Tire Size vs. Speedometer/Odometer

[QtheGenius]

I recently bought a non sport 99 528it. Currently there are 235/45-17 tires on the car all around. On the highway, the car will show 80mph on the dash, yet my GPS states 74mph. This is a huge difference and one that I want to fix. So logically, the easy fix is to put larger tires on the car, but I'm not sure that's the best idea. Factory equipment is 225/55-16 tires, yet Tire Rack states that equivalent tires are 235/45-17 and even 245/40-18. The difference between the factory sized tires and what I currently have is only 0.3 inches. I don't believe that this would acocunt for this much of a speed descrepancy between the speedo and the actual car.

The only other information that I have it that the car was bought in AZ, then imported to Canada, had the readouts switched to Metric (while on 18" wheels), imported back to the US to Cali, switched to 17" wheels, then had the readouts switched back to Miles. Now I don't know, but I'm assuming that when you switch the readouts, it doesn't do any recalculating of the speedo. I've also heard from other members with e39 wagons that their speedo is off by 3-4 mph at speed. I'm concerend becasue calculating MPH will ALWAYS be way lower than actual, and I'll rack up miles faster that i'm actually driving.

What are my options for fixing this issue?

[MeNoo]

There are speedo recalculation kits, the only one I know of comes in a kit form, so you'd have to solder it up. I don't have a link... But I'll get you a video.

[Critter7r]

I recently bought a non sport 99 528it. Currently there are 235/45-17 tires on the car all around. On the highway, the car will show 80mph on the dash, yet my GPS states 74mph. This is a huge difference and one that I want to fix.

While you may want to modify it, that amount of variation is within specs. No need to fix it, nothing's broken.

So logically, the easy fix is to put larger tires on the car, but I'm not sure that's the best idea. Factory equipment is 225/55-16 tires, yet Tire Rack states that equivalent tires are 235/45-17 and even 245/40-18. The difference between the factory sized tires and what I currently have is only 0.3 inches. I don't believe that this would acocunt for this much of a speed descrepancy between the speedo and the actual car.

Putting larger tires on the car will actually reduce the speed shown on the speedo. At a given speed, a smaller diameter tire spins faster than a larger diameter tire.

The only other information that I have it that the car was bought in AZ, then imported to Canada, had the readouts switched to Metric (while on 18" wheels), imported back to the US to Cali, switched to 17" wheels, then had the readouts switched back to Miles. Now I don't know, but I'm assuming that when you switch the readouts, it doesn't do any recalculating of the speedo. I've also heard from other members with e39 wagons that their speedo is off by 3-4 mph at speed. I'm concerend becasue calculating MPH will ALWAYS be way lower than actual, and I'll rack up miles faster that i'm actually driving.

What are my options for fixing this issue?

BMW says :

"Most speedometers have tolerances of approximately +10%. A “speedometer advance" is necessary to compensate for negative tolerances in tire diameter, electronic controls, tire temperature, tire pressure, vehicle load and other factors. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle. If you are using a known accurate GPS system as a reference speed, you need to use the formula below to calculate the maximum permitted speedometer advance, which is 10% of the actual (input) speed plus 2.4 mph. Example: Actual speed = 50 mph … 50 mph x 10% = 5 mph … 5 mph + 2.4 mph = 7.4 mph … Therefore, the permissible displayed speed is 50 to 57.4 mph. (Note: The displayed speed must never be less than the actual speed.)"

[QtheGenius]

Thanks critter, that statement from BMW is basically a "Cover Your A$$" statement. Of all the cars i've driven, I've never seen teh speedo off by more than 2-3 mph. You would think that BMW, of all manufacturers, would be able to create a speedo that is damn near accurate as they have in all teh other cars I've driven.

[Critter7r]

I don't really think it is. The Service Bulletin that contains the statement I posted originated in 1984. It was recently updated to include the line about using GPS as a reference speed, but other than that, they were saying the same shit back in the mid 80s.

[QtheGenius]

Putting smaller tires on the car will cover less distance, resulting in me moving slower. The speedo readout won't change depending on the size of the tire, because all it reads is revolutions. A larger tire would cause my car to go faster, which is sort of what I want since the read out says I'm going faster than I am. I need to move faster to match the speedo.

[Critter7r]

Putting smaller tires on the car will cover less distance, resulting in me moving slower. The speedo readout won't change depending on the size of the tire, because all it reads is revolutions. A larger tire would cause my car to go faster, which is sort of what I want since the read out says I'm going faster than I am. I need to move faster to match the speedo.

notsureifsrs.jpg

At any given speed, the speed that the vehicle is actually travelling has zero to do with the tire size. However, the number displayed on the speedo IS affected by the diameter of the tire.

With me?

So at a given speed, say an actual speed of 60mph, the tires are spinning at X RPMs and the speedo shows 64mph. Putting smaller diameter tires on the car and moving at that same actual speed of 60mph will require the tires to spin faster to cover the same distance within the same time.

[QtheGenius]

you are correct in your statement, but it won't fix my problem. Changing tire sizes won't change the speedo readout. I need my car to move faster to match what the speedo says. If I'm driving and the engine is at 3000 RPM in 5th geara and I'm moving at 75mph, and I put a larger outside diameter tire on car, I will physically be moving faster, but the speedo and tach will read the same.

[Imolarot2]

Jonathan,

I'm running oversized rears (~2% larger than stock) on my ZHP and my speedo reads accurate now. I know there is a way to code the E46 to your actual wheel/tire diameter and reduce the 'overestimate' but I just went the mechanical way. There may be a way to do the same for your new car but I don't know coding all that well. Did you try messaging Jackson Chin and see if he has any ideas?

JP

[QtheGenius]

no, I haven't tried him. The time to do that would be when I get new tires. I'll PM him and see if he has any experience with that.

And what was your default stock tire size and what are you running now?

[Critter7r]

you are correct in your statement, but it won't fix my problem. Changing tire sizes won't change the speedo readout. I need my car to move faster to match what the speedo says. If I'm driving and the engine is at 3000 RPM in 5th geara and I'm moving at 75mph, and I put a larger outside diameter tire on car, I will physically be moving faster, but the speedo and tach will read the same.

If your vehicle is currently travelling slower than the speedo indicates, and putting larger tires on it causes you to "physically be moving faster" while keeping the speedo readout the same, isn't that exactly what you're trying to do?

Example:

current tires : Actual speed = 65mph ; speedo = 70mph

larger tires : Actual speed = 69 mph ("physically be moving faster") ; speedo = 70 mph (speedo stays the same)

[MeNoo]

I never thought about this... Do you have an OBC? In an E34, mine can adjust speedo readings.