80 series tires vrs. 90 series tires?

I have a '79 KZ1000E and have since lowered the bike. Instead of the factory (taller) 90 series tires I went with (shorter) 80 series tires. The damage is done ($$$ so to speak) but I would like to know all the differences between the two other than the obvious (height)? I know I'll have a lower top speed and lower gas mileage, but how much lower? Am I going from 40 miles to the gallon to 20, or is it much more minimal? I'm not a top speed guy, but am I loosing 10 MPH, or 30? Are there any other adverse side affects going to an 80 series tire that I haven't discussed? Are there any benefits?

Your thoughts and ideals please, and thanks!

See link below. Ed

www.tacomaworld.com/tirecalc?tires=120-90r18-120-80r18

You can go with a smaller back sprocket, Ed 's info says you gain almost 30 rotations per mile so the tires will wear out a little quicker.
What I wouldn't like is having the higher rpm's at highway speed, that's were you burn up the fuel and put more strain on the engine.

The difference will be minimal. A few extra rpm on the road won't cause any significant extra stress on the engine. I'd go ahead and ride the bike and if you don't like the higher rpm and (very) slightly lower mpg get 90 series at the next tire change.

That is only 3.6% difference. If you hit redline at 120mph before, the new tire will bring that down to about 114.6. If you were getting 40mpg before, it might now drop to 38.
Destruction of the planet is unlikely.

bluej58 wrote: You can go with a smaller back sprocket, Ed 's info says you gain almost 30 rotations per mile so the tires will wear out a little quicker.
What I wouldn't like is having the higher rpm's at highway speed, that's were you burn up the fuel and put more strain on the engine.

LOL! Changing my back "sprocket" may be a little difficult!

Nessism wrote: The difference will be minimal. A few extra rpm on the road won't cause any significant extra stress on the engine. I'd go ahead and ride the bike and if you don't like the higher rpm and (very) slightly lower mpg get 90 series at the next tire change.

So far (mind you, I may have put 10 miles on it) I like the smaller series tires. They do feel a little more tight on the road, not as sloppy. I have not done any highway riding as of yet.

TexasKZ wrote: That is only 3.6% difference. If you hit redline at 120mph before, the new tire will bring that down to about 114.6. If you were getting 40mpg before, it might now drop to 38.
Destruction of the planet is unlikely.

LOL, I've never hit redline. I baby that thing! I could in the past get 40mpg on her, if she is down to 38 now, that's no biggy for me. I don't ride very often. I fill up maybe once every 2 weeks.

Your speedo may be a tad off compared to before. Assuming an MM90 vs MM80 front wheel, an MM90 spins about 775 revolutions per mile, while an MM80 would spin 800 revolutions per mile, so you'll have a slightly higher indicated speed. 62 mph instead of the actual 60 mph. your odometer will be off as well, so the fuel consumption calculation will be off a little. 35mpg instead of the actual 36mpg. The speedometer may not be 100% accurate, so these calculations are just kind of theoretical & the difference isn't that significant.

...

And now for the " way back machine". When I was a young man in the 70's , we went from 78 series tires on our cars to 70 series or 60 series. That was to compensate for the existing rear end gearing to get better acceleration in a drag race ( stop light to stop light) or for cruising down Main street (or Hertel Ave or Furhman Blvd ). Gas mileage!? Who cared.

Serpent7, since you have a shaft drive, it is the easiest and maybe the only way to get faster accereration. I think the ST was the Touring model for 79-80 so I feel you are on the right track. The rest of us can easily put on a larger rear sprocket .
Good luck with your bike.

78z1Joe wrote: And now for the " way back machine". When I was a young man in the 70's , we went from 78 series tires on our cars to 70 series or 60 series. That was to compensate for the existing rear end gearing to get better acceleration in a drag race ( stop light to stop light) or for cruising down Main street (or Hertel Ave or Furhman Blvd ). Gas mileage!? Who cared.

Serpent7, since you have a shaft drive, it is the easiest and maybe the only way to get faster accereration. I think the ST was the Touring model for 79-80 so I feel you are on the right track. The rest of us can easily put on a larger rear sprocket .
Good luck with your bike.

Thanks 78zJoe! Other than a few odds and ends, it is exactly where I want it to be! I really like the shaft drive. I thought about getting rid of it early on but have really grown to love it! It is a more comfortable ride. very little differences (feel of the ride) in switching to 90 series to 80 series tires. A little more "controlled" feel to it. I just wanted to make sure I wasn't causing any damage or encouraging long term, ill-will to my KZ.

My wife has a shaft on her V-Star , very smooth, low maintenance but it is a good idea to change out the gear lube every now and then, also it is way easier to change tires.

But if you did have a chain, a larger rear sprocket would raise the rpm's