Do you know how fast or how far you are going?

[gbw]

Posting this here because I think it is a technical question. Please let me know if this is in the wrong area.

Have any of you ever compared the speed or distance shown on your bike's gauges to a GPS? If so, what are the results?

I have a Garmin Forerunner 305 GPS watch that I use when I run (https://buy.garmin.com/shop/shop.do?cID=142&pID=349). For kicks I decided to take it with me on a ride on the 77 GS750. I was surprised to find quite a difference between the speeds indicated on the bike's speedometer and the watch. At speeds below 35mph the difference was only a few mph, but when the bike's gauge showed 55 the watch only showed 50. When the bike showed 75 the watch showed 68. So, at least I know this now and can adjust for it on the road.

I talked to a friend who has a BMW R1200C and he confirmed similar behavior.

However, I started wondering...if the speedo is off a little, what impact does that have on the Odo? I'm using the trip odo reading to calculate my gas mileage. How far off do you think I am on mpg? My bike has about 25k miles showing on the odo...wonder how many actual miles this is?

Hmmmm...

[DanTheMan]

My Garmin ETREX show the same results. My bike speedometer will read 75 mph when in fact, I'm going 69.7 mph.

[lurch12_2000]

My Garmin ETREX show the same results. My bike speedometer will read 75 mph when in fact, I'm going 69.7 mph.

What did it read when your speedo showed 100mph?

[DanTheMan]

I don't know. I had the Garmin showing 100 mph once, but the speedo was only reading 85 mph.......:-D

What did it read when your speedo showed 100mph?

[Steve]

I think that we have one of the lucky few accurate units on my wife's '82 850L. At an indicated 60 mph, the GPS is showing 59.3. \\/

We did a trip to West Virginia last year over the Labor Day weekend and had a chance to compare the bike odometer to the GPS. The GPS was on my bike, and I only did two short errands where my bike moved, but not hers. The errands were from our motel to the motel where the rest of the group was staying. The motels were about a mile apart, so the GPS went about 4 miles farther than her bike. At the end of the trip, her odometer showed 1184 miles, the GPS showed 1174. Remove the extra 4 miles, the difference would be about 18 miles, over a span of 1180 (+/-), giving an accuracy of +1.19%. I really do believe that I can live with that. 8-[

We are going back to WV over Memorial Day weekend, and she will have her own GPS, so we will be able to compare a little bit better.

.

[Craiger]

Are the tire diameters stock? If not, there's your answer.

Odometer will be greatly affected over time. Taller tires make for a faster real-time speed. For example, I have 33" tires on my Jeep. Before switching my speedo gear out to one that's accurate for my tires, my speedometer would read 55 when I was really doing closer to 62. That's something you really have to watch out for. Not a big deal once you get used to it, but the thing that bothered me most was that my odometer was not adding up accurately...and I didn't want that.

Smaller tires will have opposite results.

[Steve]

Are the tire diameters stock?

Actually, going to a larger front tire corrected the inaccuracy that is so common.
The stock tire size for most of our bikes is 90/90-19. We have a 100/90-19 on there. 8-[

[DanTheMan]

Hey, Craiger,
I'm running the 100's on my bike, but the readings are still off. BTW, when I'm running the 35's on my Jeep, I'm going 5 mph faster than the speedo reads. With the 29's, the speedo is dead on. The gears to drive the speedo are engineered for a set tire size, I understand that principle, however, on some of these bikes there seems to be a cumulative effect on the output of the speedo as compared to speed over ground (SOG). I think that'sthe question being addressed here.

Are the tire diameters stock? If not, there's your answer.

Odometer will be greatly affected over time. Taller tires make for a faster real-time speed. For example, I have 33" tires on my Jeep. Before switching my speedo gear out to one that's accurate for my tires, my speedometer would read 55 when I was really doing closer to 62. That's something you really have to watch out for. Not a big deal once you get used to it, but the thing that bothered me most was that my odometer was not adding up accurately...and I didn't want that.

Smaller tires will have opposite results.

[lurch12_2000]

I have the 100 tire on my front too. But when I've hit 100mph on the speedo I don't feel like I'm going that fast. Yeah, maybe feels like 90+mph but either the bike is very smooth or the speedo does read higher than actual?? 10% off?? any other opinions based on some factual experience??

[Craiger]

Actually, going to a larger front tire corrected the inaccuracy that is so common.
The stock tire size for most of our bikes is 90/90-19. We have a 100/90-19 on there. 8-[

That's good to know. Perhaps I need to check my tire size and run the bike with my GPS to see how accurate my speedo is.

[Craiger]

BTW, when I'm running the 35's on my Jeep, I'm going 5 mph faster than the speedo reads. With the 29's, the speedo is dead on.

That's what I was getting at....the taller tires will cause the speedo to read slower than the actual speed the Jeep is going. I think the stock speedo gear is calibrated for a 29" tire. I don't remember, though....it's been too long since my Jeep was stock.

But we're talking motorcycles....so I won't get too off-topic.

[DanTheMan]

Hey, I was being somewhat facetious with my 85mph answer, because my speedo is the 1980 speedo that only registers 85 mph, so the GPS was the only basis I had for my speed at that time. Sorry if I mislead you, but yeah, if your speedo is indicating 100 mph, you're probably only going about 93-95 mph.

I have the 100 tire on my front too. But when I've hit 100mph on the speedo I don't feel like I'm going that fast. Yeah, maybe feels like 90+mph but either the bike is very smooth or the speedo does read higher than actual?? 10% off?? any other opinions based on some factual experience??

[Steve]

...any other opinions based on some factual experience??

Not a Suzuki, but it was a motorcycle...my Kaw Voyager 1300 had a digital display that was VERY optimistic. When it displayed 75 mph, GPS showed 67. :shock:
Ironically, though, the odometer was relatively dead-accurate. 100 miles on the trip meter showed as 99.6 on the GPS. 8-[

My current bike, a 2000 GoldWing, is rather accurate. No changes to tires or anything else, an indicated 70 mph shows as 70.3 on the GPS.
Checking odometer accuracy on our WV trip last fall, bike showed 1178 miles, GPS showed 1184. I can live with that. \\/

.

[lurch12_2000]

Hey, I was being somewhat facetious with my 85mph answer, because my speedo is the 1980 speedo that only registers 85 mph, so the GPS was the only basis I had for my speed at that time. Sorry if I mislead you, but yeah, if your speedo is indicating 100 mph, you're probably only going about 93-95 mph.

Now I get it#-o
Another stupid question- Do you tell the cops they're wrong for clocking you at 100mph when your speedo only said 85mph?:-D

[Misterj1953]

Motorcycle speedometers are like politicians, they only lie when they are working.