MPG and Speedo error.

[Oldhway]

Ok, so I have had my 95' RT for awhile now and I am curious about a few things.

Is the speedometer grossly optimistic or am I really riding a whole lot faster than it feels like. I haven't had a chance to check it against a GPS but i am apparently a lot more comfortable on it than I realized :blush .

What kind of gas mileagae should i expect from my bike. Again, it's a 95' R100RT w/ the stock windshield. Lat tank was at 38 mpg. It's been cold here so the warm ups have been longer and I haven't done any longer trips beyong my commute to work (12 miles) but it seems like the mileage should be better. New spark plugs, wires, and air filter already and the carbs are clean.

Any thoughts?

[kevinedwards]

My experience is that the speedometer is generally 5 to 8% faster than actual speed, check against a GPS to determine.

Fuel economy depends on how you ride, tire pressure, load, etc. Keep an eye on it with warmer weather approaching, see what you find. Keep good records on mileage, temperature, load, this will help you figure it out.

[Manfred]

Most all the articles I've read since buying my first BMW say that airhead (perhaps boxers in general) have "very optimistic" speedometers. Mine, on the other hand, seems to be pretty close, based on traffic in sight of cops.

Here's an article on this topic: http://home.jtan.com/~joe/speedo.htm

Regarding fuel mileage, don't know about your bike. Mine is its smaller brother and low to mid 40s is expected. And delivered, in my case.

[Guenther]

My first speedometer was going ca. 5 mph too fast (checked with a digital bicycle speedometer at that time...before GPSes). The odometer seemed to be pretty acurate. Then it fell apart (the odometer rings). The new one did exectly the same thing and now checked with GPS. Rumour has it that BMW does this on purpose not to get sued in case it shows a speed too low.

38 mpg seems a bit at the low end. My R100GS goes between 40-50 mpg. There are a lot of things to influence this. I know from experience on a summer day at 90F, no head or tail wind, going 65mph on a leveled Interstate my cow goes 45mpg.

Assuming you have checked/adjusted all the typical things like valve clearance, ignition timing and carb synch it could be something that have deteriorated: compression (how are the spark plugs), bearings (can you easily turn the rear wheel while on the center stand? and clonk noises?).

Or, your tank was not filled up to the same level. Sometimes different brand/quality gasoline can make a difference.

Joint the torch-a-tank nutsoes. Note odometer, fill'er up, ride till it's near empty, fill'er up and ride home and fill'er up again and calculate your mpg. This way it compensates for wind, temperature and you had a full day riding fun. :laugh

Guenther

[PGlaves]

I've had BMW Airheads where the speedometer read 10 to 12 mph too fast and where the speedometer read 7 mph too slow - both at a real 60 mph. I suspect yours will fall within that range.

But I would suggest that you check yours because what it is actually doing is far more valuable to you than my thoughts on the topic.

40 mpg on a good day is what I think your bike ought to do on today's gas.

[swall]

I would have to say that BMW undercalibrating the speedometers to prevent a lawsuit is urban legend. There is an SAE spec on speedomoter accuracy (I did not review it for this response) that allows zero underage, i.e. showing a speed slower than actual) and about 4% overage (showing a speed faster than actual).

[108625]

I'd agree the speedometers tend to err towards optimism, so you'll have to check it against a reliable control (GPS, etc). I average similar mileage with mine, the best I've ever seen was 44 mpg. State of mind has at least as much to do with mileage as state of tune.

[Oldhway]

Thank you to all.

FYI: Plugs look great and no running or driveability issues. I'm going to guess that my cold Connecticut warmups combined with short trips puts the mileage about where expected. Gonna pop the Tom Tom in the Tank bag this weekend to check the speedo. 3500 RPM seems to equal 64MPH; I don't how that compares to others.

[HexST]

If your fuel is laced with Ethanol mileage will be less.

[Geoff]

I just recently took my cluster in to my friend's speedo shop and they have this really old school calibration machine that they use to calibrate the actual speedo function against a "known accurate" reading. My '84 was dead on and I previously thought it was reading about 5MPH under. This is what I learned... Think of your odometer and speedometer as 2 separate devices, because they are. Your odometer is gear (or mechanically) driven while your speedo needle is driven by an aluminum speed cup resting on a jewel, and is responding to "eddy currents" from the magnetized shaft. Not a mechanical drive at all. To test your odometer, just find a 10 mile stretch of highway and compare the mile markers against it. For the speedo, its all about cable delivery. Tire size and gearing changes can affect this, as well as the amount of magnetism of the internal shaft. If your speedometer was off the bike and thrown into a milk crate with a giant speaker sitting in it, this could dramatically affect the magnetism and screw up your true reading. That's my story and I'm sticking to it.