Speedometer Error
Thread Starter
Lexus Test Driver
Joined: May 2006
Posts: 1,155
Likes: 7
From: TX
Speedometer Error
Okay, this may have been discussed, but I didn't find it.
I have long suspected my speedo is reading higher than my actual speed - partly because I have passed numerous fixed radar speed displays that always show me going slower than my speedo says.
So I finally took my wife's portable Garmin GPS (which displays speed constantly) and went for a drive. When my cruise is locked onto 30mph, the GPS says I'm doing 28. At 50mph on my speedo, the GPS says I'm barely doing 47, and at 60, the GPS says I'm doing only 57.
In short, it appears obvious my speedo is reading 5% faster than my actual speed. That's within specification, and within the requirements of the DOT, I know, but it was still enlightening.
Anybody else checked theirs? Is this pretty much how they're set?
Well, the good part is that I can speed a little and not worry about exceeding the limit.
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I have long suspected my speedo is reading higher than my actual speed - partly because I have passed numerous fixed radar speed displays that always show me going slower than my speedo says.
So I finally took my wife's portable Garmin GPS (which displays speed constantly) and went for a drive. When my cruise is locked onto 30mph, the GPS says I'm doing 28. At 50mph on my speedo, the GPS says I'm barely doing 47, and at 60, the GPS says I'm doing only 57.
In short, it appears obvious my speedo is reading 5% faster than my actual speed. That's within specification, and within the requirements of the DOT, I know, but it was still enlightening.
Anybody else checked theirs? Is this pretty much how they're set?
Well, the good part is that I can speed a little and not worry about exceeding the limit.
.
.
No Substitute
Joined: Nov 2005
Posts: 2,830
Likes: 77
From: RI
Hey there,
FWIW my 2006 2IS was exactly how you describe. In fact I went thru the same process and could have easily posted the same.
Separate but related, when I replaced my tires earlier this year I wanted a little better ride quality so I went up one size from the 45 series to the 50 series tire (added 4/10 of an inch to the sidewall) and now my speedometer imatches my Garmin and the roadside radar!
FWIW my 2006 2IS was exactly how you describe. In fact I went thru the same process and could have easily posted the same.
Separate but related, when I replaced my tires earlier this year I wanted a little better ride quality so I went up one size from the 45 series to the 50 series tire (added 4/10 of an inch to the sidewall) and now my speedometer imatches my Garmin and the roadside radar!
Last edited by TripleL; May 4, 2009 at 10:06 AM.
Pole Position
Joined: Mar 2007
Posts: 2,730
Likes: 26
From: Sakon Nakhon
The 18" OEM tires are "slightly" smaller than the 19" OEM tires. Since Lexus probably uses the same speedo system (pickup, cable & readout) for both tire sizes, and these are interchangeable, I would bet dollars to doughnuts they calibrated the system to be accurate for the larger tires, and let the smaller tires be off a bit. And this is validated by TripleL's experience above. I did some math in a previous thread that correlates this, except my analysis had to do with extra distance as opposed to speed. IOW, your smaller tire's smaller diameter translates into a marginally slower rate of travel, than indicated on your speedometer.
Last edited by Johnny; May 4, 2009 at 07:07 PM.
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Thread Starter
Lexus Test Driver
Joined: May 2006
Posts: 1,155
Likes: 7
From: TX
Yeah, right. But if I were an attorney, I think I'd give it a try just for the fun of it.
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Pole Position
Joined: Mar 2007
Posts: 2,730
Likes: 26
From: Sakon Nakhon
In theory you get better MPG using larger tires. This is because you gain extra distance per wheel turn (dia. x pi), with a very slight offset for the increased inertia to rotate that larger wheel.
Driver School Candidate
Joined: Feb 2009
Posts: 12
Likes: 1
From: Wisconsin
There was actually a class action law suite filed against Acura for this same reason and the defendants won. Acura ended up extending their warranties by 5% longer to compensate.
Thread Starter
Lexus Test Driver
Joined: May 2006
Posts: 1,155
Likes: 7
From: TX
IOW, if a given mfr offers both 18" and 19" wheels on a given model, they will typically either choose a tire size that yields the same overall diameter, or alter the gearing slightly, to compensate for any difference. Therefore, Car X will get the same mileage whether it has the 18's or the optional 19's.
Just to clarify ...
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Pole Position
Joined: Mar 2007
Posts: 2,730
Likes: 26
From: Sakon Nakhon
Ideally, that's what should happen. For the OEM Bridgestone EL42s 18s & 19s, that is not the case, and my previous calculations bore that out. I haven't checked Dunlops, Michelins or others tire sizes. Even if the tires are out a quarter inch, the long term difference is significant in terms of speed and mpg, but not enough for manufacturers to build individually accurate tolerances for each size. Further, users may later opt to change their tire sizes, either increasing or decreasing sizes to say nothing of tire wear. Manufacturers cannot design to these unknowns. So your case evidently falls into the "tolerable" category.
