108 chassis tire size, revisited - PeachParts Mercedes-Benz Forum

jas2wa · #1 10-29-2003, 05:13 PM

Okay, turning to the experts here at the Vintage section: please see thread of the same title in Wheels and Tires section (is it possible to post threads in multiple sections?) that I started-

to recap: from current 195/75/14 on stock steel wheels, what size tire would fill out the wheelwells a bit more- was thinking 205/70/14 on my steel rims, but have been presented with the following opportunity: 225/60/14's on a set of 6.5 inch wide bundt style M-B alloys.

Any thoughts on fit, aspect ratio/gearing differences on the highway with a 60 series tire?

Thoughts appreciated!

Thanks,

JAS

psfred · #2 10-29-2003, 07:03 PM

The only thing that really matters is diameter, although the 205/70/14 fill MY fender so well they rub a bit on hard left turns at slow speed -- I need new springs up front, I think.

Peter

Tomguy · #3 10-29-2003, 10:36 PM

I have 205/70R14's on mine, and I get NO rubbing EVER. And I drive pretty hard at that, especially on those corners. I feel the tire provides awesome handling and is pretty close (I would imagine) to the "perfect" tire.
Remember that the first series of numbers is completely unrelated to the 2nd. The first set (205 in my case) is the width of the tread pattern (in mm). A tire too small will provide poor traction. A tire too wide might rub fenders or shocks. I would think a 225 would be pushing it, you might get some rub! The 2nd set is height (from the rim to the tread). 70 I believe is pretty close to what was stock back then. 60 would be very "Tiny," resulting in speedometer inaccuracies (you'll think you're cruising at 70 when you're doing 60).

jas2wa · #4 10-30-2003, 10:00 AM

I am thinking I should maybe put the 6.5 alloys with 225/60/14's on the 84 TD, and then go with the 205/70/14's on the 108 (on the 6" alloys pirated from the TD). Seems to me the more modern (relatively-speaking) 123 chassis would benefit from higher performance tires more than the old-tech 108--- what do you think?

For the 108 I just want a larger footprint and good solid ride and handling- and was wondering if a 205/75 might relax highway cruising a little, in terms of rpm's. Probably not enough to be even noticeable, right?

-JAS

neileg · #5 10-30-2003, 10:34 AM

Quote:

The 2nd set is height (from the rim to the tread). 70 I believe is pretty close to what was stock back then. 60 would be very "Tiny," resulting in speedometer inaccuracies (you'll think you're cruising at 70 when you're doing 60).

Not really. The 60 and 70 are the tyre height as percentages of the tread width.

195/75 has a height of 146.25
205/70 has a height of 143.5mm
225/60 has a height of 135mm
205/75 has a height of 153.75mm

Therefore, the 205/70 is almost exactly the same diameter as the stock tyre, the 225/60 is a 8% drop and the 205/75 a 5% increase.

#6 10-30-2003, 11:46 AM

Someone is now offering on EBay a set of the rarer 15" X 7" Bundt Cake wheels, which might be of great interest to you vintage fans. I emailed the fellow and he affirmed that they were indeed 15" X 7" and he didn't seem to know that these were any more valuable than the regular 14" X whatever, but he does have a hint now. Now I _almost_ regret having made a committment to go with AMG 15" X 7" for my 300TD.

By the way, the aspect ratio is the percentage of height versus _overall_ width, not tread width.

Tomguy · #7 10-30-2003, 01:02 PM

If they're the ones I've seen then they're 14 rims - 15" from outside of the rim to outside of the rim yes, but 14" from bead to bead, which is what is measured in rim size.

205/70R14's are 642.2mm high (from ground to top of tire).
225/60R14's are 625.6mm high (from ground to top of tire).
This is a 16.6mm difference, or 0.654 inches.

Using circumference = 2pi*r, or pi*d, this is a rotating difference of ~2 inches. This is about 7.91% rolling difference. In other words, a 205/70R14 tire will have to rotate 8% less per revolution to cover as much ground as a 225/60R14 tire. The other way around, a 205/70R14 tire is 8.12% larger than a 225/60R14

Converted to a speedometer difference, your speedometer would read 60MPH with a 225/60R14 tire when, if you had a 205, it would say (a more accurate) 55MPH. In other words, you'll be going slower than your speedometer says. So, you'll also have to burn more gas (higher RPMs) to go the same speed, and your top speed would be lowered (assuming your top speed is 130MPH, although your speedo would say 130, you'd actually be going about 120).

#8 10-30-2003, 01:35 PM

1. My e-conversation with the fellow with the 15" Bundt Cakes seemed to establish that he did know how to measure rims -- I don't think this is the same guy as the one who was advertising some 14" as 15" a week or two ago.

2. If you want to use the '70s type of 6 1/2" X 14" rims (ET not marked, but a hair _under_ 30mm) with 225/60 tires on a W123 chassis car (300TD, etc.), you have to use thin spacers on the front or else the sidewall will contact the rubber boot of the tie rod end under some wheel movements. What you really need is the later, '80s versions, which are ET25 or thereabouts. I have 6.5mm spacers on the 6 1/2" wheels I have for my "85 300TD. I suspect that the sidewall of a 225/60 will get really close to the boot of the tie rod end even on 6" ET30 rims. None of this is a problem on the earlier chassis cars, as far as I know.

#9 10-30-2003, 06:18 PM

about whether the wheels advertised in EBay item "2440142724" really were 15" and not the ubiquitous 14", when I got home, I took to doing some measuring on the various rims in both sizes that I have there. By measuring across two bolt sockets which are not adjacent, but separated by one, center to center, I got 107mm, and an overall diameter of 390mm for the 14" rim and 417mm for the 15" rim, which means that the proportion of bolt separation to overall diameter is roughly 27.44 %for the 14" rim and 25.66% for the 15" rim. Then I printed out a copy of the advertisement's picture of the wheels and did a little caliper work, which wasn't easy because the picture wasn't straight on and there are parallax factors to worry about; nevertheless, the proportion for the wheel pictured turned out to be 26.03% -- much closer to that of a 15" rim than a 14". Of course, my measuring is not up to two decimal places of accuracy by any means, but I do wish I had some money to bid, and I hope that if these really are genuine 15" Bundt Cakes, they go on a car worthy of them.

Does anyone want to buy any of my 14" Bundt Cake rims and tires??? -- 6 of them are 6" and 4 are 6 1/2", most needing refinishing, and mounted with everything from Bridgestone 195/70 to Michelin 225/60, depending on which pair you select.

neileg · #10 10-31-2003, 11:39 AM

Quote:

By the way, the aspect ratio is the percentage of height versus _overall_ width, not tread width.

Dead right, my mistake. But, since the tyre size is what is referred to in the spec, not the tread width, my maths is still right!

Tomguy, your maths is good, but the reference to gas consumption isn't. Gas consumption is not affected very much by small changes in gearing. The engine may be turning faster, but it's doing the same amount of work (except for some power losses in the drive train).

Tomguy · #11 10-31-2003, 12:22 PM

The engine is turning faster to go the same speed: If your speedometer says 60 and you're going 55, so you raise your indicated speed to 65 in order to account for that difference, your engine is turning faster to go the same speed, therefore it's using more gasoline. It isn't so much the work done as how much the engine turns to do it - overdrive is very tough on an engine going uphill at 2K RPM, yet since it turns so much less than it would otherwise, you save a ton of gas.

neileg · #12 11-03-2003, 05:34 AM

OK, Tomguy, one last shot.

The engine burns gas in order to create power. You need power to drive the car along. You need the same amount of power to drive the car regardless of the engine speed.

If you lower the gearing, the engine turns faster for the same road speed. But, the only things that use extra power for that increased speed are the friction losses in the engine, transmission and the final drive.

So, you are quite correct that increasing the engine speed for a given road speed will increase the gas consumption. However, the amount of increased consumption is not directly proportional to the increase in engine speed. What I mean is that increasing the engine speed by 5% won't worsen your consumption by 5%

Tomguy · #13 11-03-2003, 05:15 PM

Well, neil, it'd be something like this:
If you increase your engine speed 5% you usually raise your vehicle speed by some similar amount (which, of course, depends on your final drive ratio). With an overdrive transmission, raising your engine speed 5% probably raises vehicle speed 5%. If you increase your vehicle speed, you aren't "Wasting" those RPMs - your engine turns more but you go faster, covering more ground in less time.
But if you put smaller tires on the car, you'll have to turn 5% faster to go the same speed. That's what I'm saying. If you have to turn 5% faster to go the same speed, sure - you won't lose 5% efficiency. It could be anywhere from 3% to 8%! Less rolling resistance and making the engien work less would be "raising" efficiency, but making the engine turn more RPMs would lower it.

If your car turns 3K RPM at 80MPH, 8% less (as my calcs's showed it would be) would result in having to turn an extra 240RPM. Sure doesnt sound like much, but 240RPM over 1 hour of highway driving is often the difference between 20MPG and 18MPG!

Tomguy · #14 11-03-2003, 05:20 PM

neil: You're saying that it doesnt take extra energy to go that speed, even though you're turning more RPMs. If that were true, cars wouldn't need overdrive to save gas. Also, if it were true, putting a brick on your gas pedal and letting your car run in park would run it out of gas no faster than letting it idle at 750RPM.
You have to remember - when an engine turns more, it sucks more gas to do so!

jas2wa · #15 11-03-2003, 05:28 PM

Okay, so tell me what size tire I should be putting on my V8 108!!

With all that power, there's no reason to have the engine turning faster at a particular highway speed- following that logic, I should be looking at maybe as wide a 75-series tire as will fit? like 205 or 215/74/14's?

Ideally, 15" rims would be great, but too much $

Let me tell you, I don't need my tire choice to end up lowering my fuel mileage from an already dismal point...!

I am going to have to drive a Toyota Prius for years in order to get into heaven I fear!

JAS

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