Can Tire Size Affect Shifting? - PeachParts Mercedes-Benz Forum

d2bernhard · #1 01-19-2003, 02:21 AM

Car is my ’86 300E. 156k on the clock. This used car came with about +2.75 tires on it. 225/65/15 I think. Tires rubbed all over the place. The wheel actually puled in the direction of a turn during parking lot maneuvers. They were lousy Falken Ziex, the worst tracking, sloppy tires I have ever experienced -- just sloppy pigs all around. (Certainly all that extra sidewall contributed to this, I know, so maybe the tires were not entirely to blame.)

Now I have the correct size on the car and all the negative tire issues are over with. I also noticed it upshifts MUCH more smoothly — correctly actually, instead of the abrupt way it did with the old tires. This includes powering up freeway on ramps with the pedal on the floor and shift button depressed all the way. Simply an amazing difference with the right size tires on the car. My question is, does this make sense? Can tire size affect the tran adjustment?

sbourg · #2 01-19-2003, 09:42 AM

What the larger diameter tires would effect is the effective overall ratio between engine speed and road speed. Transmission shift points are chosen to occur at specific engine speeds to maximize smoothness, but this will be the case only if the corresponding road speeds are correct.

If your tires are now the 195/65R15, they are 1.5" shorter in diameter than your old ones. This would cause a noticeable difference in shifting and reduced accelaeration performance. Since the old tires were way too wide for the 6.5" rims, this would be the main cause of the poor handling. The heavier tires would also deteriorate handling, and make the ride harsher if inflated to the pressure recommended in your manual.

Steve

stevebfl · #3 01-19-2003, 02:01 PM

The smaller tires will actually increase acceleration performance by effectively lowering the gear ratio (relative to what you had).

Big heavy, aggressively treaded tires also take more power to roll down the road.

I'll bet some kids idea of what makes a car look fast resulted in them disliking the car and getting rid of it. How lucky the car got another life.

haasman · #4 01-19-2003, 10:52 PM

In California it has been "standard" to automatically replace the standard recommended tires with 225/65/15 sizes. A lot of 300Es have this done.

When I first got a 300E and it needed tires to replace the standard 195/65R15 that were worn out, I took it to a tire store and was told it would be ready in an hour or so. When I came back the brand (Bridgestone) and the model (RE950s) were correct but the sizes were 225/65/15. The car looked weird but what was even more weird was the tire guy's response: "Well everyone ALWAYS gets the Tire Upgrade on their Mercedes."

I was the "first" to request the "smaller" tire .... he said since he can remember, and he had been the business in a long, long time.

I had to laugh. I told him that the "standard" sizes were just fine (as long as they had the right load rating). I wanted to keep the standard "smaller" size because I wanted good fuel mileage, power and handling. Here is where the argument came in. He said the large tire offered a greater tire contact patch and thus better corning. I said that if I was racing around a track I might be concerned about a tire patch, otherwise I need the excellent balance of less unsprung weight, fuel economy and acceleration.

The point of the story, last week I was in for tires once again. I noticed a 300E getting tires and my tire guy came out and proudly told me he had recommended the "correct" tire size for the customers car …. He could tell from my face I was wondering …… "The 195/65R15 size of course".

d2bernhard · #5 01-20-2003, 02:25 AM

I would like to add that I do have smaller than stock, but wider tires on my 300TE. 205/60/15. 0.3 inch smaller diameter, shoulder width 0.3 inch wider but tread with 0.5 wider. Also gained from 89H to 90H in load rating. This all in a Bridgestone RE950. BEWARE: Not all tires gain any tread width in this small size change -- some are identical. I am very very happy with this size on my wagon, and will use this size on my sedan as well (right now I have stock size as I got a swinging deal on barely used tires and original MB wheels.)

d2bernhard · #6 01-20-2003, 03:00 AM

Yes, I think they are a LOT of tire for the money. How do you like them?

I cannot make them plane in the wet, even when trying > (Test Day was 2 3yr olds in the car, on a Sunday vacant, 4 lane road during a torrential over an inch of rain day, 40-45 MPH hitting 4-6 inches of water. Car was rock steady. The kids were in heaven.) I have them on my ‘92 300TE and my ‘80 VW Rabbit Pickup (the one with the good motor).

I posted an ‘opinions please’ on this site which went to 2 pages. Sure there was some brand loyalty issues and some puffing going on, but not one person who actually owns them had a negative to note. I found this amazing. Very highly rated on Tirerack by the consumers.

RE950 Post

haasman · #7 01-20-2003, 03:46 AM

David

Are you liking the RE950s?

d2bernhard · #8 09-07-2003, 11:52 AM

A final note here. Sloppy, piggy rear bushings and mounts can also affect shifting. With my new rear end, I have done away with some shift idiosyncrasies I used to think were from my aging tran.