psi depending on the day and where i am driving. I'm averaging about 22mpg i've noticed this Onstar app will show my tire pressure at 28psi sometimes. i sure hope that is wrong

MTB · September 2, 2014

I've had my truck serviced for reading low pressures on one particular tire. Not sure what they did to fix it but it appears to be functioning now. I think I'm going to run out and grab a paint pen this evening and see what is really going on with my tires. MPG maximization = more play dollars in my wallet...