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post #1 of 22 (permalink) Old 03-22-2011, 06:18 PM Thread Starter
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Speedo Variation

Hey guys. just got a reading of 55+ MPG over a course of 145 miles! besides that, how much do the speedos vary from bike to bike? My dad's '86 at 4k rpm, is ~60 mph. my 2001 at 4k rpm is 55mph. how much will new tires vs. worn tires effect it? My bike has a NEW front and rear. my dads has a NEW rear, worn front. what are some of your speeds at various rpms. I really wanna check mine with a gps.

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2001 Vn750
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spline lube
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otherwise all stock
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post #2 of 22 (permalink) Old 03-22-2011, 07:09 PM
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Speedometers read about 10% high. Odometers are fairly accurate.

Size,type and wear of the FRONT tire can make a difference on speedometer and odometer readings. Rear tire would not make a difference other than speed to rpm figures.... and not by much.

You can't really take mpg figures as gospel if your calculation only deals with one tank of gas. You need to find an average over several fill ups.


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post #3 of 22 (permalink) Old 03-22-2011, 08:25 PM
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Quote:
Originally Posted by 1986vn750 View Post
Hey guys. just got a reading of 55+ MPG over a course of 145 miles! besides that, how much do the speedos vary from bike to bike? My dad's '86 at 4k rpm, is ~60 mph. my 2001 at 4k rpm is 55mph. how much will new tires vs. worn tires effect it? My bike has a NEW front and rear. my dads has a NEW rear, worn front. what are some of your speeds at various rpms. I really wanna check mine with a gps.
According to my calculations both the 150/90-15 and the 170/80-15 rear tires have nearly identical diameters and circumferences.
At 4k rpm both should give a true speed of ~58mph.

The vn750 speedo has been reported by many riders here to read about 10% high with the stock 100/90-19 front tire.
The odometer is fairly accurate however.

Changing out to the 110/90-19 nearly corrects the speedometer, but the odometer will now read a bit low.

The ~10% difference in speedo readings between your father`s bike and your own could be explained by these different sized front tires. The worn front tire on dad`s bike will cause a few more revolutions per mile and may add an extra mph or so to the reading as well.

There is also the possible difficulty of making very accurate readings on two gauges at speed while riding. Other than these two scenarios, I`m not sure how else to account for the different readings, as all previous reports that I recall have verified the approximate 58-60 mph at 4k rpm.

Both readings seem to fall within a reasonable margin of error, given the conditions under which they were made. <shrug>

True calculated speed using gear ratios for the vn750 is 14.52 mph per 1k rpm in 5th gear, but will vary a little with tread wear.
Simple multiplcation calculations will give the speed at any rpm.

Gordon

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Purchased May 16, 2008
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post #4 of 22 (permalink) Old 03-22-2011, 08:43 PM Thread Starter
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My speedo reads 55 at 4k. If it is reading 10% high. 10% of 55 is 5.5 mph. so if it reads 55mph, really its about 50mph? that cant be right, because everyone says about 4k is 58-60 mph. I really need to just test everything with a GPS. and see. is there anything that can cause a tach to read wrong?

1986 Vn750
18,800 miles, and counting
Bone Stock
New cam chain tentioners
all fluids changed
Spline Lube
windshield
Engine Guard
B-stone S11 RWL-stock sizes
*wanted*
New grips

2001 Vn750
13,100 miles, and counting
spline lube
Bridgestone S11 100/90
Shinko 170/80
rear end oil change
aftermarket mirrors
windshield
otherwise all stock
*anticipated mods*
Engine Guard
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post #5 of 22 (permalink) Old 03-22-2011, 08:50 PM
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55 mpg seems really high, I usually get about 42, but I stay on my throttle too...

Last edited by km60155; 03-22-2011 at 09:07 PM.
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post #6 of 22 (permalink) Old 03-22-2011, 09:18 PM
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I've kind of resolved to the idea that no two speedometers read the same these days, especially from brand to brand, and even two bikes like yours and your dad's from different years. You'll only know for sure once you ride with a GPS, and I highly recommend it so you'll know how to read your speed off your speedo. My 750 was about 10% high like everyone has said, til I put the 110 on the front, then it ran about 3% high.
Strangely, my 1500 is really close to dead on with a GPS and I'm running the stock tire sizes. A buddy's Shadow 750 reads more than 10% high which we verified with a GPS on a road trip, so it is best to just check it yourself.

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post #7 of 22 (permalink) Old 03-22-2011, 09:19 PM
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Quote:
Originally Posted by 1986vn750 View Post
My speedo reads 55 at 4k. If it is reading 10% high. 10% of 55 is 5.5 mph. so if it reads 55mph, really its about 50mph? that cant be right, because everyone says about 4k is 58-60 mph. I really need to just test everything with a GPS. and see. is there anything that can cause a tach to read wrong?
Is your front tire the stock 100/90-19 or oversize 110/90-19?

The tach reads off the front cylinder coil or plug wire.
Not sure if the tach sensor or wire could read only some of the signals or not.

Gordon

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/showthread.php?t=9127
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post #8 of 22 (permalink) Old 03-22-2011, 09:57 PM
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55 is completely doable. I usually don't hit that anymore because too much of my current commute is at 60-65, which these bikes really don't like, but 45 mph backroads for a tank can be expected to yield deep into the 50's to low 60's.
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post #9 of 22 (permalink) Old 03-22-2011, 10:23 PM
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I think what this really boils down to is......what does it fkn matter?

If your speedometer says you are going 90, when the cop pulls you over
And writes your ticket for going 82....you just saved yourself some green.

And most of the other times you might not even get pulled over, because in fact you aren't even speeding. They are just numbers and don't mean much
unless you are breaking the law.......and you should already know if you are doing this without looking at your gauges.

My advice is just to ride the bike and enjoy yourself, and be glad the speedometer is off in the direction that is to your advantage.



KM


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post #10 of 22 (permalink) Old 03-22-2011, 10:50 PM Thread Starter
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Quote:
Originally Posted by km60155 View Post
55 mpg seems really high, I usually get about 42, but I stay on my throttle too...

This was for 145 miles. about 90% of those were about 60mph on level highway.

1986 Vn750
18,800 miles, and counting
Bone Stock
New cam chain tentioners
all fluids changed
Spline Lube
windshield
Engine Guard
B-stone S11 RWL-stock sizes
*wanted*
New grips

2001 Vn750
13,100 miles, and counting
spline lube
Bridgestone S11 100/90
Shinko 170/80
rear end oil change
aftermarket mirrors
windshield
otherwise all stock
*anticipated mods*
Engine Guard
1986vn750 is offline  
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