Tire Pressure for 245/35/19 & 275/30/19

[mrgizmo04]

Yes max inflation of this tire is 50 psi and I think I've been running them at 45 psi. Which is too high and I'm grateful to the OP for asking the question. Knocking them down should take out some harshness. Incorrect, yes, but 45psi is not skiddy and oversteery.

The max inflation pressure is cold so the comment around higher pressures after driving is an improper comparison.

Depends on how/where you drive. From my driving, my experience has been that running canyons or track (mostly any time of year because of CA weather) tires build up extra 5-10psi from cold. When they start getting closer to 38-40 (hot) they get slidey. I can not imagine running 45 cold in rear. Your contact patch diminishes and I think that outweights the stiffer sidewall you are trying to benefit from by keeping the pressure up (that high anyway) so then the middle of the thread gets very overworked and overheated carrying all the weight and doing all the work, traction is diminished and is less predictable since higher pressure in rear tires also stiffens that axle, ride comfort is diminished and tire wear/life is diminished, which results in more frequent tire purchases and $$.

While, yes, what is stamped on the sidewall is cold max, if you build up pressure from there, that gets into some hairy territory I would not want to find myself in hoping that the plies and cords hold up under the stress I put my tires through.

That said, ymmv, so if it works for you, 👍.

[Speed Monkey]

Now that everyone who’s knowledgeable has weighed in on the topic, it’s time for the peanut gallery (me 🤪) once again to provide my 2¢ worth of opinion. 🙃 I’ll present some pics, then move onto general blatherings, and opinion.

The Pics
On the transition between the most outer tread block and sidewall, there is usually a marker of some sort. To use a Forum favorite as an example, the Michelin P4S, this indicator is identified by the white arrow. I use this factory marker to judge tire pressure. Next, is how I do this.




Blatherings & Opinion
A tire only works best when the tread blocks make full contact with the tarmac, that is with all other factors being within their parameters, like having gas in the tank, not being drunk or stoned, proper alignment, etc. In cornering, the most important tread block is the outer tread block. It’s the tread block that does all the work of biting into the tarmac, not the sidewall; if one’s tire has wear/scrub markings beyond the tread block and onto the sidewall, that’s no bueno.

I use the factory marker to judge maximum allowable outer tread block wear from the tire rolling over. From trial (and lots of) error, 38 psi on all four corners works for me as one who views autoX as a hobby. Perhaps it’s my dim view of matters, but I use the KISS principle (Keep It Simple Stupid), so 38 psi is what I run all the time. 🙃 I like to say, one dances on their feet soles, not on the side of their foot. At 38 psi, the M3’s front end always feel sharp, ready to dance, and ready for action!

[D-O]

On staggared setups, the rear PSI should be higher because the profile is lower to match the front rolling circumference due to wider rear wheel. So that bias does make sense.

Jesse, seems like you should go the other way, higher in the rear given that the tire is 30 series as opposed to front 35 series.

Personally, on MPSS, I run low PSI (30), but I don't care about tire wear. MPSS also turn into bricks once above low 30s.

OP: Try something like 35 F 40 R

The rear 30 series tires have a circumference of 80.11 inches, while the front 35 series tires have a circumference of 81.053. You could run the rear pressure higher to increase the circumference, but I do not believe the less than one inch difference in front to rear circumference is the reason BMW specifies higher pressure for the rear tires.

Higher tire pressures will, within reason, result in a bit more grip. More rear grip means a bit of push, which is certainly safer for the average driver. More pressure will also reduce the likelihood of aquaplaning.

I am shooting for even tire wear without upsetting the car's current balance.

Jesse

[Tbonem3]

The reason we step down in profile in the rear is to match circumferences since the front is 30mm narrower of a tire (225/255)

Matching circumference is what your DSC expects. You don't do it for tire pressures or anything nor do you adjust tire pressures to get matching diameters/circumferences.

So, you'd increase the rear PSI to match wear with the front, not lower it.

[D-O]

Not so, good sir!








front circumference: 25.8 * 3.14159 = 81.053"

rear circumference: 25.5 * 3.14159 = 80.11"

I do understand that the DSC expects similar circumferences front to rear and that to achieve this with different section width tires front to rear the front will require a higher aspect ratio than the rear.

My contention is that the recommended tire pressure difference front to rear plays no part in matching front to rear tire circumference, but is instead intended to reduce the likelihood of oversteer.

Jesse

[jbfrancis3]

Depends on how/where you drive. From my driving, my experience has been that running canyons or track (mostly any time of year because of CA weather) tires build up extra 5-10psi from cold. When they start getting closer to 38-40 (hot) they get slidey. I can not imagine running 45 cold in rear. Your contact patch diminishes and I think that outweights the stiffer sidewall you are trying to benefit from by keeping the pressure up (that high anyway) so then the middle of the thread gets very overworked and overheated carrying all the weight and doing all the work, traction is diminished and is less predictable since higher pressure in rear tires also stiffens that axle, ride comfort is diminished and tire wear/life is diminished, which results in more frequent tire purchases and $$.

I'm with ya on all those good points. Since the OP was asking about daily driving, I was trying to take the edge off with suggesting that the higher pressures were out-of-this-world nuts or dangerous in every day driving. But incorrect, like we've said.

[jbfrancis3]

Now that everyone who’s knowledgeable has weighed in on the topic, it’s time for the peanut gallery (me 🤪) once again to provide my 2¢ worth of opinion. 🙃 I’ll present some pics, then move onto general blatherings, and opinion.

Blatherings & Opinion
A tire only works best when the tread blocks make full contact with the tarmac, that is with all other factors being within their parameters, like having gas in the tank, not being drunk or stoned, proper alignment, etc. In cornering, the most important tread block is the outer tread block. It’s the tread block that does all the work of biting into the tarmac, not the sidewall; if one’s tire has wear/scrub markings beyond the tread block and onto the sidewall, that’s no bueno.

I use the factory marker to judge maximum allowable outer tread block wear from the tire rolling over. From trial (and lots of) error, 38 psi on all four corners works for me as one who views autoX as a hobby. Perhaps it’s my dim view of matters, but I use the KISS principle (Keep It Simple Stupid), so 38 psi is what I run all the time. 🙃 I like to say, one dances on their feet soles, not on the side of their foot. At 38 psi, the M3’s front end always feel sharp, ready to dance, and ready for action!

🤣 A great read and great info. Post=💯

[mrgizmo04]

I use the KISS principle (Keep It Simple Stupid), so 38 psi is what I run all the time. 🙃 I like to say, one dances on their feet soles, not on the side of their foot. At 38 psi, the M3’s front end always feel sharp, ready to dance, and ready for action!

Monkey always with pictures ftw!

Right, but Monkey, 38 works for you and and your style of driving and your car setup - what combination of suspension components you run and how much body roll your car has, how much camber you have dialed in and what tire/how sticky/how stiff of a sidewall, etc.

KISS is good, but we can't generalize 38 for everyone for different reasons. Watching that marker is a good single input into a multivariable system. Grocery shopping on fully stock car setup you don't need 38 because you will never get over that sidewall marker. Autox or tracking on an otherwise stock car except with super sticky rubber with weak sidewalls, no amount of tire pressure will be enough to keep you above that marker and in fact you will probably see wear half way (if not further) down the sidewall (see my example below). So amount of body roll your car has, the amount of camber dialed in, how sticky the rubber and how stiff the sidewall (not all are made equal) all also factor in. With that marker, there is also the opposite that can happen (which I also demonstrate with my second example) - I dont see any wear past the marker on the sidewall, so I'm all dialed in - kumbaya, champagne all around - no.

2 somewhat extreme examples:
I did a run with my dad in his car today (1975 Fiat spider) in the mountains. By nature of how his suspension is set up it requires shimming his control arms to gain camber. He can only net about 1 degree until he runs out of threads that keep the nut securing the control arm + shims to the subframe. Suspension is stock so the car sees a lot of body roll. He ran 38 (! magic number) psi cold in the tires. He runs Conti extreme sport (I think they are 300 or 340 TW). The way I was pushing the car, after we pulled over for a break, we saw that the tires were rolling over about half way(!) down the sidewall (all 4). And very quickly got greasy in sweepers/not super sharp turns and the ride was pretty bouncy due to high pressure. I didnt measure the pressure during the break but I'm pretty sure we got to mid 40s.

When I'm bored (which is a lot) I take either my M3 or my S2K up the same mountain (10 mins away, lucky me). Both run MP4S/MPSS tires and both are set up pretty aggressively with Ohlins, swaybars, a bit more camber than they should (M3 runs about -4 front right now, don't ask ). I run them around 34 cold, and after a run I use a pyrometer and also look at wear across the thread. I am nowhere near the mark on the sidewall in terms of wear and the outer block is a bit fresher and doesn't get as high of a temp with a pyrometer. Now, yes, that means that I could push it harder to get more rollover, but it's a mountain with residential areas and I shouldn't and I don't. I don't need more pressure, i could run less to have the sidewall flex more, but that's not optimal and gets squishy. I know I need to dial out some camber .

Good discussion. I expect more pictures . And let's keep using KISS (or not) and take nationals next year.

[mrgizmo04]

Reading through other posts on circumference and DSC, there can be a difference between front and rear axle, but tire pressure monitoring system will freak out if on same axle left is different from right. It does take adjustment of tire pressure to make sure circumference is the same left to right per axle. Also, I don't remember exactly how much the allowed variance is between front and rear, but if the rear circumference is "much" lower than the front, DSC will freak out because it will think that rear tires are spinning relative to front and kick in.

[Tbonem3]

The dsc circumference thing was a misunderstanding. I was referring to a lower profile tire running a higher psi and how our rears are lower profile due to being wider to have matching circumferences (basically). DO took that way too literally and busted out the exact circumferences and it took off on a tangent which, regrettably, is something we seem to have been able to port over from m3f. Forget dsc, forget circumferences. That can be another discussion as Gizmo started. It was only ever that you'd increase rear pressures, not lower them, following the discussion of the thread.

[mrgizmo04]

The dsc circumference thing was a misunderstanding. I was referring to a lower profile tire running a higher psi and how our rears are lower profile due to being wider to have matching circumferences (basically). DO took that way too literally and busted out the exact circumferences and it took off on a tangent which, regrettably, is something we seem to have been able to port over from m3f. Forget dsc, forget circumferences. That can be another discussion as Gizmo started. It was only ever that you'd increase rear pressures, not lower them, following the discussion of the thread.

T, no worries, I'm just catching up on posts in threads. This thread has taken so many tangents already, haha.

[0-60motorsports]

36 psi all around here on conti's 245/275 19s

[D-O]

Okay. Here is my original response to the OP:

Continental Extreme Contact Sports 245/275 19's

32 front
32 rear

The rears did not wear evenly (center wore noticeably faster that the edges) so I have dropped the rears to 30. We shall see.

Jesse

Accelerated center wear is generally due to the air pressure being too high, or the rim being too narrow. The rear rims on my car are 19 x 10, which seems appropriate for a 275 width tire.

I am interested to hear ideas on how I can achieve more even wear across the rear.

Thanks.

Jesse

[mrgizmo04]

Okay. Here is my original response to the OP:




Accelerated center wear is generally due to the air pressure being too high, or the rim being too narrow. The rear rims on my car are 19 x 10, which seems appropriate for a 275 width tire.

I am interested to hear ideas on how I can achieve more even wear across the rear.

Thanks.

Jesse

Only center of tire wearing faster or center and inside?

[D-O]

Only center of tire wearing faster or center and inside?

Center only.

Jesse