Would going to a slightly larger tire diameter, (plus 1.5") translate in slower acceleration and lower engine rpms at highway speeds, lets say 75 mph vs the engine rpms with the oem tires???

Thanks for the insight.

Regards, ;)

yes, and your speedometer/odometer will display slower/less than true speed/distance.
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Definitely slower acceleration. I would get bigger rims with smaller tires; this would still detrimentally affect your acceleration, but at least you would make an aesthetic improvement.

I think this will also throw your navi off, it uses your speed to calculate your position. Has anyone come up with a way to recalibrate the speedo?

A few things to keep in mind about upgrading wheels is the rotational mass. You want to keep the wieght of the new wheel as close as to the stock weight. If you keep it fairly close than you won't notice too much performace loss. Also look into SAE approved wheels as well. Even though 20"+ wheels have come up there are still some that can negate the safety of your vehicle and increase wear. Again this goes back to rotational mass and how much inertia and power it would take. A heavier wheel can lead to an increase in stopping distance and more stress on your braking system and in acceleration it can hinder your tranny and axles (the reason why suburbans and explorers need to have ecu tunes when adding a wheel bigger than 22") In the safety side (I use to work for Volvo so I just know this stuff) As an example a 40lb child in a head on collision at 35 mph can propel him at the force of almost 2800lbs. So the average 18" tire and wheel combo weighs about 40 lbs. Ifyou are involve in a front end collision that's the amount of force of that wheel being pushed. If the wheel doesnt collapse or break like it should it can crush your legs or in rare cases kill you. GM did a lot of research on this so of course it makes sense for all of us to do research on these item s as well

I think this will also throw your navi off, it uses your speed to calculate your position. Has anyone come up with a way to recalibrate the speedo?The navigation system uses GPS signals from satellites & is unaffected by any vehicle modifications.

The navigation system uses GPS signals from satellites & is unaffected by any vehicle modifications.

I believe the nav uses a combination of sat and speed/compass calculation to track your exact position in real time. I know it uses your speed and direction when the GPS sat signal becomes intermittent.

I believe the nav uses a combination of sat and speed/compass calculation to track your exact position in real time. I know it uses your speed and direction when the GPS sat signal becomes intermittent.

Nope. ARH1956 is correct. Navigation systems use satelites alone to locate your position. Speed has nothing whatsoever to do with it. The system uses 3 satelites to triangulate your position. Most system can always see at least 8 or so, this is why you usually do not have a problem.

The satelites can; however, tell you your speed much better than the speedometer based on the same triangulation.

Nope. ARH1956 is correct. Navigation systems use satelites alone to locate your position. Speed has nothing whatsoever to do with it. The system uses 3 satelites to triangulate your position. Most system can always see at least 8 or so, this is why you usually do not have a problem.

The satelites can; however, tell you your speed much better than the speedometer based on the same triangulation.

"The navigation system receives its GPS, or global positioning system signals, from up to eight satellites to display the position and direction of the vehicle. A gyroscope and the vehicle’s speed sensor enable the system to display the present vehicle position even in locations where a GPS signal may not be available."

http://www.wkjeeps.com/wk_nav.htm

"The navigation system receives its GPS, or global positioning system signals, from up to eight satellites to display the position and direction of the vehicle. A gyroscope and the vehicle’s speed sensor enable the system to display the present vehicle position even in locations where a GPS signal may not be available."

http://www.wkjeeps.com/wk_nav.htm

OK, but this won't "throw off" your navi much as you are very rarely unable to pick up at least three of the 24 GPS satellites orbiting the earth (maybe when you are in a parking garage, or driving through a dense forrest).

The radio waves used to triangulate your position are traveling at the speed of light, so the signals are being picked up at a mere fraction of a second. For instance, if a GPS satellite is located right overhead, the signal would reach us every 0.06 seconds! Therefore, as soon as you pick up the three satellites again, your true position will be calculated and displayed.

So, the long and short of it.......don't worry about tire size screwing with your Nav system :)

The Nav system is calibrated to the exact tire size so that it remains accurate, for example, when you are travelling through a long underground tunnel. On our Jeep the tire size is selectable only using a StarSCAN. On a new Mercedes you can do it yourself in the nav setup menu, and it even asks if the tires are new or used, for extra precision. Combined with the gyroscope, the nav system should be able to detect which direction you are going through an underground intersection.

But still, it doesn't matter. You could put on 35" mud tires and the nav would work fine.
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The Nav system is calibrated to the exact tire size so that it remains accurate, for example, when you are travelling through a long underground tunnel. On our Jeep the tire size is selectable only using a StarSCAN. On a new Mercedes you can do it yourself in the nav setup menu, and it even asks if the tires are new or used, for extra precision. Combined with the gyroscope, the nav system should be able to detect which direction you are going through an underground intersection.

But still, it doesn't matter. You could put on 35" mud tires and the nav would work fine.

So you can have the dealer update it using StarSCAN? How about the speedo, can/will the dealer recalibrate that? I ask because I'm thinking of going to a larger rim myself.

A few things to keep in mind about upgrading wheels is the rotational mass. You want to keep the wieght of the new wheel as close as to the stock weight. If you keep it fairly close than you won't notice too much performace loss.

Yep, larger Rotational Mass + Moment of Inertia = Less performance. Also, if you increase the tire diameter you are hurting your effective gear ratios, making them higher. Higher gear ratio = slower accelleration.