triplecore

it is very inaccurate, isnt the rx limited at 120mph because my GPS said 110 before it retarded the throtle

Grumpa72

According to my TomTom GPS, my RX reads 60 when the GPS says we are doing 57. Identical numbers for my one year old BMW 3 series. The funny thing is that my 15 year old Acura Legend shows only about 1.5 mph faster than the GPS. Fwiw, we had a similar discussion on the BMW forum. I could care less as long as it doesn't read high. That is probably not defensible in traffic court.

Orzel

iTrader: (1)

Our Garmin 660 indicates that we are dead on at 60 and 70 MPH, and slightly slow below about 50. Could be because we went up one tire size when getting rid of the OEM Goodyear’s - at least that's the only logical explanation that I can come up with.

triplecore

I think i read that one. Is that the one that said Sports cars speedos are purposely set so it seems like your traveling faster then your are. hence your speedos read 70, but your actually only going 65. I am pretty sure I have read some theory like that somewhere

CTtech

I posted this same issue, as my 08 rx350 is off by about 2 MPH at 50+. It does err on the sid eof caution, that is to say when my speedo says 70 I am actually at 68. It may not seem like a big deal to some but having never seen this in any of the vehicles I have driven in the past, it was a great disapointment to me when I noticed it in my brand new Lexus. I also don't believe that for what we paid for our vehicles and with todays technologies, there is a reasonable excuse for this. It will be one of the things I consider when I purchase our next new car and I will be sure to bring along my wifes Nuvi 660 to test the speedo on any test drives. That said I deal with it. Even though it bugs me, I still love my RX over all and this is only one of the cons on a list of many more pros.
My 2 cents.

jfelbab

If Lexus says they can't adjust the speedo for accuracy and it bothers you a lot why not consider a Plus size tire upgrade the next time you need tires?

You could replace your 235/55/18 OEM'swith a 255/60/18 which would give you about 5 %. Maybe a Kumho Road Venture APT which is well regarded.

If you only wanted 2-3% change you might choose Kumho Solus or Goodyear Eagle F1 in a 255/55/18 size.

These changes on your OEM 18 rims will increase your vehicle height by a 1/2 - 1 inch but should not harm the ride or performance.

Lil4X

~3-4 mph of speedometer optimism is considered acceptable in the auto industry - for the reasons noted. jfelbab is entirely correct, uprating to a slightly larger (diameter) tire will make your speedometer read a bit higher for a given GPS speed, bringing it closer to your actual groundspeed. This can, of course, be overdone.

When we upgraded our RX300 from the factory 225/70-16 Goodyear Integritys to the Michelin CT's at 235/70-16, our speedo accuracy improved considerably. If you look at Tire Rack's data, the smaller GY has a diameter of 28.4", while the (wider) CT indicates a slightly larger diameter at 29.1". Of course depending on the year model of the tire and inflation, your actual results may vary, but if you want to improve your speedometer accuracy, consider going to a "larger" tire.

howdybob22

Did not even think about the speedo accuracy until I saw this topic. I noticed that on my IS350 when I drive 39-40 and the road side radar said 37. I just assumed it attributed to my worn tires. +1 with Jim on the Kumho Venture, as it has been a great set - much better than the OEM Goodyear RSA before it. The bad part is that I did not go with the Plus size, maybe next time - good info.

Audriuss

Censored. Bad attitude. Moderators will. Sorry, my bad.

howdybob22

For what it's worth, it did save my butt a few times. On a few occasions, I got laser pointing at me while driving 70 (as shown on my speedo) on a 60 mph zone. I guess that few (2-3 miles off) was the difference for the cop to not go after me.

Who_Me

I am sure that the other gauges (tach, engine temp, cabin temp, voltage, etc) also have allowable tolerance levels. I never expected a precision gauge on any car I ever owned, so it is no big deal to me. I know I am driving somewhere in the neighborhood of the posted speed limit and that is close enough for me. How does one know if their GPS is accurate? Maybe the speedo is accurate and the GPS is wrong.

Arbitrage1

Wonder if this affects the Odometer?

5% overestimation at 48,000 means Ive actually only traveled 45,714 miles... thats 2300 miles that should be covered by the warranty, that arent.

salimshah

^^ I think one can make a reasonable case and most probably, Lexus does take this factor into account for things that fail just out of warranty.

Salim

AV8R_BOB

True, if the circumference of the tire is increased proportionally to the mph error, that could eliminate the speedo error by making the wheel spin slower at any given speed.

A 2 mph error at 70 is 3%, so the circumference needs to increase by 3% or diameter by 1%. The OEM tire is about 28.5" dia so adding about a quarter inch should do it, whatever size that would be. However, the speedo error would only be "fixed" at the target speed, 70 in this case. At 30 the 2 mph error is 7%, so increasing the circumference 3% will remove only half the error.

AV8R_BOB

Well said, it pretty much reflects my thoughts. After driving cheaper GM vehicles my whole life and being accustomed to the speedometers being dead nuts accurate, it seems inexcusable to me that that the expensive Lexus that's loaded with high tech features can't give me an accurate speed read out.

I'll add that my initial post was off a little. With more miles under our belt and more testing with a portable Lowrance GPS it seems to be off 2 mph at every speed, not 3. It's also interesting that it's 2 at every speed, not a fixed % which would make the mph error greater at greater speeds. That leads me to believe it is an intentional offset they programmed in.

I plan to do some data recording to address the odometer error issue. That is, I want to determine if Lexus is overstating miles driven, robbing us of warranty miles and resale value. Should not be too hard using the portable GPS... zero your odometer/trip meter and GPS trip data and drive a stretch down the freeway and compare the odometer with what the GPS says. Or with the mile markers on the freeway. Maybe some of you can do the same and post your results.