Speedometer Off?

[seohenning]

I was recently driving a loaner 2009 IS250 AWD without the Navigation and was using a handheld Navigational unit which has a speed function. I noticed that above 70 MPH the cars speedometer seemed to be off from what my Handheld Navigation was indicating. I know the cars speedometer does not have exact numbers on it but it was a noticeable difference.

For Example here is what I found between the cars reading and the Navi's:

Car - Navi

70 - 70

72 - 71

74 - 72

76 - 73

78 - 74

80 - 74/75

80+ I didn't test any further as I have a company car and speeding tickets = no more company car.

I also double checked this with my 06 IS250 AWD and virtually the same issue happened. I tend to believe the Navigation is correct?

Anyone else notice this?

Note: I do have snow tires on, but they are the same size as the standard tire and I will follow up when I get my summer's on in a month.

[pauljcl]

I was recently driving a loaner 2009 IS250 AWD without the Navigation and was using a handheld Navigational unit which has a speed function. I noticed that above 70 MPH the cars speedometer seemed to be off from what my Handheld Navigation was indicating. I know the cars speedometer does not have exact numbers on it but it was a noticeable difference.

For Example here is what I found between the cars reading and the Navi's:

Car - Navi

70 - 70

72 - 71

74 - 72

76 - 73

78 - 74

80 - 74/75

80+ I didn't test any further as I have a company car and speeding tickets = no more company car.

I also double checked this with my 06 IS250 AWD and virtually the same issue happened. I tend to believe the Navigation is correct?

Anyone else notice this?

Note: I do have snow tires on, but they are the same size as the standard tire and I will follow up when I get my summer's on in a month.

Most speedometers are 'optimistic'. A previous post stated that someone - who had an RX - had a very optimistic (+6.5%) speedometer, checked with Lexus, and was told acceptable tolerances were 0 to +7%. I have checked my own new RX350 with the GPS, and I seem to be about 2-3 mph 'fast' at 70 mph, which is within the previously mentioned tolerance. -- The numbers you mention astonish me a little: 1. That you are 'accurate' at 70 mph (maybe because of the snow tires?) 2. That the 'deterioration' in accuracy is so quick: from 0% at 70 mph to +/- 6.5% at 80 mph.

[CrunchySkippy]

I would tend to agree that the GPS is probably more accurate.

But it should be noted that GPS' tend to have a slight lag/delay when changing speeds. Did you set your cruise control at each interval while travelling on a flat stretch of road (and also waited a few seconds before taking the reading)?

I've also heard that altitude changes affect short term accuracy of the GPS as well...but I'm not sure of the definition of "short", though. I'm guessing its more like split-seconds.

[seohenning]

Yes I agree with what you are saying and I am sure they can't be 100% accurate, but that is also the reason why I made the post. I couldn’t figure out why that range was particularly off in comparison to others (granted I haven't checked every range). This range is my major concern as a 1 or 2 mph difference makes a big difference in a ticket, points and fine.

As for the driving circumstances, I was traveling from NYC to Albany, NY. I don't think altitude has much to do with it based on location and that is about as straight and flat as you can get in NY. I did have the cruise set at "74" which when I turned on the NAV I noticed it said "72" and that started everything. I also allowed ample time for the NAV to adjust.

I guess I'll keep a calc. handy and in the event I get pulled over I can build in that 6.5% "tolerance" factor into my argument. "77? I swear my speedometer only said 72!" Somehow I don't see that working.

[bartkat]

You can always go on an Interstate and time your car at a set speed for say 10 miles of mile markers. That's pretty accurate.