RPM-Gear-Speed conversions?

My speedometer is running really slow, so I'm hoping to pop the back of the cluster and lube everything up, hopefully fixing it. What I need is some sort of benchmark so I can say "If I am in X gear, running at Y RPM, my speed should be Z." Of course I plan on double checking against my wife driving my car to be sure, but I at least want to get close on my own. It's a completely stock 1982 XJ750 Maxim with no accessories, I weigh about 200 lbs, and I live at about 3000 feet above sea level (if it matters). I remember my old XS11 would match the speedometer to the tach when in 3rd gear, traveling at 30mph for 3k RPM. Any help would be appreciated.

 

wal mart has bicycle speedometers for about 12$ with a clock, avg speed and all kinds of nonsense
a gps, a smart phone with gps

 

if you have a smart phone there are all kinds of free gps programs that give you true and accurate gps speed.

or you can guy a cheap car gps tom tom or something.

you can hook up a 12v cigarette socket and a phone dock for under $20.


there are all kinds of solutions. all you need is the creativity. and using the wife isn't one of them. LOL.

i had the same problem with my maxim 650. i wired up a 12v socket from harborfreight. and found a phone dock on the internet. already had the car charger so that wasn't a problem. my latest creation has the same phone dock. same 12v cigarette socket but now i have a adapter that plugs into the socket. it has 2 sockets and 2 usb plugs. it comes in really handy for those people that have usb charge type phones and forgot to charge them. i also have a 12v electric tire pump.

my overall idea though is to not only use my phone as a gps navigation. but also as a stereo system.

 

I think the gearing is a little different...but I also get 30 mph at 3k rpm in 3rd gear on my 82 750 seca.

 

The magazines used to have a chart for speed at redline in each gear.
Oiling the internals may help you - but my 900 reads 9% fast (a bit annoying)

 

OT: snowy, I'm progressively getting to your level only with my battery tender outlet rather than a cig lighter plug. In my tool bag are a couple of splitters I wired up and a number of useful cords all wired for the tender. Net book cable, phone charger, generic USB power, 12v mini air compressor.

 

Thank you ManBot. I don't have a Smartphone, nor GPS (anymore), and I would like to keep this bike stock.

 

Update: I reviewed the Haynes manual checking to see if there was a transmission difference between the Maxim and the Seca. The transmissions are the same, but the J model has a different final gear ratio, so mine will be a little off. Thanks for the thought, but if someone with a J model could post it would be awesome.

 

there's an article floating around on here somewhere on how to fix the speedo. but it has a loud warning that it's not for the faint of heart.

it's not an easy task.

i found one shop in the whole country. they wanted $125 + $25 shipping.

there are local speedo shops but they won't touch a bike speedo with a 100 foot pole. they stick with cars.

 

You can, if you are patient, find a speedo/tach combo on ebay for pretty cheap. I got one for $30 plus a little bit of shipping.

I'll be honest, it looks good but I don't know if it works yet. But it looks like it works better than the one I have. I needed the tach, btw.

Don't forget to check your dog drive, on the hub on the wheel. Mine didn't work at all when I got it, but it was because the little $3.00 gear that drives the whole thing was chewed up. Apparently it's not uncommon.

Even my BMW (car) doesn't exactly reproduce the speed that I get from my GPS. You seem to live in Reno, if it's a big enough city, you don't need it to be all that accurate, anyway. Right???

Once I get home, if the question isn't answered, i think I did the calculations. I'll send you the file if you'd like (Excel). But keep in mind the Tach is much less precise than the speedo, so it really didn't help me much.

 

I got kinda the same question.....

I picked up this bike and the speedo didn't get any signal, though I'd swear at one moment it was showing maybe 10mph while I was doing 50+... or it rain hitting my face at speed as I forgot to change my shield to clear.

Haven't had a chance to inspect it yet but what should I look for to know if it's the dial, the cable, the $3 gear????

thanks

 

Check the dog gear. If it's worn it will be obvious, because the two teeth won't reach to the speedo hub to grip it. If it looks good, then check to see if the cable spins easily in the sleeve. If so, then you might have to put the cable into a drill chuck and run it, to see if it picks up speed indication. If it does, look at your dog gear again. You probably think it should work but isn't.

 

Day7a1, that Excel file would be sweet. As for a replacement, I'm trying to stay original and not spend more money than I have to (nagging wife). As for Reno, I live north of there and have to commute into the city on the freeway, with a cop station between me and town. Snowwy, I found that article and downloaded the .pdf form of it, stuff like that doesn't scare me, I trained to work on helicopter avionics in the Army. You want to talk about a piece of junk from the 80's, OH-58D is pretty high on that list.

 

Bump for some help?

 

I forgot where I got this, but my calculated values are pretty close, just not as precise. Sorry about the format, but the post won't maintain the tabs.

1 2 3 4 5
0 0.0 0.0 0.0 0.0 0.0
500 2.4 3.4 4.5 5.5 6.4
1000 4.7 6.9 9.0 11.1 12.7
1500 7.1 10.3 13.4 16.6 19.1
2000 9.4 13.8 17.9 22.1 25.4
2500 11.8 17.2 22.4 27.7 31.8
3000 14.2 20.7 26.9 33.2 38.1
3500 16.5 24.1 31.3 38.7 44.5
4000 18.9 27.5 35.8 44.3 50.9
4500 21.2 31.0 40.3 49.8 57.2
5000 23.6 34.4 44.8 55.3 63.6
5500 26.0 37.9 49.3 60.9 69.9
6000 28.3 41.3 53.7 66.4 76.3
6500 30.7 44.7 58.2 71.9 82.7
7000 33.0 48.2 62.7 77.5 89.0
7500 35.4 51.6 67.2 83.0 95.4
8000 37.8 55.1 71.6 88.5 101.7
8500 40.1 58.5 76.1 94.1 108.1
9000 42.5 62.0 80.6 99.6 114.4
9500 44.9 65.4 85.1 105.1 120.8

 

I can extrapolate it myself, thanks for the data! I doubt if I will be able to see THAT accurate, but if I can get within a couple MPH I will be pleased.

 

there are very very few speedos that are actually accurate. usually they are 5 - 10% fast.

 

Time your speed between highway mileposts. 60 seconds between posts = 60 MPH. gives you a ballpark estimate where youre at.

 

your speedo is accurate with the odometer.

all you have to do is track the odometer between mileposts on the highway.

go one mile on the freeway. but car says you've actually gone 1.05 miles. your odometer is 5% faster. your speedo will also be 5% fast. your car will say 63 mph when you actually going 60.

it's also worth noting that if your odometer is 5% faster. that means it actually adding up miles at a rate of 5% faster then what you've actually driven. for every 10 miles you've actually driven. your car will add 10 1/2 miles to the odometer.

at 100,000 odometer miles your car actually only went 95,000 miles.

you can put on a larger diamter diameter tire and that will slow the speedo down. instead of having a 175/70-14 you would put a 185/70-14.

pay attention to your motorcycle speedo. a brand new tire will have a larger diamter then a bald worn out tire. your speedo will spin faster as the tire wears down. your tread pattern is say 8/32 deep. the overall diamter is 16/32 or 1/2 inch. wear that tire bald and that's 1/2in. diamter you've lost.

 

Makes sense. Maybe that's why the bike only says 15k miles...good thing I'm planning an oil change and fix this before I go far.