I got a 2009 RX last month, I noticed what my old car, 1999 4 Runner, tracked as 14 miles on the odometer, the RX says is 16. Is it possible that the speedometer/odometer are off? it feels like when I am doing what the RX says is 65, is more like 60. Has anyone had or heard of these issues? I will try to find a freeway with mileage markers to do a test. I was wondering about this, if they cranked it to shorten the warranty.

 

Get or borrow a GPS to use as a comparison value. Also, confirm the information on Google Maps or MapQuest regarding the correct value.

Odometer will never be exact as it varies even as your tires wear.

 

What size tires you running? Maybe your other vehicle was off.

 

In a prior thread the OP mentioned he had Hankook 235/55-18 installed by the dealer at time of purchase.

 

yes, Hancook 235/55-18 current tire. The 4Runner I had for 13 years, do not think it was off. The Lexus has a Nav system in it, should i trust it when it says how many miles when i plug in the address i'm going to? is that a stand alone system, not tied into the odometer/speedometer?

 

Good idea, i will see what Google maps says for mileage, and compare.

 

My RX300 along with others reads +2 mph than the actual speed I am traveling. BMW's have a bigger margin of error. Auto manufactures do it on purpose, not exactly sure as to why.

 

Not sure if it applies to cars, but I know on motorcycles, all speedos are very optimistic.

Most of the cars I've had including several Lexus, Acura and BMW have been very accurate. They are all electronic these days (even on motorcycles).

I was told on a bike forum that its to reduce liability, to seem you are going faster than you really are, but that seemed a bit of stretch!

Machog

 

One reason has to do with optional tire sizes. The outer diameter difference between a set of 17" vs. 18" on the RX could be 0.5" or more.

Another reason would be due to the tire diameter difference over the life of the tire. As the tire wears it covers less ground with each revolution.

The whole system is a compromise. Each manufacturer just compromises in a different way and by different amounts. I also believe there would be (or would have been) a number of lawsuits should the speedometers read slower than actual speed. First time someone got a ticket for going 38MPH in a 35MPH zone they would blame it on the car manufacturer if they could. So, they error on the side of saying your are going faster than actual.

 

Interesting replies on the speedometer. My scenario is the reverse of the ticket scenario mentioned. If my speedometer is reading faster than what I really am going, when speedometer says I'm going 35, I am actually going 30 and would not get a ticket. So there would be no lawsuits over slower set speedometer. But faster set, yes. I think I'll have my friend drive side by side with me in her trusty 13 year old Ford Mustang and have us both do the same speed per our speedometers and see if there is a difference.

 

isn't this something that is checked during the yearly inspection at the DMV? on that roller thing?

 

what yearly inspection at the DMV? I've never gone to the DMV for a yearly inspection. You mean the smog test? That isn't yearly, and I don't know if they are verifying speedometer. that checks the emission system.

 

well... not in any real sense... the rolling diameter of the tire is a function of strut clearence... a typical tire for an 18" wheel is roughly 27 in in diameter for a circumfrence of roughly 84.75 inches. if you wore it out to cord that would make the diameter no less than 26.5 which gives a circumfrence 83.2 thats or about 1.5" of circufrence difference... or about a 1.5 % difference
awith a 1.5 % difference , if you have an indicated 65 mph on new tires , you have a true speed of 64.03 with tires that are worn to cord.

if you put the tallest tires on the car you could possibly fit without strut or fender interference those MIGHT be 28" rolling diameter which is about a 2.5 % difference which means an indicated 65 would actually be 66. 2 mph or 2.5 miles LESS on the odometer per 100 miles traveled than you actually went.

on cars where you can actually fit MUCH taller tires, then yes... you can drive the speedometer error into an area where it might be an issue but on these cars where the strut clearence is really the defining element, you can't make any meaningful difference.

if your speedometer or odometer is wrong it can be a lot of things but the tires aren't going to be the cause.

 

stevesxm, I think you missed the points I was trying to make for the question I was trying to answer. You make some of the same points as myself yet discard some of your own calculations to say it doesn't make a difference. I say they do and it adds up. I can further the points I was trying to make without even having to go outside the two OEM tire size choices for the RX.

Take a look at the Yokohama Prada-X in the OEM sizes for 17" (225/65R17) and 18" (235/55R18) tires. Listed as having a half inch diameter difference when new and a difference of twelve revolutions per mile.

http://www.tirerack.com/tires/tires....wtpackage=true

This does not take into consideration tire flex due to the sidewall construction. Will throw that out for now. FWIW, the revolutions per mile numbers above for the Prada take into consideration sidewall flex and TireRack notes this is not a calculated number:

http://www.tirerack.com/tires/tirete...jsp?techid=203

Thread depth for each size is 11/32". If you run them to 3/32" you would lose 8/32" off each end of the diameter for a total of 16/32" or 0.5". For the 17" tire, the total diameter would drop from 28.6" to say 28.1". For the 18" tire the diameter would drop from 28.1" to say 27.6" From max value to min value for the OEM tire sizes the tire diameter needing to be considered might vary from 28.6" to 27.6". One full inch of the diameter. A new 17" tire would need approx. 705.2 revs/mile while a worn 18" tire would need 730.7 revs/mile. For argument sake the worn 17" tire would need 717.7 revs/mile. Gives us values of 1.8% difference over the life of a 17" tire, the same for the difference between new 17" and 18" tires, and a potential 3.5% difference between the min and max values on OEM tire sizes over their lifetime.

So, what am I getting at...

3.5% is quite a bit to compensate for here. Your vehicle does not know if you have new 17" tires installed or used 18" tires. This is one point I was making. The base and +1 tire sizes are just not close enough in diameter. This possible 2+ MPH difference (at 65MPH and nearly 3MPH difference at 80MPH in TX) cannot be discarded. The manufacturer has to take this into consideration during the design process making the speedometer and odometer to be compromises at best and brings a particular bias to the equation depending on the company. While they may approximate very closely at one extreme, they will be off on the other. As they do not want to understate your speed for legal reasons (it would be OK to overstate your speed) that also factors in as well.

Depending on the choices Lexus/Toyota decided to make with the RX regarding speedometer and odometer accuracy -- when, where and how much -- plus which tire you select and its size will affect the overall accuracy of these systems. And, during the life of your tires you can estimate on 1MPH off from beginning to the end just due to tire wear. All these affect distance calculations. They affect MPG calculations. Will your GPS exactly match the distance traveled according to your odometer? Over a long distance, probably not.

In this example I decided to throw out any affect of sidewall flex. I also did not take into consideration any information from other tires other than the Prada. They just happen to have listed the specs for the two sizes needed. Selecting different tires from different manufacturers may exacerbate this problem or it may bring the numbers closer together.

 

I guess the items of the car inspection is state / country dependent. I just had mine done here in HK and they tested both speedometer accuracy and also braking effectiveness.

lets just say i was thoroughly happy that my car is definitely road worthy....except i almost failed cause i didn't use a certified license plate vendor. =D