Blew one of the older rear tires on my 240, so bought some new ones for the rear. Went with much larger tires overall for additional top speed and possible fuel economy.

In the past, I have found that this kind of caper causes lots of debate, whether the increased weight of the tire = more fuel being burned, and the larger tire increasing road friction, and so on.

This time I recorded some data on the tires before mounting them, and the info was pretty interesting, figured Id post it if anyone else was curious about this.

The tires that were on the 240 originally were Mastercraft A/S 4s in the 185/70 R14 size.

I bought new rear tires in the same brand, and went all the way up to 205/75 R14s. then I compared the new tires with my still new spare in the 185 size.

I measured outside diameter between them, and for each revolution, the larger tire will cover an additional 5"s of ground.

The 185 tire weighed in mounted and balanced on the bundt at 32.5 lbs.
The 205 tire weighed in mounted and balanced on the bundt at 36 lbs even.

Heres the crazy part, on the same brand, the smaller tire has 5.50 inches of tread touching the road while the larger tire has 5.25 inches!

The much larger tire actually touches the road with less material and only weights 3.5 lbs more than the smaller tire! For an additional 5 inches per revolution on the highway, I figure its worth it, but I don't think the big tire will affect fuel economy at all really.

Not sure what it will do to acceleration, but the car is so sluggish anyway I figure I won't notice any difference.

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This post brought to you by Carl's Jr.

Two things affecting the amount of tire touching the ground: Tire pressure, and rim width. If the rim were widened, or the air pressure reduced, your tire patch would become wider.

This is one thing that I started to notice when I tried a couple of non-Michelin tires on my coach: different tire (section width) widths didn't necessarily mean different tread width. I found that the Michelin tire for my coach had 1" wider tread than the Dunlop in the same width, size, and weight rating on the same rim. I then noticed that the 205mm Pirelli P-700Z tires on my quattro, had a wider tread than the 235mm Goodyear tires on my van, and the van rims were actually wider!

There are many ways for a tire company to cut costs. Most times you get what you pay for.

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Gone to the dark side

- Jeff

Well, to my surprise, it had a pretty negative effect on fuel economy, by almost 3mpg! The car got 33-34 mpg at 70-75 and now got 30-31 mpg. I will verify this with some more runs.

There was also a pretty noticable jump in topspeed to the larger tire, but now I am in a dilemma between keeping the tire and maintaining a higher topspeed, or driving slightly slower and getting better fuel economy.

A version of the old lead foot vs wallet problem.

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This post brought to you by Carl's Jr.

You are listing that 80 240 D as a five speed. Are you counting reverse as a speed? If you consider reverse a speed, do you consider it as first gear or fifth gear or something in between?

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

Fyi his car has a custom 5 speed gearbox. It's a combination of a 190e 5 speed, and a 240d 4 speed. It's a real 5 speed.

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light

Then that makes it a Craigslist 6-speed LOL.

Sounds nice.

__________________

Gone to the dark side

- Jeff

?????? Im trying to figure out if there is something major I have done to make you act so obnoxiously in this and other posts? Let me know, I usually like to deserve something if I receive it, and I don't like having unexplained enemies on forums. Makes no sense to me. If you have an ax to grind, let me know what it is and we can discuss it over PMs perhaps.

I am assuming these acidic sarcastic responses are because you think I am a fool for my joke over on the diesel forum where I temporarily forgot exactly how powerflow worked . Let me assure you, despite the idiocy of that joke (written fast and when tired), I have several ASE transmission and diesel certifications, so im not a complete idiot, only partially and sometimes temporarily.

But to respond to your post, its a real 5 speed that I adapted, as mike-81-240D said, here is a link to the build thread- http://www.superturbodiesel.com/std/how-adapt-w201-speed-om616-om617-applications-t-965.html

Here is a link also to a test drive video- http://www.youtube.com/watch?v=3vZEJbGeuSk

Its a 717.411 1984 190E gearbox which has a .8 overdrive, I am in the process of testing it for longevity. I was hoping to bolt it to a turbo motor, but the wagon in question has so many issues that I went with a 240 first. Im working on a second adapted gearbox though to put in my wagon (which also needs an engine, but I have several to try out for that)

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This post brought to you by Carl's Jr.

Your 240D originally was fitted with 175-14 (175/80) tires.

300Ds were originally fitted with 195/70-14 tires and WIDER WHEELS to match.

If you've now got 205 tires on the original narrow 240D wheels, they are way too big for your wheels.

175-14 and 195/70-14 provide the same diameter and consequently same speed readings. When you had 185/70-14 you had smaller diameter than required and now with 205 you have larger, so you've never had an accurate speedo/odo.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles

I have the 205s mounted on bundts I got off of a 1985 wagon. The speedo is hopelessly inaccurate anyway with the different tranmission. Its about 8-10 mph off past 30, and between 5 and 6 in the lower speeds. For the moment, I am measuring accurate speed with a GPS unit, which is where I am getting my numbers.

If you bought a 240 way back when, could you order bundts on it? I bought the car with 4 steel wheels on it, I know there is a 15 inch bundt, but are there a lot of different width bundt options? They all seem the same in the yards to me, but i never measured them

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This post brought to you by Carl's Jr.

In 1983 the 240d got bundts stock off the line. I'm not positive but before that I don't think there was an option for them unless it was a euro...

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1981 300TD 4 speed manual
Euro bumpers, zender valance and skirts, H&R springs, billy HD's, leveled sls, real AMG Pentas 16x8 et11, vdo boost/egt gauges intergrated into ash tray, eurolights, led 3rd brake light

Silly question: You say that mileage went down with the larger tire, how did you compensate for the different odometer calibration in your mileage calcs?

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83 300TD, 260,000 miles, aka "Dusty"
6 vehicles, 2 cup holders (both in the dump truck)

The odometer on this car is broken.

Twice a week I drive to and from base cities about 300 miles apart. I clocked 330.4 miles between two diesel fuel stations at both ends of the route with another car and a trusted odometer.

What I have been doing out of curiosity is filling up right before I leave, and driving directly to the other station, and filling up when I arrive. Then i have been clocking average speed with the GPS and comparing it to the amount of fuel I use.

Im trying to find just the right combination between tire size, and relatively high cruising speed that would give me an acceptable fuel economy. I have a lead foot problem, and cannot tool along at 60 for 300 miles, id be tearing my hair out.

So far, I have been very pleased with the cruising speed of the car, and the best mileage I have managed to achieve was about 33.4 mpg at 70-75 mph.
The diesel 240 is getting approximately twice the fuel economy of my 93 nissan HB pickup I used to do this run with, and its about 15 mph faster on the top end and more comfortable to boot. The nissan could break 70, but you would know it, and several times the fuel economy dropped down below 13 over the whole trip as I got impatient.

Im about to do another run today, the second one with these larger tires. What I really want to do is put a tach in this 240, thats the next big change I have planned.

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This post brought to you by Carl's Jr.

Ah. Very repeatable situation. Should make for good comparisons. Thanks for the data.

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83 300TD, 260,000 miles, aka "Dusty"
6 vehicles, 2 cup holders (both in the dump truck)

Here are some factory specs on MPG-
Mercedes spec

for 123.123 (240D) with OM616.912
.................................city 56mph 75 mph
240D with 4 spd manual: 24.7 / 32.6 / 23.7
240D with 5 spd manual: 24.7 / 37.3 / 27.0
240D with automatic:.... 24.7 / 30.5 / 22.2

Since the last time I posted, I have made a number of runs with the big tires. Im starting to learn that I get considerably better fuel economy going one direction over the other, this is because I discovered there is a fairly long grade one way (after doing some topography research since is pretty hard to notice), and for about 150 miles, the route is down a long valley with pretty directional winds. All my worst and average runs have been upstate, and the best runs have been coming downstate. The big tires are actually making a fairly measurable difference in fuel economy too I think, about 3mpg. (I really need to do this for a year to really generate excellent data)-
best- 9.36 gallons 35.25mpg
next best- 9.43 gallons 34.99mpg
typical- 10.00 gallons to 10.80 gallons 33 to 30.5mpg


I did some math which you may find pretty interesting to try and figure this out and back it up. I went out and re-measured the circumference of the stock tires I have on the front, and tall tires I have on the rear.

I am enjoying increased fuel economy over the stock specifications of a 240 with a 5 speed, and here are my figures at to why-

The stocks are 74" around, the talls are 80.75" around. its actually a 6.75" inch difference between individual rotation. I was wrong with the earlier measurement of 5 inches.

I looked up that there are 63,360 inches in a mile.
To travel a mile with the stock tires requires 856.21 revolutions vs 784.64 revolutions with the oversize tires.

Heres where the fun numbers come in- ,

so there are 20908800 inches in 330 miles.
Stocks are 282551.35 revolutions to cover that distance.

the same number of revolutions would carry the oversize tires 22816021.50 inches, or a further 30.10 miles over the stocks.

So instead of 330 miles stock, the large tires with the middling extra weight, same ground speed, and same footprint cause the vehicle to burn about as much fuel as it would to move the stock tires 299.90 miles.


back to the factory figures, 299.90 divided by my best 9.36 gallons equals about 32 mpg, much closer to reality. The second best would have been 31.80mpg, and the more usual 10 gallons would be around 29.9mpg.

Since at 75, a 240 with a 5 speed is recorded at 27mpg as a factory spec, between the 37.3 mpg and the 27mpg, there are 19mph gained. I figure that every additional 4.75mph gained there are an additional 2.57mpg lost between those two mpg factory benchmarks at 56 and 75.

If I were traveling at 70.25mph, I would have been getting 29.57mpg according to the factory specs, which is pretty close to the 29.9mpg I would have recorded had I been rolling stock tires.

On average, my actual fuel burned is giving me a fuel economy which is too high according to factory spec, so it must be a gain in the tires themselves, which gives me the 3mpg average increase figure for these tall tires.

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This post brought to you by Carl's Jr.

I have it figured out, ... the taller tires in the rear mean that your car is always going down-hill!

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Gone to the dark side

- Jeff