I need to take my car in to get the speedometer calibrated. I ordered the car with 21" wheels and have noticed that the speedometer is incorrect. Just want people to be aware of this if you have 21" wheels because it seems that it came out of the factory calibrated for the 20" and was never adjusted.

 

Seems rather common https://mbworld.org/forums/cle-class...eedometer.html

 

It is not uncommon to be a couple mph off at higher speeds such as 70mph or more. How far off is yours and how are you measuring the accuracy of your actual speed? Your phone GPS is not always correct. My car with 20" factory wheels shows 2mph high at 80mph when measured against my phone GPS showing 78mph. The phone will sometimes fluctuate and show the same speed as the car for longer than a couple of seconds but it's pretty consistent at that speed. To me, that is acceptable as it could prevent a ticket, but it does mean that my odometer will show a higher distance traveled than what I have actually done over a long period of time.

 

Just read that thread. Now I am anxious to hear what the dealership will say or if they will/can adjust it. From what it says, the odometer is independent from the speedometer, which was my biggest concern, but I will check that for myself to see if that is accurate. If they are independent than I guess I can deal with an inaccurate speedometer, I just don't want the car to rack up miles that aren't actually being put on.

 

It's right around 4-5mph over running at 80mph according to Waze, and 2'ish mph over at much lower speeds. As to your comment about the odometer, the thread posted above says they are independent, but I haven't verified that yet myself.

 

I think it depends on the reason for the inaccurate speedometer. If the car manufacturer purposefully increases the speedometer reading then they can certainly choose to omit the odometer from that adjustment to have it more accurate. If the cause is because tires are not within the original design spec then both the speedometer and odometer would be affected. In the end, it's probably a little of both which would mean the odometer is off by a lesser degree than the speedometer. It would be hard to tell exactly by how much without doing a test.

 

By the way, just because a wheel is 21" verses 20" or 19" doesn't mean the outter diameter of the tire mounted on that wheel is larger. It depends on the actual tire. Normally when changing wheel diameter, the goal is to match the outer tire diameter by making the sidewall shorter. This gets the lower profile look and helps in cornering due to a stiffer sidewall. Keeping the outer diameter the same is primarily to keep the speedometer from being off, but also to prevent the tire from rubbing on the wheel well. You could intentionally reduce the outer diameter of the tire to make the adjustment. That would mean a shorter sidewall for the same size wheels or reducing the wheel size while keeping a similar sidewall profile to what you have now.

 

This is a very important point to consider when it comes to calibrating the speedometer, thanks for pointing it out.

 

Perhaps give it a shot.

 

I marked a measured mile on a street in my neighborhood using GPS. Then I checked the accuracy of my car's odometer against it. Not surprisingly, both of my cars with factory wheels & tires were off by several feet.

 

My E53 is fitted with 20" wheels and the speed is overstated by about 2 mph at 60 mph. If the odometer is off by the same percentage, the warranty is ending 1500 miles early.

 

Is the speedometer displaying faster than GPS or slower than GPS?

 

I didn't check for speed, only distance. Checking for speed on a residential street course of 1 mile is difficult.

 

True true, fair enough.

 

How did you do this? The odometer only shows 10ths of a mile if using the trip. That would mean you could only measure to an accuracy of 528 feet. Since the odometer is a slow moving measurement, I would think that you would need to travel much further than 1 mile to determine accuracy. One way would be to drive 60mph on an interstate for exactly 60 minutes (and zero seconds). Then compare the odometer trip since that has 10ths of a mile. If it is anything other than 60 miles, it's off.

 

I measured it with a surveyor's measuring wheel.

 

Classic

 

Ah, I see. Could you explain further what you did exactly? I have some thoughts on how this could be done but wondering how you did it.

 

Before going crazy, this is all about averages. Every tool has a margin of error and when using multiple tools to come up with data, you have an accumulation of error. There is a reason that police that act as humans don’t pull you over for being 10% above the speed limit. A measuring wheel isn’t perfect unless the ground is glass flat, real GPS has a margin of error of a couple feet, and phone GPS has an even greater potential error. Accordingly, since part of this conversation is talking about the distance being off at speed, you’d have to hit that mile in your neighborhood at 70 miles an hour….The error value may be different at speed, but the percent error should be the same.(sure this can depend on the range of the tool where the error is greatest.)

For some of us with larger tires on older trucks, we use a speedometer adjustment device and can get the info dead on.

Regardless of any of this, a brand new vehicle with stock tires should be dead on at all speeds considering this was achievable decades ago before they stuck leashes in our asses to track us while we pay them for the opportunity.

 

I marked my starting position on the street with chalk; walked 1 mile as indicated on the wheel; marked the ending position with chalk. I'll note that the street is 2 miles of very smooth asphalt. I then drove from the starting position along the route until the odometer measured 1 mile. I measured the distance between the car and the chalk mark. It takes longer to explain it than do it.

 

 

How many feet were the cars off by?

 

I don't recall exactly, but it was nearly 6 feet short, an insignificant amount. IIRC manufacturers over report speed (slower than indicated) rather than under report speed (faster than indicated) because under reporting speed is illegal in Europe and many other jurisdictions. The US Federal requirement for accuracy is ±5 mph @ 50 mph. The Florida requirement is ±3 mph at any speed.

 

The car being 6ft short of 5280 feet is basically a bullseye and is within 0.1% of the expected outcome. The wheel you used is likely not accurate to 0.1% and slight variations in temperature, the path taken, and the technique used could easily account for that amount of error. Other factors are that the speedometer and possibly odometer may be less accurate at significantly slow speeds which you would have to be doing to stop exactly at 1 mile or risk overshooting an actual mile by many feet depending on your speed. It's an interesting idea and good attempt to measure the accuracy of the odometer but it's quite inaccurate. I do applaud the ingenuity and effort though.

A better, but still inaccurate method would be to drive at 60mph on an interstate for exactly 60 minutes and start a timer while resetting your trip just as you pass a starting reference mile marker. At exactly 60 minutes look at the odometer and take note of the mile marker you just passed or are about to pass and do the math for the distance traveled. If that is close (within 1/3 of a mile) to the odometer than your odometer is likely pretty accurate. If your speedometer is off by +2mph at 58mph and your odometer was equally off, you would expect to be passing the 58th mile marker at 60 minutes while your odometer says 60 miles driven. There are many factors that will cause this to be imprecise such as the true distance of mile markers on the interstate, your ability to reset your trip and start a stop watch at the exact same time, the fact the odometer trip only shows accuracy to 1/10th of a mile (528 ft), and your ability to estimate how far you are from the nearest mile marker at exactly 60 minutes. That's why you would need to travel a long distance such as 60 miles to make these factors insignificant. You could increase the distance to say 120 miles to reduce the impact even more. By the way, I wouldn't recommend anyone try this without a co-pilot for safety reasons.

 

In my metro area driving any steady state speed let alone at 60 mph that is above every speed limit on our bumper to bumper I-75 and I-4 roads is virtually impossible. My method of driving a measured mile on a residential street where speed doesn't matter is actually doable. The measured mile is the static parameter and the odometer is the variable being measured. The OP wanted to validate his speedometer and odometer both. My method evaluates the odometer very accurately. Your method does a poor job of evaluating actual distance because of the margin of error travelling at 88 feet per second. Typical human reaction time is about 300 ms or more than 26 feet at 60 mph. Keep in mind that mile markers along highways are approximate, not exact. They may be off by hundreds of feet due to infrastructure such as an interchange or overpass.