Gauges way off

I installed gauges on my 1979 LTD 1000. The gauges are from a different bike (don't know what model and year, speedo goes to 160 mph). Now I realized the speedo is approximately 15% off, showing 85mph, when my friends brand new Harley speedo shows 75mph also confirmed by GPS. The tach is also off, but can't tell how much. I revved it up to the point where it scared me and the tach only showed 7,000 rpm.
I thought all gauges/drives had the same ratios for the kz 1000 models, at least until 1980, but I think I am wrong....does anyone have more insight? Appreciate the help, thanks.

If you have access to a dwell meter you can check the accuracy of the tach. Pretty much all dwell meters have a tach function that is very easy to use. Ed

I just have the cheapo multi-meter, that doesn't have the dwell/rpm function. Maybe time to invest in a better one ? Or can I make it work with a regular multi-meter? Thanks!

I don't know if a multimeter can be used as a dwell meter or tach. My dwell meter was made specifically for that purpose. They can be found on eBay, and they don't cost much (otherwise I wouldn't have bought one because I'm cheap :laugh: ) I bought mine (a used MAC tool) to set the points. Ed

If your meter has a HZ setting, you can see it. At 1000 rpm you should get 16.67 HZ. At 3500 about 58.3 HZ

Ok, time to get a decent multi-meter Mine neither has the dwell or the Hz setting. Will go out and get one in the next days, however even without measuring, I know the tach is off. I rode again today, revved the bike up really high and the tach didn't even show 7,000. I am sure measuring will confirm that.
Are you aware of different ratio tach drives or tachs out there? Could it be that I need to change the tach drive? Or can the tach be calibrated? Again, thx for all the help. It sucks to have a tach that is unusable...


.

AFA the speedometer, it sounds like the donor bike had a different diameter front wheel.