Close, but not quite right on reading what the tire size means Chuck A.
Second number is sidewall height expresed as the percentage of tread width.

Sidewall height of a 150/90-15 tire would be 150 mm X 90% = 135 mm
Move up one size to a 170/80-15 and the sidewall would be 170 mm X 80% = 136mm.

A 1 mm difference in sidewall height (2 mm difference in tire diameter) should be so small as to be impossible to tell a difference in RPM at a given speed.

The OEM front tire is 100/90-19. 100 mm X 90% = 90 mm sidewall height.
One size up is 110/90-19. 110 mm X 90% = 99 mm sidewall height.
The 9 mm increase in sidewall height is enough to notice a difference in speedo readings that most members report when changing up.

I totally understand those thoughts, as Id like to myself.....to accomplish that, you'd need a rear of a larger OD...the front does nothing for gas milage, as it is not the drive wheel, unless you own a Rokon....

For the VN, you'd need to measure the space available with the rear shocks fully compressed....you could actually get away with a 16" rim if you replaced the shocks with non-compessable struts. Hurts my azz just to think about it...

And now, to decrese RPM at same speed, you'd need 1)to change final gearing...forget it, I looked into it. 2) add a higher 5th gear (possible, but cost not worth the result). 3) rigid out the rear and go to a 16" rear wheel...I'll pray for yer kidneys. 4) go with just a slightly larger diam rear tire, but you will have less "off the line" performance, and honestly, the result wouldnt be worth the expense IMO....if you mostly ride hiway, then ya...go for a higher profile rear tire....but you'll find out you may bog some from a stoplight (then you'll slip the clutch, and burn that out)...

For a VN to be more fuel efficient ?...go with an FI (fuel injection) system instead of the carbs, and update the antiquated ignition system for a guaranrteed good spark at all RPM ranges.

Ive gotten as much as 59 mpg with my 85, stripped and modded properly....
..a larger diam rear tire might raise me to 64 tops.

Ok, so bottom line here is that the RPM's are essentially the same; when I noticed my RPM's higher than they were with the old tires, it really meant that I was actually rolling faster than the speedo was reading, correct?

Thank you

Yeh, you're right. Thought about that after I had left for work. Chaulk that one up to a brainfart. One other thing to concider on the width though is the wider the tire on the same width rim it will lower the diamter of the tire once aired up due to mushrooming the tire sidewall, not alowing it to stand up as intended. I've seen car tires at popped off the rim in a curve cause of the tire wider than the rim.

If your RPMs were higher at the same indicated speed, the only thing that could cause that would have been a smaller OD rear tire. BUT....since you changed out the front too, the comparison wouldnt be visible.

Do yerself a favour...get clocked by a cop at 3 different speeds.

Your larger front, will show you going slower at same RPM....larger rear, should decrease RPM at same speed....shouldda done one at a time, lol....rear first...that'd tell ya the actual RPM drop at same speed. Then a larger front would correct the speedo error.

Sorry...I did a lot of acid back in the day....sometimes a lil tough to git the gist of what Im sayin.

LOL - we are both saying the same thing, bro, just backward from each other. Maybe the pot got to me, too LOL YES, the same RPMs are now showing a slower speed. I was just curious if anyone knew exactly what the difference was. I'll have to get clocked, hopefully not in a bar.

This will have to do for now. Thanks, Chuck!

According to Cycle World's road test they did on a new VN700...30 indicated was 29 actual....and 60 indicated was 55 actual. I suppose a math whiz could figure out the exponetial differences and apply a percent formula. But just a guess, 45 indicated would then show 42 actual, and inversely, 90 indicated would show 82 or 3 actual. I dunno, I did better in chemistry, lol.

So, it is close to what an old post which "Diane" wrote....approx. 10 % over.

WilliamTech

Lil closer to 8.5-9% though methinks...too tired to figure it out tonight, lol.