VicRoads have strict conditions on wheel modifications, in particular, the new wheel diameter.
I use a satnav (TomTom) to know what speed I'm actually doing.
I have a mental correlation of the factory speedo needle position when compared to the satnav reading for when I'm driving through tunnels with speed cameras and the Melbourne CBD when the tall buildings block out the satellites.
I too just set the speed on the dial around 5km lower than posted speed and from sat nav's and phone GPS speed that seems about right, just wanted to adjust it so when renting out the van it is working more like stock. Also as for Vic roads, I'm still within the allowed 15mm increase and using correct weight rated tyres. getting stuck on a grassy hill was enough of a push to find some half-decent all-terrain tyres for her
Jaycar have a module that you wire under your dashboard and it acts as a speed corrector, able to change the speed up or down by a factor of 0 to 99%. Around AUD$50 if I recollect, and comes as a complete box with wires hanging out - you just have to intercept the wire to your speedo and power and earth. Quicklock connectors work fast, and no soldering needed. Zip tie to closest panel inside the dash with a bit of foam to prevent future rattles, or use double sided tape to mount it, once you have got the settings right..
Originally featured in Silicon Chip Magazine a few years ago. Uses a PIC chip to count pulses and then sends out new ones based on a stored correction factor you have set during setup on a little rotary dial inside the box.
By the time you find somebody with a functional Merc HHTWIN programming unit that knows how to use the software, you might as well use this. Easily transferable to other vehicles if you decide to sell.
How do I know? Two speeding fines for exactly 7% over the limit, and I swear I was doing the speed limit as I was carefully watching the GPS as I went past the radar. Happens to exactly match the ratio between 14"/15" tyres. First was a shock, the second wasn't.
Dealers no help - 'we dont know how to do that', but are willing to put their apprentice to learn on your vehicle and your dime...
Bonus annoyance - your fuel economy and odometer also are wrong until you fix this.
On the subject of speedo correction, I have a 2000 312D. I discovered it had a speedo problem after first getting it and driving it on a trip. The speedo was reading slow by quite a bit. Others that I have spoken too with a similar vehicle have also noticed this ??? WTF went wrong at the factory for these vehicles. Dangerous to the wallet and points at about 10% low ! Lucky i did not drive hard on the speed limit.
I bought a speedo correcter unit that I wired in to the speedo wire under the PS seat where the wire was. I did not buy it from Jaycar but the supplier in the area. HOWEVER my vehicle does not have ABS and thus has a speedo sender on the rear of the gearbox. I do believe other models use the ABS signals to get the speed signal so you might need to find the correct wire near the speedo instead.
The corrector has some odd things about the action. The speedo jumps a bit at set points as it 'rescales' or whatever it does. But basically it does give the correct speed and distance since I set it. I used a GPS to adjust it, quite some years ago now.
Good luck Jaahn
PS I have never heard of HHTWIN, Xentry/DAS. What is it, maybee you should explain what it is for the ignorant here. We usually talk about another MB clone software here that some people do have.
Referring to the instrument panel electrical wiring diagrams, the signal comes from the speed sensor, is processed inside the instrument panel, and then fed onwards to other modules. You need to break the signal before it arrives at the instrument panel to correct the readings. Just behind the instrument panel you will find the three necessary signals where they arrive at the plugs - speed, ground and switched power 12volts. Cut the speed one, and tap into the other two, and insert your unit, right there in the large empty area behind the instrument panel. To remove, re-instate your cut speed signal and disconnect the power and ground - voila! The hunting you have experienced as your speed corrector comes up to speed is part of the method the software algorithm uses to calculate what pulses to output once enough input pulses are counted. The source code is downloadable from the Silicon Chip website and quite easy to follow. There are quite a few tricks in it to make it work as well as it does.
A bit of history: HHTWIN is the software equivalent of the HHT - Hand Held Tester - WIN for WIndows, which was used prior to OBD-II connections to program different modules, including the instrument panel. OBD-II came into force around the turn of the century and is now a obligatory diagnostic interface to all vehicles worldwide. The HHT could perform diagnostics on different electronic modules, and change parameters as required. Mercedes used a lot of electronic control units quite early for the motor industry, and these were the early diagnostic tools used to program them. As dealers had less and less older vehicles, these units were rarely used and most have been abandoned or disposed of. If a dealer has one somewhere on their shelf, the personnel who know how to use it in anger have long retired. Given that the English version of WIIS has a lot of the HHTWIN instructions and diagrams still in German, this also makes it very difficult to follow. I cannot imagine Mercedes going back and updating these half-translated sections for a small market.
One of the items that could be modified was the divisor ratio used for the speedometer, so different diameter tyres could be fitted, and the division ratio updated on the instrument panel to suit the new tyres. This took the pulses from the speed sensor, divided them by a stored value, and used that to display the speed, and pass it onto other vehicle control units such as the ABS and ECU. The divisor value was stored in the instrument panel memory chip, along with a number of other parameters and values, including the mileage. Some versions of the HHT has the wheel diameters selectable as a dropdown value, and others needed you to calculate the divisor value and set it directly.
I even approached some of the larger tyre dealers that specialise in trucks and vans, choosing the oldest person they had on the premises to speak to. "Oh yes, I remember those, but we haven't see one for decades" is the usual reply.
The Hand Held Tester is a rare find these days, and has been incorporated into a software emulator that connects to the ECU via the round 38pin and 14pin diagnostic connector found in earlier vehicles, and thence to the instrument panel. Later Mercedes interfaces such as the Star C3, C4, etc incorporate adaptors for the 38pin connectors and software such as HHTWIN to enable older pre-OBD-II models to be serviced. The same interface has ODB-II connections for the 14 pin interfaces for more modern vehicles, and the SDS/Xentry/DAS software takes care of the rest. Beware the cheap 14 to 38 pin adaptors available for sale on the Internetz, as they only translate the wires, and do not provide the analog to digital interface required for OBD-2 software to communicate with the older vehicle modules.
To emulate/duplicate functionality of a HHT in hardware, using one of the recent $5 micro controller chips might be a useful and interesting project, and worthy of further investigation. The 38pin diagnostic connector pin connections are not a secret, published everywhere, but secrets of the handshake signals and the parameters involved are out there somewhere, a somewhat closely held secret, and I'm sure if they were released some enterprising young whippersnipper could program them in an afternoon. For what they charge at the dealers to learn how to access historical records, learn how to do it, read and understand the part German instructions, and then futz around, all on your dollar at an hourly rate, one of these might make a wise investment. Anybody that has access to these parameters and codes or has reverse engineered them is welcome to contact me. Alternatively, somebody that has an actual HHT that is willing to clone it would also be an interesting exercise.