Cruise Control/speedometer Issue

[MBC2008]

We live in Corpus Christi and have taken our RX330 to Houston twice since purchasing the vehicle. Each time, with the cruise control set at 70 mph, we have been cited for going +10 over the limit. The first time, I thought it might have been that I put my foot on the pedal, or wasn't paying attention, or, yes, we were on a hill and it surged suddenly to get up to 70. The second time, however, we had just come over a hill and started down. The cruise was set, and the speedometer said 70. Ideas? or should we take it to the dealer? Needless to say, it's confined to city driving until it's diagnosed. Thanks for any help !!

[PK_3]

Don't they still have mile markers in your neck of the woods? Just count how many you pass in 20 or 30 minuets and do the math.

PK

[dgorrie]

I have a '99 RX300 acquired used, and over the years have determined that the speedometer will typically read up to three mph FASTER than I am actually going. That also translates to issues with the on-board display that shows miles/gallon, making it APPEAR that I am getting about 3 mpg more than I am actually getting. Aggravating.

In your case, determining actual speed is obviously important at this stage. Mile markers may be useful if you can find them, but I don't always always find them available. One option that might be worth considering is the Escort 9500ix radar detector. I played with one a few months ago on Escort's 30-day trial program, and noted that it will display your actual speed, either all the time or just when the detector goes off (based on settings). You could always get one under the trial program, see what your speed actually is, then decide how to proceed. But be forewarned -- even though the 9500ix is expensive, it can be addicting (it works great).

Another option might be to ask your local police department if they have any of the units around town that show the speed of passing cars. These units aren't very bright, so it's hard to tell if the displayed speed is yours or someone elses. But if you can pass by one when traffic is light, you should be able to get a fairly accurate reading. In-town measurements may not be what you are looking for if the problem is a highway issue.

Another option - have some friends clock you in their car. True, their speedometer could be off, but similar readings from 2-3 friends should make the point pretty well.

Good luck. and if the dealership or a reliable source can show that the your speedometer is actually busted by 10 mph, maybe you can get out of a ticket. B)

[smooth1]

Maybe the speed limit is 60 mph there!? :P

Did you change the size of your tires?

[pauljcl]

Maybe the speed limit is 60 mph there!? :P

Did you change the size of your tires?

Yes, Smooth1, seems to me that it is an issue of speedometer reading rather than cruise control. I am assuming that, after setting the cruise control at a certain speed, it would be evident if the speed increased - the speedometer would read higher. If it doesn't, the original reading was 'wrong'.

[mikey00]

Maybe the speed limit is 60 mph there!? :P

Did you change the size of your tires?

Yes, Smooth1, seems to me that it is an issue of speedometer reading rather than cruise control. I am assuming that, after setting the cruise control at a certain speed, it would be evident if the speed increased - the speedometer would read higher. If it doesn't, the original reading was 'wrong'.

If you changed your tire size, use one of the online tire size calculators to determine how much you may have knocked your speedometer off. Here's one:

http://www.miata.net/garage/tirecalc.html

Also if you have a portable GPS compare your speed using the speedometer which is built into it. If you don't have one borrow one. It seems like eveyone has one these days.

[cduluk]

Is the air pressure low in your tires? That would result in your speedometer thinking you're not going as fast as you really are (thinks you're covering less distance/time than you really are).

If your tires were just a little bit low, you'd lose ~1 inch in the radius of your tire. One rotation of a fully inflated tires yields ~69.08 inches at 11" radius. At 10" radius one rotation yields only 62.8 inches. Multiply that a few times, and that could account for 10mph difference at high speads...

[pauljcl]

Is the air pressure low in your tires? That would result in your speedometer thinking you're not going as fast as you really are (thinks you're covering less distance/time than you really are).

If your tires were just a little bit low, you'd lose ~1 inch in the radius of your tire. One rotation of a fully inflated tires yields ~69.08 inches at 11" radius. At 10" radius one rotation yields only 62.8 inches. Multiply that a few times, and that could account for 10mph difference at high speads...

Actually, I believe it works the other way. The speedometer - indirectly - measure sthe number of whell rotations in a specific time. The smaller the wheel, the more the speedometer will overstate the actual speed, and vice-versa, the bigger the wheel, the more the speedometer will underestimate speed, which seems to be the problem here.

If the cruise control is set at a specific indicated speed, and after being set the speed creeps up (easy to see by the speedometer), then the problem may be cruise-control linked. If the indicated speed remains constant, the problem may be the speed indication given by the speedometer: either it misreads, or the wheels are larger than it believes (i.e. larger than standard).