Author Topic: Final drive ratio suspect (Read 9706 times)

dante53 · « **on:** October 11, 2013, 14:39:13 »

I suspect that one of the previous owners of my 230 changed the final ratio gear to 3.46:1. I was indeed surprised to notice that at 60 mph speed (as stated by speedometer gauge), corresponding to about 3.000K in 4th gear, my GPS indicated a real speed of 110 kmh, i.e. 68.75 mph. The difference is as much as 14.58% wich is almost equal to the difference between 4.08 US standard final gear ratio and 3.46.
Could this explain why my car is reluctant to go over 4.5k RPM in 4th gear but loves to run between 3k and 4k RPM?
Is there a way to find out what final drive gear is installed on my car without lowering and opening the differential?

jameshoward · « **Reply #1 on:** October 11, 2013, 15:36:02 »

Look at the ratio, which is stamped on the casing of the rear diff. You'll see a machined flat area and the ratio is there.

dante53 · « **Reply #2 on:** October 11, 2013, 23:25:09 »

Thx for the hint. I imagine that the ratio stamped is the one originally installed on the car So, I will need to make calculations anyway to find out if it was changed or not. Am I right?

GGR · « **Reply #3 on:** October 11, 2013, 23:56:10 »

Though crown and pinions can be swapped, people usually swap the whole rear axle.

It may also be that your speedo was replace at some point with the wrong one. The other issue is that these speedos demagnetize with time, and end up indicating a lower speed. You should check if the the odometer is indicating the right distance. I think 230s in Europe were fitted with a 3.75:1 rear ratio.

Singer · « **Reply #4 on:** October 12, 2013, 02:10:27 »

I have an original Daimler-Benz AG Service Manual for Maintenance, Tuning, Unit Replacement for Passenger Cars starting August 1959. It's about 2 inches thick with assemblies drawings, pictures, the works. I'm told that not many of these volumes exist. I'm sure I have your answer, but I'm just a girl.

jameshoward · « **Reply #5 on:** October 12, 2013, 06:35:04 »

Agree with GGR - highly unlikely that someone would try to change the crown wheel alone, so it's more likely you've either got a new rear end, or a dodgy speedo.

And as for being "just a girl", Singer, you shouldn't be so hard on yourself. We'd be lost without your team!

dante53 · « **Reply #6 on:** October 12, 2013, 07:15:19 »

Thank you all. Being accostumed to Porsche 911 world, where you tear down transmission and change gear and pinion to modify final drive ratio, I dod not realize that on Pagodas is much more easy to change rear axel being a separate piece. I will inspect rear axel case as soon as I can. Blondie, what have I got to do to have you retrieve my answer from your book? I live too far to invite you for dinner.

jameshoward · « **Reply #7 on:** October 12, 2013, 12:17:41 »

The info on the ratios and speeds/RPMs is on the site already. Alfred Esser (66andblue) has a post, I think, that contains a graph showing the detail you need. Try the wiki first, then do a few searches. You'll learn more than you wanted to know about the rear diff. Trust me.

dante53 · « **Reply #8 on:** October 13, 2013, 07:34:18 »

Thank you James. I downloaded Alfred's graph. Later I will record my speed/RPM!

dante53 · « **Reply #9 on:** October 13, 2013, 10:27:52 »

Well, my speedo appears to be exactly calibrated for a 4.08 final drive ratio while the real speed corresponds to a 3.75 or 3.69 ratio. I will investigate further as soon as I can have the car lifted.

jameshoward · « **Reply #10 on:** October 13, 2013, 10:41:37 »

No need to lift the car. Just need a torch. You should be able to see it just by crawling under from the rear. Or getting someone else to crawl under...

GGR · « **Reply #11 on:** October 13, 2013, 11:55:34 »

Is your odometer working? Have you checked distance? If distance is off by the same % as the speed then your speedo/odometer and rear axle are not matched. If distance is ok then your problem is inside the speedo. What is your tires dimensions?

dante53 · « **Reply #12 on:** October 13, 2013, 12:30:29 »

Hi GGR, I checked speedo vs. tachometer vs. Gps. Then compared my readings with Alfred Esser graph. I run 185R14 standard tires.

GGR · « **Reply #13 on:** October 13, 2013, 12:45:26 »

Then it looks to be a mismatch between the speedo and the rear axle. I guess the build card of your car will tell you what rear end ratio it was equipped with originally. Is your transmission original? Speedo gears in Getrags 262 and 265 (the latter being a popular upgrade on Pagodas) have the speedo read 15% slower.

You can either change the speedo (expensive unless you're lucky to find one cheap), or put a ratio adapter. See the thread I started here: http://www.sl113.org/forums/index.php?topic=19312.0

The company I found can make whatever ratio you want, but it uses 7/8" fittings, I don't know if this will fit the MB speedo and cable. May be fittings can be swapped. Or you could source a company in Europe with metric fittings that would fit the MB speedo. If you do so I would be also interested as I'm having a similar problem.

Benz Dr. · « **Reply #14 on:** October 13, 2013, 17:38:14 »

Or, you could simply watch the tach. Once you know what speed you are traveling relative t RPM, it's pretty easy to look at your tach and see
how fast you are going.
I changed my rear axle ratio from 4.08 to 3.92, run 195 R 14 tires with a ZF 5 speed. There are so many things that effect the speedo on my car that it's easier to watch the tach. I know that 3,000 RPM is very close to 60 MPH and that every 500 RPM increase is another 10 MPH. Checked against a GPS, it's far more acurate than the speedo which seems to be about 15% too high. I never look at the speedo and haven't for the 20 years I've owned this car.

dante53 · « **Reply #15 on:** November 07, 2013, 07:49:22 »

Almost everything is clear now. Somebody in the previous life of my car swapped the rear axel with one from a 250. Not only I have a 3.92 differential case but also have rear brake discs.

What should I do now? Replace the real axle with an original one for originality, or keep the one I have and enjoy the better brake efficiency of the four discs system???

Bonnyboy · « **Reply #16 on:** November 07, 2013, 14:10:48 »

My suggestion is to drive your car - that will turn your upside down face right side up. Disk brakes are easy to maintain and having another set of brakes within for parking brake just adds to the cool factor.

Who looks at their speedo anyways. You could always put a couple little stickers on the speedo glass to remind you where the proper speeds are. Up here in Canada every second vintage car has stickers on the speedos harking back to the days when we switched from miles per hour to kilometres per hour. If someone asks just tell them that German mph was different than American MPH - just like the difference in imperial and UK gallons.

dante53 · « **Reply #17 on:** November 07, 2013, 20:38:46 »

Hi Ian, I'm not worried about the speedo reading as I almost never look at it but about originality.

garymand · « **Reply #18 on:** November 08, 2013, 19:29:32 »

I'm sure someone here would be happy to trade drums for discs. One of the reasons I bought a 250 was 4 wheel discs. Just enjoy the ride.

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Author Topic: Final drive ratio suspect (Read 9706 times)

dante53

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Singer

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