jer

I am just wondering if anyone has had an issue with regards to their speedometer vs. actual MPH. I have a Garmin that I have used in my 04 RX and it shows MPH along with the route's speed limit, and if I go over it goes to red. The MPH on the Garmin and the 04 RX match up perfectly. When I switch it to my 14 RX (19 inch tires) when my speedometer is at 50MPH, the Garmin is only showing 48. It seems to be a couple-3 miles off. Not sure if the Garmin/04RX is off or the new one is off. Any thoughts?

RX330inFL

Not unusual for a speedometer to read a bit slow these days. Almost expected. Also, as your tires wear your vehicle will be going even slower yet when compared to what shows on the speedometer.

JDR76

Both my IS and my RX read 2 mph slow. I think it's just a Lexus thing...

RX330inFL

Not just Lexus. Have driven many vehicles in my lifetime. Few if any were perfectly on and as your tires wear they go off even more. Some German vehicles were off as much as 4 MPH. The best you can hope for is when the tires are brand new.

They will never show you going slower than actual. If that were the case as soon as one person got a ticket there would be a lawsuit.

corradoMR2

Refer to this thread on some additional findings.

RX330inFL

and this...

http://www.caranddriver.com/features...ometer-scandal

PeteInGilr

My '14 RX350 doesn't have HUD, but in my past experience ('13 Avalon Limited) I found the digital display perfectly accurate and the speedometer to be about 2 mph slower than my actual speed.

This was verified by Garmin, portable trailer mounted radar speed displays on city streets, and permanently mounted freeway radar speed displays.

jer

Thanks all for the replies. Something I won't worry about. Just thought it strange the 04 is spot on w/the Garmin & the brand new 14 is a bit off.

raylor4

The way I see it, tire wear will cause higher rpm's for a given speed, therefore a slightly higher reading on the speedo. I think tire wear would affect the speedo very slightly.

Ray A.

websailor

So this means all this time I've thought I've been speeding, maybe I haven't been (as much). I've probably been saved from a few tickets by that!

RX330inFL

Perhaps you would believe Click and Clack...

http://www.cartalk.com/content/how-m...ometer-reading

raylor4

Hi RX330,
Thanks for the link. I'm a big fan of Click and Clack. They concur with my statement that as tires get smaller, speedo readings go up. Loss of ½" of tread depth seems like a lot, but for the sake of argument, I let it ride, so to speak.

Copied & pasted from Click & Clack:
"TOM: That's probably not enough to change a speeding ticket, Todd. Besides, it goes in the WRONG direction. As your tires shrink in size, the car is actually going slower than the speedometer reads, not faster. So while your speedometer read 80, your ticket only cited you for going 77. But you already knew that."

In other words, the speedometer is reading faster than the car is going.

Back in the day, I'm talking 50's & 60's, the mechanical speedos tended to read high. Maybe this was to keep the manufacturers from being responsible for speeding tickets.
I think every traffic court judge in the country has heard the speeding excuse "I put bigger tires on my car and didn't realize how fast I was going."
Great topic for discussion.
Thanks again for the link.
Ray A.

RX330inFL

Yes, I believe we are saying the same thing in just different ways. In my example, the speedometer would never show you doing 45 MPH when you were really doing 50 MPH. Instead, it would show you going 50 MPH when you were actually only doing 45 MPH.

FWIW, I am of that vintage you speak of above. I wonder how many of the kids today would even know what a manifold vacuum gauge was even for or how to start a vehicle with a manual choke?