Speedometer accuracy
Road Master
Joined: Dec 2009
Posts: 1,223
Likes: 6
From: Las Vegas, NV
I've wondered about this......here's my understanding for cages and bikes....
Changing tire sizes will alter your speedo if your speedo is calculated from the wheels.....
.......but if it is calculated from the transmission (I've heard some vehicles do this, I had a '76 Vette that had this set up IIRC) it doesn't matter the wheel size...
Anybody else heard of this?
Changing tire sizes will alter your speedo if your speedo is calculated from the wheels.....
.......but if it is calculated from the transmission (I've heard some vehicles do this, I had a '76 Vette that had this set up IIRC) it doesn't matter the wheel size...
Anybody else heard of this?
Elite HDF Member
Joined: Mar 2008
Posts: 3,906
Likes: 145
From: SE Louisiana
I've wondered about this......here's my understanding for cages and bikes....
Changing tire sizes will alter your speedo if your speedo is calculated from the wheels.....
.......but if it is calculated from the transmission (I've heard some vehicles do this, I had a '76 Vette that had this set up IIRC) it doesn't matter the wheel size...
Anybody else heard of this?
Changing tire sizes will alter your speedo if your speedo is calculated from the wheels.....
.......but if it is calculated from the transmission (I've heard some vehicles do this, I had a '76 Vette that had this set up IIRC) it doesn't matter the wheel size...
Anybody else heard of this?
Banned
Joined: Jul 2009
Posts: 3,719
Likes: 1
I think maybe ashmarc may have gotten that backwards. On the older cars with speedo gears at the tranny, we would have to change out the gear at the tranny to compensate for the change in tire size. Revolutions at the wheel hub would remain the same no matter the size of the tire, I believe? Kinda confuses me at that point. Larger wheels move more ground at the contact point, and the reverse would be true for smaller wheels, same revolutions at the hub...
Sorta seems to me that wheel size would affect either?
Sorta seems to me that wheel size would affect either?
Last edited by CroK; Feb 21, 2010 at 05:04 AM.
Elite HDF Member
Joined: Nov 2009
Posts: 3,886
Likes: 3
I think maybe ashmarc may have gotten that backwards. On the older cars with speedo gears at the tranny, we would have to change out the gear at the tranny to compensate for the change in tire size. Revolutions at the wheel hub would remain the same no matter the size of the tire, I believe? Kinda confuses me at that point. Larger wheels move more ground at the contact point, and the reverse would be true for smaller wheels, same revolutions at the hub...
Sorta seems to me that wheel size would affect either?
Sorta seems to me that wheel size would affect either?
Of course none of this really means anything. It's a rare day when I am running less than 5 mph over the posted limit, and I've blown by hundreds of cops with radar over the past 40 years, and never been ticketed for that kind of minor excess velocity....when I've been popped it's been for more like 20 to 30 mph over the limit.
Last edited by krusty1; Feb 21, 2010 at 11:24 AM.
Road Captain
Joined: Jul 2009
Posts: 583
Likes: 2
From: Eaton, OH
Actually wheel diameter does not directly affect the accuracy of a speedometer.
What affects it is the circumference of the tire. If you change wheel size on a car (what they used to call going +1 or +2) and change the tire to a lower profile (going from a 70 to a 60 or a 45 to a 35), the circumference could stay the same.
If the circumference of the tire changes (the distance around the tire or the distance it rolls to make one complete revolution) then the accuracy of the speedometer will change.
I've compared my car and bike to the roadside speed checks and they are both fairly close. One thing to keep in mind is the speed you see on the display may or may not be the speed of your vehicle. It could be displaying the speed of another vehicle. Only way to be sure it's yours is if you're the only one on the road.
What affects it is the circumference of the tire. If you change wheel size on a car (what they used to call going +1 or +2) and change the tire to a lower profile (going from a 70 to a 60 or a 45 to a 35), the circumference could stay the same.
If the circumference of the tire changes (the distance around the tire or the distance it rolls to make one complete revolution) then the accuracy of the speedometer will change.
I've compared my car and bike to the roadside speed checks and they are both fairly close. One thing to keep in mind is the speed you see on the display may or may not be the speed of your vehicle. It could be displaying the speed of another vehicle. Only way to be sure it's yours is if you're the only one on the road.
Road Master
Joined: Dec 2009
Posts: 1,223
Likes: 6
From: Las Vegas, NV
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Ultimate HDF Member
Joined: Sep 2005
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From: Ozark Country
I've had a Deputy Sheriff I know check a number of our bikes to get a better idea of how accurate the speedometers were on various Harleys.
He used certified radar on some, and 'pacing' with a certified speedometer on others.
On average the Harley's read about 5% slow, which means that a 50 MPH reading was actually a verified speed of around 48 MPH, a 100 MPH reading was an actual speed of 95 MPH, etc..
None of them were 'dead on', and none of them read lower than the actual speed that was clocked on radar.
That's not bad, as most 'metric bikes' checked were off by up to 10%.
He used certified radar on some, and 'pacing' with a certified speedometer on others.
On average the Harley's read about 5% slow, which means that a 50 MPH reading was actually a verified speed of around 48 MPH, a 100 MPH reading was an actual speed of 95 MPH, etc..
None of them were 'dead on', and none of them read lower than the actual speed that was clocked on radar.
That's not bad, as most 'metric bikes' checked were off by up to 10%.
