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Quote:
Originally Posted by Chuck A.
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The speedometer is read off the front tire so the rear doesnt change it at all, just the engine rpm's per mph. On the tire size, the first number is the wideth is millimeters, second is the tire height is mm's, third is the wheel size in inches. The second number on the rear went lower which means you put a smaller diameter tire on, hence raising the rmp of the rear tire as the front stays the same. The wider the tire the lower the height is regardless if the wheel wideth stay the same. But as for the how far off is the speedometer, mine is 5 mph off at 55 acording to my GPS.
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.

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)
H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15
TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/11-vn750-general-discussion/9127-top-ten-items-you-would-suggest-new-owner-do-his-new-ride.html
Last edited by OlHossCanada; 05-21-2016 at 05:20 PM. Reason: spelling correction
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Registered User
Quote:
My main issue here in moving to bigger tires is getting lower RPMs at the same speeds (better gas mileage),
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.
Quote:
Originally Posted by Wolfie
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Ive gotten as much as 59 mpg with my 85, stripped and modded properly....
Thank you
Dave
----------------
2005 Vulcan 750
Odometer @ October, 2019: 14,200
Purchased May, 2011 @ 2,172
Splines lubed @ 2,962 (were in good shape)
Küryakyn Daily Tour Bag
SS-28 Slipstreamer Smoke
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Quote:
Originally Posted by OlHossCanada
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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.
Second number is sidewall height expresed as the percentage of tread width.
Registered User
Quote:
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?
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.

Quote:
Originally Posted by Wolfie
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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.
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.



Dave
----------------
2005 Vulcan 750
Odometer @ October, 2019: 14,200
Purchased May, 2011 @ 2,172
Splines lubed @ 2,962 (were in good shape)
Küryakyn Daily Tour Bag
SS-28 Slipstreamer Smoke
Quote:
Originally Posted by Chuck A.
View Post
..... But as for the how far off is the speedometer, mine is 5 mph off at 55 according to my GPS.
Dave
----------------
2005 Vulcan 750
Odometer @ October, 2019: 14,200
Purchased May, 2011 @ 2,172
Splines lubed @ 2,962 (were in good shape)
Küryakyn Daily Tour Bag
SS-28 Slipstreamer Smoke
Registered User
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.
Quote:
Originally Posted by Wolfie
View Post
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.
WilliamTech


-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!
Last edited by WilliamTech; 12-09-2012 at 09:27 PM.
Registered User
Lil closer to 8.5-9% though methinks...too tired to figure it out tonight, lol.
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