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How To Regear The Speedo For Larger Tires

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kurt
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Re: How To Regear The Speedo For Larger Tires

Post#11 » Thu Feb 21, 2019 10:14 am

Here's the calculation I used, based on HansHehl explanation. Only thing that I added is an assumption to reduce the tire diameter from the theoretical 33 inch for example, to a real life situation, where the tire is compressed under load and gets used, hence reducing the diameter a bit. I took 3% for this => 33 inch * 0,97

Screenshot 2019-02-21 19.09.01.png


The cells in yellow are input cells. the green cells is the result. Here'e the excel https://www.dropbox.com/s/es5twouzgwffu ... .xlsx?dl=0

You need to take angular gear just a bit higher than these results, as in most countries it is illegal when your speedo lags behind.

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kurt
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Re: How To Regear The Speedo For Larger Tires

Post#12 » Fri Feb 22, 2019 12:37 am

One more thing :geek:

If I take the tire specs (I found a data sheet for all tires on BFG website) for my tires (BFG AT KO2 285/75R16), they mention the number of rotations per mile in the datasheet. Calculating from this number, I obtained a angular gear ratio 3% lower than the "theoretical" calculation above, which corroborates with the 3% correction I used.

Also, the same size 285/75R16 in MT and AT have a different diameter in inch (MT: 33,1 and AT 32,8), so one type/brand of tire can be different in real diameter than another, even with the same size

AT: https://www.dropbox.com/s/lnz94e4rqczrz ... s.xls?dl=0

MT: https://www.dropbox.com/s/8snk9ic38ecs0 ... s.xls?dl=0

autonovice
Posts: 1833

Re: How To Regear The Speedo For Larger Tires

Post#13 » Fri Feb 22, 2019 8:41 am

kurt

Really appreciate you to decipher this! This item has been on my outstanding todo for 6 years now.
I couldnt wrap my brain around this.

I added to your spreadsheet all avail gear ratios from Hans Hehl website. Its a drop down list.
I also calculated for % diff ( error ?) between the gear ratio avail vs gear ratio needed.
I "assume" negative ( - ) %diff is the speedo needle is faster than expected speed?
And at what %diff is tolerable? I guess its better to convert both gear ratios into KPH and take
the diff ratio to visualize the different in speed?

Capture.JPG

Capture.JPG


Rename this file to xlsx before opening ...
Angular gear ratio.xlsx.pdf
(12.84 KiB) Downloaded 4 times

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kurt
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Re: How To Regear The Speedo For Larger Tires

Post#14 » Fri Feb 22, 2019 11:30 am

Minh, here you go

I added to the spreadsheet 2 tables where you can see the differences in km/h and %

The % is always the same, no matter how fast you drive, but of course the absolute difference in km/h varies and is relative to the speed.

Note that some angular gear ratio's are NLA. Usually, if the sites https://www.carparts-pros.com/page_1_3.php or https://www.online-teile.com/mercedes-ersatzteile/ give a price, it's available.

How much error is reasonable, depends on your personal preferences. For me, I'm happy it's would be < 3%. To calculate fuel mileage, it's easy to account for the error. I wanted it more precise when I follow tracks from roadbooks that indicate directions after a certain number of km "turn right at km 34, ..., ...", and it's more convenient to respect speeding limits ;)

Rename this file to xlsx before opening (great trick btw, thanks!)

Angular gear ratio - II.xlsx.pdf
(13.21 KiB) Downloaded 8 times


If the difference is negative (installed angular gear lower than the necessary gear), then the speedo reads lower than reality. My current angular gear is 0,421 and the speedo/odometer read 27% % lower than reality.

Make sur you understand the calculation, I wouldn't want that you order a wrong one. I can give you feedback in a good week or so, when I'll hopefully install the new angular gear.

Btw, I got a used #4: 0,476 and a #1: 0,421 which I plan to sell after re-sealing them

autonovice
Posts: 1833

Re: How To Regear The Speedo For Larger Tires

Post#15 » Fri Feb 22, 2019 12:05 pm

Kurt, you are the man! thanks for putting this spreadsheet together. Very helpful.
So to validate my %diff question ... negative ( - ) %diff is the actual kph is lower ( not faster as posted earlier ) than expected kph.

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GermanStar
Posts: 364

Re: How To Regear The Speedo For Larger Tires

Post#16 » Fri Feb 22, 2019 12:55 pm

I did calculation for my, and find correct one.
You have to know diff ratio and tire size.

I can take a photo of calculation, and you just to have to put your numbers and you will get the correct info for gearing.

I have 1 or 2 for sale, I think no.5 and no.11 or no.6.

New one is very expensive, and some of gearing is not anymore possible to order.

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kurt
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Re: How To Regear The Speedo For Larger Tires

Post#17 » Tue Mar 12, 2019 1:13 pm

autonovice wrote:Kurt, you are the man! thanks for putting this spreadsheet together. Very helpful.
So to validate my %diff question ... negative ( - ) %diff is the actual kph is lower ( not faster as posted earlier ) than expected kph.


Minh, , I think I missed your post.

If the % diff is negative (-) in the spreadsheet, it means that the installed angular gear is too low. This means the actual speed (real speed), will be higher than what you read on your odometer display.

I know it can be a twist of mind, but an example may make it clearer.
Take my case for example => I had a speedo gear of 0,421 (number 1) installed, which resulted in a much lower reading on my speedo/odometer versus reality (example: When real speed was 100km/h, I read +/-79 km/h on my speedometer.

With the new gear I just installed 0,538 (number 13), the odometer now reads +/-102 km/h if the real speed is 100 Km/h. I could also have installed gear 0,522, this would even have been more precise, but it would have understated my odometer vs reality, which is illegal in Belgium

Below some pics of the new one
Attachments
2019-03-02 11.47.10.jpg
2019-03-02 09.57.50.jpg
2019-03-02 09.57.36.jpg

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cal_w460
Posts: 1363

Re: How To Regear The Speedo For Larger Tires

Post#18 » Tue Mar 12, 2019 1:33 pm

When you are relying on the mechanical speedometer and angular gear. Make sure that you have a fresh speedometer cable. Maybe install a new one just in case if your G is old. Or lubricate the old one with thin oil that doesn't oxidize when it ages.

The teeth on that nylon gear inside the angular gear tend to break if there is a problem with the speedometer wire. Mine was damaged, but I opted for a 100mm VDO Speedometer + a separate GPS reciever from ebay that is compatible with most modern speedometers. It will give me full accuracy even if I decide to change wheel size etc.

Will do a thread on the install as soon as it gets a bit warmer here (working outside).

A trick that I will use is to install a cheap digital bicycle speedometer as a backup for the GPS speedo. When inside tunnels or in places where there is bad signal. The neodyme magnet can be placed on the edge of a brake drum, one of the prop shafts or so.
w460.333 1982 Colorado Beige 300 GD OM617.931

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kurt
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Re: How To Regear The Speedo For Larger Tires

Post#19 » Thu Mar 14, 2019 11:44 am

cal_w460 wrote:When you are relying on the mechanical speedometer and angular gear. Make sure that you have a fresh speedometer cable. Maybe install a new one just in case if your G is old. Or lubricate the old one with thin oil that doesn't oxidize when it ages.

The teeth on that nylon gear inside the angular gear tend to break if there is a problem with the speedometer wire. Mine was damaged, but I opted for a 100mm VDO Speedometer + a separate GPS reciever from ebay that is compatible with most modern speedometers. It will give me full accuracy even if I decide to change wheel size etc.

Will do a thread on the install as soon as it gets a bit warmer here (working outside).

A trick that I will use is to install a cheap digital bicycle speedometer as a backup for the GPS speedo. When inside tunnels or in places where there is bad signal. The neodyme magnet can be placed on the edge of a brake drum, one of the prop shafts or so.


Good point Cal, my speedo cable got well lubricated with TC oil ;)

But the outer guide of the cable is damaged at a certain point, after the TC - I think moisture could come in as the damage is at a low point, and could cause problems. Before the cable was "lubricated", the speedo needle was shaky, while now it is rock steady. So I'll take no chances and get a new cable.

I had also thought about the GPS speedo, but the "bad signal issue" in certain circumstances witheld me, which you seem to have solved creatively :)

Many thanks

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kurt
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Re: How To Regear The Speedo For Larger Tires

Post#20 » Tue Mar 26, 2019 8:07 am

I received the new speedo cable :) Does someone nows if I need to lubricate it or does it come lubricated from the factory?

Many thanks, Kurt
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2019-03-26 16.59.38.jpg

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