Speedometer calibration

[Geo.Lander]

Hi! I am trying to calibration my speedometer for the new 385/65 22.5 wheels and tires. I'm not able to find this screw.. 😂 any help appreciated

 

[charlieaarons]

Is the speedometer electronic or mechanical?
Including the sensor, which would be on the rear output of the transfer case.
And if the system is mechanical, do you know your axle differential gear ratio? And what tire size originally came on your actual vehicle?
Because if the system is mechanical, it’s possible that a speedometer drive gear or pair of little gears, original equipment for smaller tires but faster differential, will be correct.
The original differential ratio and tire size is often displayed on a plate attached to the “B” column of the driver’s door opening. And/or available in the EPC system, which MB has made very difficult/impossible to access for private people.
For what it’s worth, your 385s are about the same diameter as 11R22.5s, a very common size.
If on the other hand electronic, is the vehicle young enough to be hooked up to a Star Diagnosis computer?
Or maybe my post is irrelevant, perhaps there’s just a screw to turn somewhere near the dashboard.

 

[DiploStrat]

FWIW, we simply had Atkinson Vos replace ours with a new model. Probably the single most expensive thing they did, but it is great, and, according to the GPS, spot on speed.

 

[Neil]

My speedo / Tacho , which is slightly different, has a small rectangular seal type sticker on the plate behind the drop down clock face , It looks like an anti tamper sticker.
Once peeled of it revealed a very small hole and the screw was deep set in the hole .
Calibrating was a pain in the butt as you tweek it a bit and then drive. Tweet a bit more , backing off a bit etc..

I had 2 GPS devices on the go to compare with

It took many goes , so be patient

I found that doing that calibration at quite high speed , maybe 100kph was far more accurate .

If you did it at say 50, once you got to higher speeds any inaccuracy had been magnified and it was way out .

Neil

 

[Geo.Lander]

Is the speedometer electronic or mechanical?
Including the sensor, which would be on the rear output of the transfer case.
And if the system is mechanical, do you know your axle differential gear ratio? And what tire size originally came on your actual vehicle?
Because if the system is mechanical, it’s possible that a speedometer drive gear or pair of little gears, original equipment for smaller tires but faster differential, will be correct.
The original differential ratio and tire size is often displayed on a plate attached to the “B” column of the driver’s door opening. And/or available in the EPC system, which MB has made very difficult/impossible to access for private people.
For what it’s worth, your 385s are about the same diameter as 11R22.5s, a very common size.
If on the other hand electronic, is the vehicle young enough to be hooked up to a Star Diagnosis computer?
Or maybe my post is irrelevant, perhaps there’s just a screw to turn somewhere near the dashboard.

Thanks, I think I have to do this because the circumstance on the tire is essentially a higher gear. I assumed this would be easy 😂

 

[Geo.Lander]

My speedo / Tacho , which is slightly different, has a small rectangular seal type sticker on the plate behind the drop down clock face , It looks like an anti tamper sticker.
Once peeled of it revealed a very small hole and the screw was deep set in the hole .
Calibrating was a pain in the butt as you tweek it a bit and then drive. Tweet a bit more , backing off a bit etc..

I had 2 GPS devices on the go to compare with

It took many goes , so be patient View attachment 801977

I found that doing that calibration at quite high speed , maybe 100kph was far more accurate .

If you did it at say 50, once you got to higher speeds any inaccuracy had been magnified and it was way out .

Neil

Right, I'm going to take another look today. I don't really have an accurate gps yet. I had assumed I could calibrate based on tire circumstance...

 

[Neil]

Your speedo is electronic. It relies on impulses to show the speed. There will be an adjustment screw on it somewhere, you might have to remove manufactures stickers to access it on the back.
The adjustmentvwill simply advance or retard the display. Your new tyre sizes will be of little use to you.
Having found the adjuster you will have to either be on a rolling road or use a GPS to compare with.
I had two GPS devices that were totally in sync with each other.
Even so it took many attempts to get the speedo in sync with the GPS devices. Lots of getting up to a good speed ( 100kph ) and checking it. Then adjust and go again.
It's a frustrating task as only the slightest adjustment can send you way out in the wrong direction.
Reducing the impulses will make your speedo go higher.

Do you have a model number for the speedo, I can see its VDO. ( maybe on the back or side of it on a sticker )

Neil

 

[Neil]

Is the speedometer electronic or mechanical

It says electronic on the front of it

Neil

 

[charlieaarons]

It says electronic on the front of it

Neil

It may say “electronic”, but
1) the odometer appears to be mechanical
2) I was mainly referring to the sensor, which could be a pair of small gears near the origin of the rear driveshaft with either a mechanical cable, or a pair of wires attached.
If the vehicle has that pair of gears, then correcting the odometer/speedometer is best done by changing gear ratio of that pair of little gears.

 

[DarcyOlson]

Hi! I am trying to calibration my speedometer for the new 385/65 22.5 wheels and tires. I'm not able to find this screw.. 😂 any help appreciated
View attachment 801958









The speedometer calibration often involves adjusting a screw on the back of the instrument cluster or using a specialized tool. If you're sick and weary of the stress that comes with homework. It was during a particularly difficult semester that I discovered CustomWriting Service, and it has been a revelation. They offer unmatched comprehensive support to students who are struggling with their coursework. The writers' proficiency is evident in their assignments, which are intelligent, thoroughly researched writings. Meeting deadlines is only one aspect of the service; another is providing exceptional quality. Because of the affordable prices, even students on a tight budget can use it.

the speedometer calibration often involves adjusting a screw on the back of the instrument cluster or using a specialized tool.

 

[Joe917]

Out truck had the commercial speedo with the tachograpgh. when you flipped up the tachograph there was a taped seal, behind the seal was the adjustment screw. I think it was a small slot.