+Tire Diameter 4 Better Look/Lower RPM

[DSAviate]

Just bought a set of used style 87s. Had them evaluated and "spun" at an independant wheel shop - OK. Now looking at my tire options.

I'd like to address a few issues with my new tires . . . for looks, I'd like to fill in a some of the large clearance between our 29.1" tires and the fender . . . also want to reduce the high rpms of my 5speed.

Check out the pic of one member's use of 285/50 R20 tires on page 138 and 139 of "Show Us Your X" thread. His tire diameter is 2.03" more than standard issue on our E53s, 31.13"!! And he claims to have no issues, don't know how long he's been using this setup on his 200k+ miles X.

So, a few questions for the incredible knowledge base here:

1. Is the large clearance between tire and fender necessary? For seriously stressful off-roading, I imagine so. But some here have lowered their E53s and maintained 29.1" tires in widths of 275+, thereby reducing this space and clearance. The gentleman above filled the gap with larger tires. At what point would one expect to run into inner fender tire rubbing in tight turns with normal street driving?

2. Larger tires = Lower RPM. 31.13" tires have a 6.9% larger circumference than our stock. At 80 mph, my rpm could be reduced from 3800 to 3535 with this larger diameter. I know this would affect the OBC estimates for mpg, range, etc. . . . but are there any damaging repercussions if the OBC is NOT reprogrammed for the new diameter?

3. Transfer case considerations - With staggered 87s and directional tires, the tires will stay in place for the life of their service. I'm towing a 5987# load and anticipate more wear on the fronts than the rears. Would it be prudent to find a tire combination for the set that starts with a SLIGHTLY larger diameter in the front? Thus the fronts could wear down to equal diameter . . . and then only approach a diameter disparity limit when the set of four is due for replacement . . . ideas?

[TerminatorX5]

2. the MPH, the MPG are calculated from the "clicks" coming from the speed sensors. The quantity of the "clicks" per mile is programmed into the FRW file - if you can calculate the new number (you need to pull your file with NCS Expert) based on the origianl number, and program it into the car, you should be ok.

I have not done it, and have not heard of anyone on the BMW forums do it, but I have heard someone on VW forum may have reprogrammed their speed sensor "clicks"...

[JCL]

1. Is the large clearance between tire and fender necessary?

Only fitting by trial and error will determine if you will have sufficient clearance.

2. Larger tires = Lower RPM. . . . but are there any damaging repercussions if the OBC is NOT reprogrammed for the new diameter?

Not to the OBC. Your odometer will read under the actual. The speedometer won't be out as far, as it is optimistic to start, by design.

Apart from the OBC, you will have some negative effect of such a heavy tire. Consider the increased wear on suspension components, and inertial effects of the increased tire at that position on the rim, which will mean slower acceleration and braking.

3. Transfer case considerations - would it be prudent to find a tire combination for the set that starts with a SLIGHTLY larger diameter in the front? Thus the fronts could wear down to equal diameter . . . and then only approach a diameter disparity limit when the set of four is due for replacement . . .

No. Always try for matched. Don't expect that they are going to wear down evenly in any case.

[DSAviate]

I always appreciate your feedeback JCL.

My experience has been that the front tires wear more than the rear. I know this is subject to driving habits . . . but the fronts are subjected to the demands of turning friction and the lions share of the bracking friction.

If I always start with equal diameter tires in a staggered set, I'll expect the fronts to be done when the rears still have ~ 50% tread left. Has anyone here experienced otherwise?

[JCL]

Originally Posted by DSAviate

If I always start with equal diameter tires in a staggered set, I'll expect the fronts to be done when the rears still have ~ 50% tread left. Has anyone here experienced otherwise?

Haven't used a staggered set up, but both my X5 and X3 wore four tires evenly over 75,000 km (original tires in both cases). The X5 tires were still in good shape at that point (sold the vehicle). The X3 tires were replaced at 4/32", not worn out, but I wanted to replace them for another reason. I would have gotten 90,000 - 100,000 km on them, by my calculations. I never rotated tires on either vehicle.

Wearing fronts out first isn't from braking, it is from aggressive cornering with a heavy vehicle, so YMMV.

[DSAviate]

Based on all research and feedback, now looking at the following tires for my offset 87s:

General Tire Grabber UHP -

Front 275/45 R20, 29.7" diameter
Rear 295/45 R20, 30.5" diameter

Will these .8" larger diameter rear tires place any inappropriate stress on my Mechanical transfer case (NV 125)?

It seems to me the transfer case must be designed to accommodate slightly different rolling diameters front to rear. Is this not true?

[JCL]

685 revs per mile vs 699. I would try and get it closer than that.

[bcredliner]

Depending on what engine you have you may lose or gain some economy with the lower engine RPM at 80, especially when towing. While slight you will be in a different place in the engine 'power band.'

[DSAviate]

bcredliner - I'm not too concerned about the engine power band. The 3.0 has plenty of power for me and I'm not trying to win any races. I rarely tow my load over 65 mph.

At this point, I'm more concerned with differential revs per mile front drive to rear drive and any affect on my Mechanical (Non-Xdrive) transfer case.

Searching this site, online, youtube videos, my owner's and Bentley manuals, . . . I can't find a detailed explanation of the transfer case's ability (or inability) to mechanically adapt to differential front and rear axle rpms. I figure that it must be able to . . . otherwise anything that throws front to rear tire diameters out of sink (tire pressures, hvy rear load, hvy front load, . . . perhaps even as experienced during heavy braking) would cause a binding stress that would greatly reduce mpgs, or worse, strip TC gears. If this is this the case, tire pressures and absolutely even tire wear are critical.

So, what am I missing? What is the component in the drive train that allows for different front and rear drive axle rpms? If this component only allows for a "slight" differential in the rpms, what is the tolerance, and how would it most likely fail if the tolerances were exceeded?

[bcredliner]

I have read some threads that state differences in diameter can cause traction control and braking errors. Tread from new to bald would be about .68" so the X5 can handle near that for some period of time. I think you should be safe at about half that but I have not seen BMW transfer case tolerances. I wrestled with this and decided to go with a lowering kit on my 4.6. Not as easy to get there but does not mess with the computer, speedometer, mileage calc, or put greater load on the suspension. Plus is in spec for the transfer case. If you are only interested in going the tire route, I suggest you keep searching until you find a match. I don't know what component would be the problem but anything involved with the transfer case is going to be costly. If you are doing a lot of towing that is going to magnify the risk. If it helps my clearance between tire and fender lip is about 2 3/4' front and rear. Rims are 87s and tires are 275/40 on the front 315/35 on the rear, same brand, same diameter. I also have 7mm spacers front and rear that I added after I had the lowering kit in place for some time. Never any problems with tire rub. As you mentioned I replace front tires twice as much as rears so there is a period time where front to rear diameter is larger by about half the tread. Never had a problem with transfer case and have clocked a little over 100,000 mi.