108 chassis tire size, revisited

[psfred]

250/70/14 is close to the original diameter of the tires, so the speedo should be OK and you will have nearly stock behavior. 195/75/14 is almost exactly the original size, but kinda skinny for such a big car.

I've heard that 215/70/14 is perfect, but I don't think they will fit under the fenders if the front springs are saggy like mine.

I put the 205/70/14s on -- tire store (Firestone) assured me they are the correct size and I like them. Speedo may be a bit optomistic, but I can live with that. Odo seems to be just about right.

Peter

[Tomguy]

If the odometer is dead on, then so is your speedo. The speed is a relation of distance/time, it has to have that distance proper.
I seem to have no troubles with the 205/70R14's I have. Wonderful handling and great ride with no rubs or any bad side effects. I suggest the same for you!

the only ones left on the market are intended for folks who would really rather have a Buick, or your father's Oldsmobile -- they don't handle worthy of any Mercedes built after 1958. You might see if you've got enough clearance for 225/70 X 14, but it's possible that the only handling tires in that size still on the market are meant for peope with restored Z28s or SS396s, and have big white letters on at least one side. The disappearance of real handling tires with really instant turn-in response, etc. in 14" from the market is why some of us are reequiping our W123s with 15" wheels from late '80s W126s, although some of the BFG tires in WIDE 14" are really quite good in that respect -- that's what I've got on my youngest daughter's little old '83 Toyota Celica (245/50X14 on the rear, 205/60 on the front, on 7" Supra rims).

[neileg]

Warren, thank you.

Tomguy, I have not said that there is no difference in consumption caused by a higher engine speed, I just said it was small.

And I'm not having a go at you, Tomguy, but it is perfectly possible to have an inaccurate odo and an accurate speedo, or the other way round. Both may be driven by the same cable but the conversion to speed or distance is quite a different mechanical process.

[wbain5280]

Originally posted by Tomguy
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!

Neil's right. It sounds counter-intuitive but the overdrive is there to reduce engine rpm but more importantly open the throttle more so it becomes less throttled. Unthrottled gas engines are more efficient.

Any car needs a certain horsepower for any spedd and load. It requires the same horsepower at 60 mph in 3rd, 4th or 5th. Pumpling losses will increase gas mileage as will a more throttled engine.

[psfred]

Speedo is a magetic drag device, the odometer is a mechanical gear driven device, no connection between them except that the same cable drives them. No reason to expect the speedo to be as accurate as the odometer after 31 years, either!

I'd rather have it a bit fast than slow anyway.

Peter

[Tomguy]

Assuming your speedometer isnt broken and was correct when originally made, since it and the odometer are driven by the same cable and the distance and speed are calculated by how much your driveline is turning, if your odometer is correct, your speedometer therefore should be as well. I'm not saying because it's 32 years old that it will be, but it SHOULD be dead on if the odometer is.
I am not arguing that an engine needs a certian power to go a certian speed. But I am saying that an engine turning 6K RPMs will suck more gas than the same engine turning 3K RPMs at the same speed. I know for a fact you don't need 500HP to go 100MPH unless your car is very, very, VERY heavy. More torque might help you get there quicker but that's for another day...
If you're going 70MPH and your engine only needs to use 50HP to do it, but you're turning 4500RPMs, producing 200HP, you're wasting 75% of your power, and burning a lot of gas to do so. That's where I am coming from.

[neileg]

OK, Tomguy, I give in. Whatever you say.

[psfred]

Tom:

The speedo is much more likely to be off than the odometer, provided the zinc gear isn't slipping. Reason is the speedo works by applying force against a spring to rotate the needle via a magnet on a stick, literally. Very small changes in the strength of the magnet or the distance between the magnet and the driven disk (I think it's a disk) will cause the needle to move more or less for the same rotational speed, and the spring can also get weak, etc.

The odometer, provided it's intact, is gear driven, so the distance wheels will move the same amount per rotation of the drive cable no matter what the speed is, completely separate from what the speedo reads.

Given that, speedo errors are usually pretty small, and wrong size tires will change the speedo and odometers by the same percentage.

On the subject of horsepower, horsepower is a measure of work done, and if you ain't doin work, you ain't makin horsepower. This is why the horsepower ratings of top fuel dragsters have gone from 1000 or so in the late 60's to 6000 or so today -- they are both "made up", appearantly from calculating "horsepower" from the potential energy contained in the fuel. It only takes about 450 hp, more or less, to accelerate a 1500 lb vehicle to 300 mph in a quarter mile, so they cannot possibly be making more than that. After all, a 50,000 lb Chance Vought Corsair accelerated from 0-60 in 600 ft on 2200 hp during WWII on aircraft carriers......ditto for the even heavier Mustang. A fully loaded 747 (about 750,000 lbs, 90,000 hp) accelerates from 0 to 150 mph in less than 8000 ft....

Fuel consumption vs hp vs engine speed is a complex subject, but usually a more heavily loaded engine at a lower rpm will consume less fuel than a less heavily loaded engine at a higher rpm, all other variables constant. This is indeed probably a result of higher charge density from a less restricted throttle. There are other considerations (less loss of heat to the cylinder walls from more efficient expanstion, etc) that only a good automotive combustion engineer can explain. I sure wish MB would have put a four speed tranny and a 2.78 rear end in the 280! Serious gas pig!

Peter

[tosbt]

I wish gas was .25 a gallon again and I wuld not give crap a about gears or RPMs!! Its all relative.

[psfred]

Tobst:

Yeah, I remember being pissed once when I'd paid $.22 a gallon and it was $.18 across the river, as I discovered when we went to visit friends. Car only got 5 mpg, 8 on the highway (62 Olds Dynamic 88 with a sloppy tranny).

Peter

[Tomguy]

Blah, cheapest I can remember gas being was under $1.00 a gallon (0.99 cents, I believe) for 93 octane. At least I don't live in Cali - I might care a LOT more about my heavy foot there!

I would love a 4-speed tranny. Rear-end gearing is fine, if you ask me. I expect poor mileage on a V8 that's 32 years old. And it gets better mileage than I even expected. It's still better than an Expedition!

I think I'd get better mileage if I checked my tires again. They look a bit low. That and the fact that due to the screws coming off of the plate that holds the breaker points in the distributor (it only had 1 and it came off at some point), I was way too advanced (and inconsistent) with my timing - causing me to set it stinking rich (as I found out when I found the problem, rectified it, and re-timed it). Probably getting closer to 14MPG now, with the way I drive, that's pretty good - you can ask PaulC about that one

[wbain5280]

Originally posted by Tomguy
If you're going 70MPH and your engine only needs to use 50HP to do it, but you're turning 4500RPMs, producing 200HP, you're wasting 75% of your power, and burning a lot of gas to do so. That's where I am coming from.

Tom, not to belabor the point, but you're confusing engine speed with horsepower. Just because the engine is turning 4500 rpm doesn't mean it's producing all of its potential 200 hp. It really depends on the load. A car doing 4500 rpm down a hill is not producing 200 hp but going back up the hill, depending on how steep it is, might just produce 200 hp.

A car with a headwind is going to have to produce more horsepower than one with a tailwind.

I hope this helps.

[jas2wa]

Here it is: 6.5x14" alloys, with Yokohama S/T 215/70/14's.

The difference in ride and handling is amazing- feels much more glued down now, and not as choppy and under-tired as the old tires made it feel. Very happy with the results, and the pretty mean looks, too- looks not unlike a 6.3 now... (if you cover up the decklid badging anyway...)

Just thought my outcome might be helpful for those trying to decide on tires or wheels for a daily driver 108-

Check out some pics, it was foggy this morning:

JAS

[jas2wa]

up closer: