RPM = Speed- - Speedometer is off?

Sorry all, but yet another "newbie" question.......

I believe my speedometer may be "off" on my '82 XJ750 Maxim.

I know engines will vary slightly for RPM to speed, but I'm wondering anyone an tell me "approx" how many RPM's I should be running to run 55mph? How about 70mph?

Does anyone have a chart showing what approx RPM's equal what speed in each of the gears?

This morning was my first "real" ride on this bike. Either my speedometer is way off or EVERYONE was in a hurry. Everything from cars, trucks, semi's and other bikes were passing me like I was headed the opposite direction.

My bike would show me going 55mph @ 3500 RPM.

If it helps, I weigh 240lbs...I know that will also factor into the RPM equation.

 

Actually your weight will have 0 factor into what your speed/rpm is. You may have to have the throttle open a slice more, but your engine will be the same rpm for what speed you're going.

Unfortunately, I haven't been on my 750 enough to memorize the speeds to rpm's yet (I know my 400's tho

But I DO know that my speedo is WAY off from what a gps will tell me i'm going, it's gunna be prolly something like, speedo says 90, i'm really doing 100+ km/h

 

On my 750 Maxim I turn 4400 RPM at 60 MPH and about 5600 at 78 MPH. Speed measured with GPS which is more accurate than most if not all speedometers.

 

There have been several of those tow behind speed signs that the police use for speed recently, and each time I roll past one it appears that my speedo reads approximately 8% high. IIRC, 4500 rpm yields 55 mph.

 

your tire size is more then likely the wrong size. (((too small)))

speedos will slow down. depending on teh circumstances. but never speed up unless something has changed to make it spin faster. like smaller tires which is the only thing that can be done on bikes.

your rear tire also makes a difference in rpm's. everytime i installed a brand new tire. my rpms were 4600 at 55 mph. as the tire got bald. the rpm's jumped to 4800. bald tires have a smaller circumference over fully treaded tires.

i started a thread for this very specific reason. it has all the rpms to speed listings. my speedo was 30% slow. i hooked up a cigarette lighter for power, downloaded a program on phone, and used my cell phone as a gpd speedo.

http://www.xjbikes.com/Forums/viewtopic/t=26177.html

 

Thanks, I will have to look into the GPS speed thing. I don't own one but I know a few people that do.

Yesterday on the way home from work. In a 55 mile zone which most people run 60mph -- I had the bike at 70mph and was being passed by everyone. Including another biker who passed me in the same lane with traffic coming from the opposite direction. Glad I saw him coming and moved over.....

Told my wife we will have to take the bike and her car out on the road and see where my speed actually is. Have her run the correct speeds with me checking the speed/tach to see if there's a difference.

I checked the tire size and both front and rear and the right size for the bike.

 

You might need to clean/lube the actual instrument, it could be sluggish.

 

The speed chart is handy, I was able to find it earlier Thanks for posting. One of the first reviews (Cycle World) had the '82 XJ 60 mph actual speed, 57.

 

i find my speedometer is completely wrong as well. its measured in Km/hr but has the conversion for Mph but if im driving at what reads 60km/hr its coincidently perfectly matched to Mph. i live in Canada so all our limits are posted in Km/hr and i'v just had to learn the conversion.

To answer your question when your trying to find the speed you should be going on a road just pace the fastest car or bike you can find. always works for me and i usually get where im going faster.

 

i have an '81 seca 750 and the speedo doesn't work at all - i believe 4000 RPMs in 5th gear is (or very close to) 55 mph and 3000 RPMs in 4th gear is (or very close to) 35 mph.

so, i just say to my self "4 in 5" or "3 in 4" for the hiway vs slow down for towns.

based on the 4000 rpms in 5 gear = 55 mph and 0 rpms = 0 mph, your speed (in 5th gear) can be calculated as follows:

speed = .01375 * RPMs

 

front tire size is 100/90 19????

you can verify your speedo and odometer on the freeway and compare to the mile markers. those markers are exactly 1 mile apart.

most speedos will register the vehicle going anywhere from 1.05 to 1.10 miles higher then the actual freeway markers. 5 to 10 percent

yours will probably say 1.20 miles or higher. 20 percent or more.

 

The tires size on the bike is:

Rear - 130/90-16

Front 3.25/3.50-19 (57H)
Using a coversion table found online - 3.25/3.50-19 is 100/90-19

The rear tire is worn but I would still classify it as road worthy. I am planning to replace it during the "off" season this winter though.

 

yeah mine reads 45 when I go 60 the higher my speed the bigger the differance

 

Well here is what the GPS is/was telling me.

GPS Speed 46mph = 3500 RPM (55mph on my gauge)
GPS Speed 55mph = 3900 RPM (67mph on my gauge)
GPS Speed 58mph = 4200 RPM (70mph on my gauge)
GPS Speed 67mph = 5000RPM (82mph on my gauge)

I'll be doing this again once my new (used) speed/tach cluster comes in hopefully this week.

I never thought of using the GPS until I asked the question in here....

Thanks!

 

gps is your most valuable tool when it comes to speedos.

you can buy cell phone mounts for 18 bucks on the internet. the same ones that are sold locally for $100.

harborfreight sells 12 volt plugs. that connect to your battery. or if your like me. be creative and create your own connection using a keyed source. all you need is the cell phone charger for 12 volts.

 

I must be one of the lucky ones, my speedo checked out to be exactly what my gps says i'm doing.