Speedometer differences with wheel size

[Montana]

Inoticed something driving on "alligator alley" in FL the other day. It's about an 80 mile stretch of highway with one turn in the middle. Since there was nothing else to do, I tripped my avg. speed on the car's puter and compared to my speedo dial.

At about 84mph on the DIAL, the puter read about 80.3 -- about 3.7 mph difference.

At 70 on the dial, it was about 3.2 difference.

At reduced speeds it was proportionally smaller differences. Or to put it the other way, the difference got bigger as the speed increased.

I noticed this on an SUV with big tires I had several years ago but forgot about the issue. The 540I sport comes with 17" wheels rather than the 16" stock.

With a little math with Pi x R squared etc., the 17" wheel is a 6.25% increase in diameter and a 12.89% increase in circumference from 16's. However, neither of those correlates with a fairly constant 4.5% increase in MPH between the two instruments at any given speed.

So I guess my questions are these:

Does BMW not recalibrate for the larger wheels on the car?

Which is the true speed...the dial or the puter? I assume the true speed is the slower, computer speed because larger wheels will cause a slower speed read out.

Where does EACH instrument read its speed on a manual tranny? (tranny,front axle, rear diff?)

Other observations:

If people are putting 19's on their cars etc. then they must assume that their ACTUAL speed is LESS than what it reads on the dial, but their TOP speed capability will be more due to more diameter per rpm. SO does this mean people should use thier tripped instant avg. speed on the computer instead of thier dial for accurate speed? I think so....or am I crazy

 

[JGM3E3632]

Yes wheel and tire size do matter. Having said that my father's 2001 M Roadster's Speedo was off about 10mph. They (Local Dealership) replaced the cluster under warranty. BMW does have a rep for their speedo's being off. Usually the speedo says that you are going faster than you really are. When I installed my silver gauge faces on my car. I drive down the road with the bezel off and set the cruise control. Hit the avg speed button reset it. Then pulled the needle of the cluster and set it at the speed the computer said I was going. I have gearing charts and they match the same speed on the computer/speedo. I have even heard Motortrend (Who reads that crap) complain about the 540i speedo being off. Set the rpm's by using the Instrument Cluster Test. Filled the car up with gas and warmed it up. To calibrate the fuel and temp gauge. Hope this helps.

 

[manuel]

check this out
http://www.miata.net/garage/tirecalc.html

 

[frolf]

sweet link manuel, rolling circumference is key.

 

[echothreezero]

Exactly, you can have bigger wheels, but same overall circumference by employing a lower profile tyre.

Also, a quick Q - your trip computers (my car doesn't have one) how do they work? Are they linked to the same system as the speedo, therefore it could just be a calibration diff btwn instruments, or are they independent (eg the computer uses GPS or some other method of calc'ing speed) in which case, which is correct - speedo or computer?

I test mine by a combination of a hand-held GPS unit off my boat, and a set of posts a set distance apart on a flat, straight road (5km apart are the ones I use - say 3miles) and a stop watch. My bimmer is negligibly out. My ute (pickup for you) is reading 15km fast at 100 (9 miles fast at 62) and about 5km fast at 50 (3 miles fast at 30) i.e. it is a non-linear error.

To maintain the open-road speed limit of 100kmh (62mph) I drive with the speedo at around 115, and since the ute has a top speed of about 130 (so in reality say 120), this is fine as it means I can almost never get a ticket in it.

 

[Montana]

Yeah that is a good link. Thanks.

The problem is I don't know where each instrument reads for input. I'm pretty sure the Dial shows the true speed though. The on-board computer has an average speed function that will show your instant speed if you are cruising at a constant speed. Set it on cruise control and hit reset onthe calculation for avg speed.

I also have a spreadsheet (lifted from another board) that calculates theoretical speed at a ny given rpm based on inputs of wheel size, tire size, rear differential ratio, and tranny gear ratios.

My spreadsheet has me at 175MPH @6000rpm based on my tire/wheel/ratios which sounds about right. It doesn't take into account wind, but with a big V8, You're gonna push through darn close to that I would think.

I had it up to 148 or so on the Dial speedo but didn't get a good read on the puter side but I bet I was only going 140 or so. The difference must be close to 10 MPH at that speed. Gee I guess I'll just have to try again. And that was only in 5th gear!

 

[Montana]

Here's the output for my speed vs. rpm...hope you can read it

 

[gizzy]

I really think that they may be using different sensors. In my car, I have noticed that the trip odometer will increase .1 for about every 500 miles. The only way I can think this would happen is if the odometer and the trip odometer are reading from different sensors.

 

[Kirby]

Yeah that is a good link. Thanks.

The problem is I don't know where each instrument reads for input. I'm pretty sure the Dial shows the true speed though. The on-board computer has an average speed function that will show your instant speed if you are cruising at a constant speed. Set it on cruise control and hit reset onthe calculation for avg speed.

I also have a spreadsheet (lifted from another board) that calculates theoretical speed at a ny given rpm based on inputs of wheel size, tire size, rear differential ratio, and tranny gear ratios.

My spreadsheet has me at 175MPH @6000rpm based on my tire/wheel/ratios which sounds about right. It doesn't take into account wind, but with a big V8, You're gonna push through darn close to that I would think.

I had it up to 148 or so on the Dial speedo but didn't get a good read on the puter side but I bet I was only going 140 or so. The difference must be close to 10 MPH at that speed. Gee I guess I'll just have to try again. And that was only in 5th gear!

Unfortunately there are 2 factors that will probably prevent you from getting even close to 175 mph. Wind resistance is exponential, it increases by a square factor, not a linear factor. It takes alot more horsepower to get to 175 mph than 140 mph. That's why only a handful of cars even with 500 hp (Corvette Z06, M5, etc) are just approaching the 200 mph mark. The other factor is that your peak horsepower occurs below peak RPM, that holds true for most engines. So as you wind it out past 5500 - 6000 RPM, you actually start losing horsepower.

But that shouldn't stop you from trying!

 

[Montana]

I agree.

Also, when I bought the car I was told that it has a software performance upgrade. To what extent, I don't know, but the fact that it will rev over 6000 and exceed 130MPH tells me that is probably the case. A stock 540i would not be able to do that as I understand it. I've been meaning to take it to my local dyno to get a baseline run and find out. It probably only has the stage 1 type where you get 10-15 HP boost.

This added will hopefully get me closer to the theoretical top speed. Now I need to scope out a suitable road situation, or preferably a track, and probably wait until I get new tires.