After speedo change out 10 mph off

[prichmon]

I swapped out a buddy's 1982 gs450T speedo with a 1981 gs650gl speedo. Both have 90/90r19 tires.

When we checked by GPs the new speedo is off by 10mph slow at 60mph.

I can only think the speedo came off another bike other than the 650. That's a huge difference in tire size for a GS.

Is there a way to correct for the error?

Thanks

RR

[rphillips]

"82" 450T part # 54600-08C01 and "81" 650 GL part # 54600-49101...Probably got to have the correct part #. I'd have figured most GS's used the same one, but I guess not.

[Suzukian]

It would be nice if there was some adjustment screw, but other than taking it to a gauge specialists, and paying 10 times more than a used one for your bike, you're probably not going to be able to adjust it. Now, I've seen people do all kinds of things, but those gauges, when I have taken them apart, have nothing to adjust, and it is real easy to mess them up. Some you can't take apart because they are "peened" together.

[Rich82GS750TZ]

I’ve just gotten used to the speedo being wrong. In my case, the speedo reeds about 65 when I’m actually running about 57 according to the Waze App. I have phone on a bar mount with Waze open and I just glance at that once in a while to check my speed.

[prichmon]

thanks guys!!!

Rich

[Cipher]

So did Suzuki use different ratios in the speedos? Is this an issue with the speedo driver gear ratio being different?
these are different part numbers
54600-34001
54600-49100

refer to https://re5rotary.proboards.com/thre...ve-differences

[Brendan W]

What was wrong with the old speedo?
It looks like if the speedo drive and wheel are a match then it doesn't matter what speedo you put on there.
All wheel and drive pairs should be sending 2240 cable rpm to the speedo at 60mph.
Have a look at this

https://www.thegsresources.com/_forum/forum/technical-forums/performance-tuning-mods/54728-speedo-drive-gear-ratios

[Suzukian]

If the wheel diameters aren't the same, they won't match. The analog speedo use magnetic discs that couple and the needle has a spring which sets the tension. Some good speed shops can take them apart and adjust, or replace that coil spring, but it's an expensive procedure, and has to be calibrated, so they take the circumference of your wheel (real world) lay that dimension out using Pi and calibrate the speedo.

[rphillips]

There's always been a lot of talk about tire size changing speedo readings, and it does. But It's amazing, to me, how "little" tire size effects the speedo reading. A 2" taller tire on a motorcycle is a lot. A 245/75/16 vs a 245/75/18 dia. is 2" taller (6.6%), cir is 6.3" more (6.6%), revolutions pr mi. 661.9 vs 621.2...Speedo reading 25 mph actual speed is 26.6, 50 mph. actual speed is 53.3, and 100 mph. actual speed is 106.5. For a tire to slow a speedo 10mph @ 60mph. he would need to use a 6" taller tire. I've always wondered just how accurate these olg GS speedo's were when they came from the factory.

[Grimly]

Mine always over-indicated by between 5% and 10%, randomly depending on the bike/car.
It's a handy way for makers to abide by the law (generally EU), which states the instrument must either be accurate or over-indicate, but not under-indicate.

[Suzukian]

I think that speedos are a bit generous with their readings.

[slayer61]

I think that speedos are a bit generous with their readings.

Yup. I always referred to it as "optimistic" Good for bench racing.

[93Bandit]

Mine always over-indicated by between 5% and 10%, randomly depending on the bike/car.
It's a handy way for makers to abide by the law (generally EU), which states the instrument must either be accurate or over-indicate, but not under-indicate.

It's the same way in the USA. It varys by manufacturer, but I believe most American made autos have a speed reading of 5-7% below actual speed, and is considered acceptable. I imagine it's the similar if not the same for the 2 wheeled vehicles as well.

Course you also have to consider the build tolerance of a 40yr old analog speedometer. I've always considered the speedo to be a guesstimate. I have yet to get in trouble with the law on my bike, so it must be close enough.