different tire size... different speed reading?

hey guys, so i currently have a '94 VFR750 and the rear tire size was a 180/55zr17 (73w) and while riding i noticed my speedometer was off by +5mph. (says i would be doing 35, was actually doing 30)

so unfortunately someone at my shitty fucking school slashed my rear tire and i had no idea untill halfway home (tried slashing it but only knicked it by a little for a slow leak)

anyways. the stock tire asks for 170/55zr17, should i get another 180/55zr17 or get the 170? would that fix the speedometer problem i was having?

Thanks guys
-Duncan

 

DwV,

Hey welcome to the forum!

Really sorry to hear about the immoral jerks. Glad you are safe after that ride as well.

Check this out:

https://tiresize.com/speedometer-calibration/

If I did it right, it is an extremely small change between these two sizes. One other factor is that the physical tire can vary from its rated size, but in this case I think you are good. I think the majority of that error is built in from the factory. Sprocket changes can exacerbate the error as well.

 

hey thanks dude!

yea it was absolutely terrifying, i was about to take this hard turn before this guy was freaking out at me telling me to pull over and honking his horn, i honestly had no idea my tire was flat because i was going straight for 20 minutes haha.

thanks for the answer, i just wasnt sure if the tire size could affect things like that.

 

Don't fret too much, the speed O error is a Honda trade mark. So one needs to not let it bug em. Yes you can fix it with a speedo healer, but heard it fouls up the accurate odometer. I have just had to get use to it. The tire size really doesn't effect the speed reading, now change your sprocket ratio, then you'll have something to talk about. As far as running the 180 vs. the recommenced hard to find 170, there's not enough difference to worry about it.

 

What was your reference for the 5mph speedo error? A GPS?

 

Seems the 170/55 tire size is gone the way of the dodo, 170/60 appears to be the new norm but it doesn't seem to be a choice in some of the more popular make/model tires. I.e. Dunlop Q3, Michelin Pilot Road series, Bridgestone Battleaxe, for example.

My '95 Ducati came with a 170 width tire but I couldn't find a 170 in the tire model tire I was interested in so I went to a 180, however, my Ducatis rear wheel is actually a 5.5" width not 5" like the '95 750 you have. So the 180 tire is more at home on this width wheel anyway.

Personally I could not tell a difference and most likely you might not either but there was a member who was in the same predicament, couldn't find the 170/55, so he went with a 180/55. At first he didn't like it but after changing the air pressure and riding with it for a while, he came to find it worked ok for him.

As far as speedo accuracy, I rather like that father Honda is off by a few MPH, especially if the speed is faster on the meter than actual speed on the road. I don't know why so many riders make a big deal of it being off, as long as you know what the difference is, what's the big deal?

 

I'm moving my rectifier under seat with a heat sink. Is this best place 2 do this. My friend his got up 2 129 degrees on side melted seliniod. and not over 79 degrees. seat with heat sink. Even put vfr 5 pin rectifier on his 750. Works great

Sent from my SAMSUNG-SM-N900A using Tapatalk

 

The best place is where it will get airflow. Not sure what kind of airflow gets under the seat but I would guess it's not as good as other places on the bike. On the 5 Gen, it's under the left side rear fairing where the air scoop is. On the 6 Gen it's on the front of the frame on the right side of the bike in the small open area between the frame and fairings. Both places seem to have decent air flow. If you're really worried about it, I've seen a few people put a computer CPU cooling fan on their heat sink, others have used the computer case fans. Either of these would be ok as they use 12v anyway.

 

I think most Speedos over read, my works van definitely does.We've got trackers on them that record speeding so when I shoot through a speed restricted area at just over the limit I get away with it because I'm going a few mph slower than it reads.Good job really.