pteam

I have 20" oem rims 275/50 R20s Goodyear Eagle LS2 runflats. The door jam suggested 32 psi front and 38 psi rear. The 32 especially seems low pressure for gas milage. Also the font and rear rims and tires appear to be the same rim and tire size and I'm not sure why they would want lower psi on the front rims, maybe for better steering imput or something? Max PSI is 51psi listed on the tires.

What do you guys suggest for max psi? The vehicle came with all 4 tires at 38psi (bought new). Would say 40 or 42 psi all around yield better results and possibly tire life? I know different people have different views on safety on psi also.

The U.S. Department of Energy says that for every 1-psi drop in pressure, you can expect your gas mileage to lower by 0.4 percent. A google search brought up mythbusters http://www.popularmechanics.com/cars/news/4199963 who state that an increase in tire psi will not increase your gas milage, but the test did the same route going 2 different directions where one was uphill more vs downhill on the other direction.

Also I would not want to reduce tire life either as that wouldn't make it worth it. I do love getting max mpg, it just kinda feels satisfying. No matter how much money you have or dont have there is no point in wasting gas and money. I took my roof racks off the 1st night I got the GL and will only use them when actually putting stuff up there. I've also noticed the diesel really decelarates when you let off the gas or turn the cruis control off. I can almost come to a complete stop when making a turn or coming to a stop light just by letting off the gas, I rarely have to use the brakes if you drive it correctly. The only concern is when other drivers are behind you I'll hit my brakes lightly sometimes just to let them know I'm slowing down.

In some of my other cars I often times would throw the tranny into neutral when coming to a stop or coasting down a hill and watch my mpg goto 99mpg even in my hummer h2! However I don't feel comfortable doing this in my GL mostly because of the way the stalk is and it doesnt look easy to throw it into neutral while driving and I wouldnt want to mess up and have a huge repair bill. This is much easier when driving a stick.

I've also noticed that the diesel right now in winter time can be upshifted one more time by manually flicking the shifter paddle and get better mpg.

imput? other mpg tips? Thanks!

Also Merry Christmas to Everybody!

justthebest

Merry Christmas!

And a couple things - I wouldn't put any stock in psi at the time of delivery. Some tech may have been airing up the tires like s/he always does, etc.

As far as increasing the pressure, I think the largest sacrifice will be ride comfort. If you can stand it, air them up to the mid-40s & keep track of your mileage. When it's time to replace the tires, make sure to look for tires specifically designed with a very low rolling friction coefficient.

Good luck

TamZ

I had mentioned the rough ride to the new mechanic at the dealer that is working on my GL now. The service dept. that I originally bought mine at had mine at 42-44 psi. The new mechanic asked me if my ride had improved as he put my pressure back down to 32psi/38psi..I DO think I noticed quite a difference in the ride. Not sure about the MPG, because I do mostly in town driving and it's never up in the high 20's that I read on here. (usually 19mpg in town) When I first got my GL, I felt like any speedbump or bump in the road, it was like hitting a rock. I do mostly in town driving and our roads are horrendous here...so my GL never has what I would call a "nice ride." But on the open freeways, smooth roads, I have no complaints about the run flats.

Maybe the fact that he keeps mine at the lower end, is why my MPG is not as high as I read others have on here.
That's my non-professional opinion on psi. hehe.

43221B

Keep 32 psi in all four unless you are loaded in the rear or you are towing. My 09 graphically explains that inside the fuel door. You might not have opened yours yet since your gl is new. When I go on a trip and i have my whole family and luggage loaded i'll add pressure to the rear. Be sure to reset the tpms though. The sidewalls are very rigid on the runflats so you don't need to over inflate for better fuel economy. I've kept my tires at 32 since new and I'm getting excellent wear. I rotate my tires myself so that I know that they are done correctly (or simply done at all) and so that my rims don't get beat up. Even though I have only rotated 3 times (I'm at 21,000 miles) I should still be able to get at least 40,000 out of the tires based on the current wear.

Keep in mind that your tires are designed for high speeds. The pressure will rise when driving at high speeds for a long period of time. The tire pressure specified on the fuel door or in the manual is going to take these fluctuations into account and help to keep the pressure within a safe range.

As far as shifting is concerned, unless you baby the engine and I mean really baby it, the transmission will learn to run a gear lower than necessary at speeds below about 50 MPH. So yes, I've noticed the exact same thing which is you can paddle shift up a gear to significantly increase you mileage when driving at slower speeds. This is the case at least in the diesel.

pteam

Tamz try upshifting when cruising. I've noticed the diesel will often be in a gear lower than it should especially when warming. EX: it'll be in 4th gear instead of 5th. Click the upshift paddle on the right once and you'll see your rpms drop and your gas milage increase

I was reading about another mercedes like the e class or c class has an "economy" mode where it will upshift faster in that mode. I really wish the GL had this, especially with 400 torque I very rarely need to be above 2000rpms. So for now the "economy" mode is my manual upshifting

justthebest

My GL has sport & comfort (aka economy) settings?

I would also note that based on my other MBs, the higher gear warm-up period is there for a reason. So definitely use it, but make sure not to (as always) hit or come close to redline before the vehicle has had a sufficient chance to warm up

TamZ

I'll have to look into that. I have only touched those paddles a few times coming down a steep but short grade we have here on the 101. Otherwise, I've just left it as is. I had paddle shifters on my last little sports car--sure miss that thing sometimes!

On the loaner C300 that I spend an AWFUL LOT OF TIME DRIVNG since I've gotten this GL...there is a sport and comfort mode. I never drove that thing without putting it into S-mode first. LOL! I'm sure that affects the MPG a bit

43221B

I agree. My old Toyota 4Runner had a similar button but I think it mainly just changed the shift points when accelerating. The diesel could also have a higher 7th gear. When you drive at freeway speeds through the mountains it rarely shifts down on steep grades which tells me that it could handle a much higher top gear on flat freeways. The way it is set up makes for a nice driving experience but there is definitely more fuel economy to be had.

StevethePilot

But that's for the suspension, no?

justthebest

It alters both the suspension and shift points

pteam

You are right. Luckily I dont cruise at 80mph or rarely 75mph but the tack would be at like 2500 rpms which is way too high for a diesel when crusing. 7th should be much higher...

fs1013

First of all, thanks pteam, for posting this subject. I've been wondering the same question for a year. The confusing part for me is the sticker on the fuel door. Although I understand what it means but I couldn't think through how it would work for me. I mean do I need to inflate the rear tires whenever I have the need for heavy loads? And deflate them right after the loads are off? That seems unrealistic because I am lazy. So, my solution was to make a compromise and set the rear at 35 psi.

The other reason I didn't want to set the rear to 32 psi as recommended was the concern of wear and fuel consumption. But I do notice the difference in ride quality when it's set to 38 psi. It's more bumpy and seems to give me less confidence in controlling the car. So I appreciate the experience shared by "43221B" and will set them back to 32 psi for daily driving.

I can not feel a difference in shift point in Normal/Sport/Comfort. Or is it because mine is a diesel (2010 GL350 Bluetec)? I read the manual and the "SPORT/COMF" switch is under the section of Adaptive Damping System. It only talks about how it changes the suspension. The switch itself even has a shock absorber logo on it. Do they do the 550 differently?

43221B

This is just my opinion but I don't think you are going to go wrong by putting 35 in the rear for daily driving. You are just going to get a rougher ride. What I do know is that my wear has been really good at 32 in all four. Keep in mind that the GL has a payload capacity of 1521 lbs which is almost 200 lbs more than a Toyota Sequoia. So, some people might be putting a lot of weight in the back. Certainly, with only three kids that don't weigh a combined total of 100 lbs, I don't have to inflate my rear tires like some might.

alx

keep 32 all around... 38 at the back if fully loaded with people and luggage.

any mpg savings realized by overinflating will be wiped out by the accelerated wear of the tire centers...

alex

BlownV8

I'm on 22's and run 48 PSI all around. Ride is good as is the tread wear. I've got over 30,000 miles on the tires and they have more than 50% treadlife.

pteam

wow 48psi is pretty high, what is the max psi of those tires?

mdadgar

Lower profile tires run higher pressures because you need more spring rate (ie more air pressure) to keep the rim off the ground in a bump.

- Mark

BlownV8

50+

Yes, if you want to keep the rims looking nice, you need the high pressure.

alx

on a 22 you can run higher pressure i assume.

on 19s running 38 in the back all the time will wear the center of the tires within 10k miles- ask me how i know...

alex

pteam

Ok thanks. I have OEM 20s that come with the diesel. What psi do you recommend so my tires dont wear in the center like yours? I'm lowering mine to 35psi right now before I get a response lol

43221B

Pteam, I have the same tires as you (why I gave you feedback in the first place).

pteam

Ok thanks for clarifying that. I'm gonna lower them all to 32 psi, hopefully I wont lose mpg.

BlownV8

I wouldn't run the 20's with less than 38-40 psi. Anything less will affect the rolling resistance and return reduced mileage.

rcf8000

I lowered the pressure in the rear tires to 32, and the ride does seem to be smoother. This forum has been very helpful. Otherwise I would not have paid any attention to the tire pressure chart on the inside of the fuel door. It's odd that the vehicle has three tire pressure charts scattered about, but only one of them mentions the reduced pressure for reduced loads.

pteam

So what is it 32 psi or 38psi? lol