Tech Q&A

Article Richard Simonds

Wheels for older cars

Q: I’m in the process of shopping for larger wheels for my 1991 420SEL, and would like some guidance with wheel offsets. What are the maximum allowable wheel offsets for a 17x7.5 wheel and an 18x8.5 wheel that will correctly fit my car? I called the franchise tire store nearby, and the manager told me 20-25mm for a 17x7.5 wheel, 15-22mm for an 18x8.5 wheel. He mentioned that these numbers were “in his system.” These offset numbers sound reasonable; I’m just looking for confirmation from the best source I can find. What can you tell me?

A: MBUSA and DAG do not provide offset data for after-market wheels – at least not that I know about. Your best bet is to follow the information from your wheel and tire store and get a warranty on the fit (no rubbing of tires and wheel-well body panels) and performance. You will likely be responsible for any potential premature wheel-bearing wear. MBUSA and DAG do not want any modifications to their perfectly engineered cars.

Member’s response: Thanks for your reply, Richard. Alas, the chore of keeping the 15-spoke wheels clean, and the lack of any high-performance tires available in size 205/65R15 are what propelled me to buy larger wheels. Per the tire store, I expect to be able to achieve an offset of 22 or 23mm, compared with the stock 25mm offset.

Premature Tire Wear

Q: I have a 2015 SL400 and my son a 2004 SL. We have each experienced rapid inside-rear tire wear; both SLs needed new tires at 11,000 to 15,000 miles. Neither of us is a hard driver. When the tire dealer and the Mercedes dealers were questioned, they responded similarly: “There is no camber adjustment” on these vehicles. One tire dealer told me of an after-market concentric bushing refit that would allow a zero camber adjustment for a mere $1,300. Do you have any history or information on this issue? In the meantime, I am enjoying my SL, but have purchased new, less-expensive rear tires and am driving them overinflated by 4 pounds to, hopefully, not wear out the inside tread so quickly. Your help or info would be appreciated.

A: Here is the response from my local shop foreman. “Fifteen thousand miles on a set of rear tires is actually good for an SL. This is the norm. People don’t like to hear that, but we see it all the time. As far as camber adjustment, that would have little effect on this condition. Toe-in is what I would be looking at for inner-rear tire wear.”

This is not the news you wanted to hear. However, this can be put into perspective by comparing your tire wear to that on performance models from other manufacturers. The Acura/Honda NSX and the Honda S2000 both used comparatively soft-rubber tire compounds for higher performance, but at the expense of tire wear. The NSX, in particular, had numerous complaints about getting 8,000-10,000 miles on a set of rear tires and only a bit better on the front tires. If you want higher mileage, look for tires with harder rubber compounds – usually a factor in the expected mileage ratings (e.g., 40,000, 50,000, 60,000). You give up the performance advantage for longer tread wear.

More on wiper blades

Q: We have a 2016 E400 4Matic, and a 2015 Tesla S85D. The windshield wiper blades on both cars shuddered while in action. Per your earlier response, I adjusted the wiper arm angle, with no improvement. The Tesla Ranger technician adjusted the wiper-blade angle twice for us and mounted new blades, with no apparent improvement.

A: A member’s response: I’ve seen a recommendation in the Tesla blogs to switch the blades to Trico Onyx blades, which solved this problem for one Tesla owner. So, I bought two sets of Onyx blades and replaced the OEM units myself. I did not find any great difficulty in doing so due to very good instructions that came with the new blades. Both of our sedans are now shudder-free. The Onyx blades ran around $50 a set, but they sure fixed this annoying problem.

European R107s

Q: I just bought what I believe is a European 1985 380SL. David Latham at Bud’s Benz thinks that mine is a 500SL. It has a spoiler on the rear deck but no model number. I am unable to clearly see all of the engine number, but what I think I see is: 116960120. Is that enough of an identification number to identify what type of engine it is?

A: Yes, it is. The 116.960 engine was used in the 380SL only until August 1981. From September 1981 on, the 380SL engine was the 116.962. If the 116.960 engine in your SL is original to the car, then it is a 1981 380SL model, not 1985.

Chassis        Engine Number     Model Number         Years       

107.045          116.960 or .962                  380SL       1980-85

107.046          116.960, .962 or .964         500SL       1980-89

Member’s response: You have made my day! Based on the chassis number, through blind luck it appears that I have stumbled into something interesting and rare. The car has only 45,000 miles, original paint, and the first-aid kit has never been opened. I had Bud’s Benz replace the seats and door panels and the car is magnificent now.

Transmission solution

Q: The May-June and July-August 2019 issues of The Star magazine had questions about transmission-shifting problems. The following is a response from MBCA member Dave Murray, detailing a solution. This was a success story for him. Will it be for you?

A: Richard, thank you for your follow up to the transmission-shifting issue. I believe I have solved the problem and wanted to share it with you and with other members. I have done a lot of research on this subject. It appears the 7-speed 722.9 transmissions built from 2010 forward required blue ATF with A89 low-friction fluid. An independent garage flushed mine using Valvoline Maxlife, which is red in color. My garage was unaware of this change in 2010. I then took my 2012 ML350 to Mercedes-Benz of Lynnwood in Washington, where they did a “super flush” – flushed it completely, twice, using M-B ATF blue A89 along with new filters. The transmission now shifts as designed. I write this in the hopes of helping other members with the 722.9 transmission built 2010 forward. My recommendation would be to take your Mercedes-Benz to the dealer to have the transmission flushed utilizing M-B fluid only.