this week on the side of the road there is a radar thing to show your speed and I notice my speedo says about 5mph faster than the radar. Anyone else? BTW I have stock sized rims and tires.

 

my spedo is dead on checked via hand held GPS up to 154 mph

 

are you positive your tires are stock size? the stock speedo is usually fairly accurate but if someone has tinkered with it it could be off. something to think about too is those speedo markers need to be calbirated also so if the agency controlling it hasn't bothered to service it the machine could be off not your car.

 

Would lighter rims through the speedo off? I have an S5 GXL and plan to buy the convertible BBS rims which I hear are pretty light, would that throw off a speedo?

 

Don't think it would,

 

Um, no, there is nothing in physics that would cause the speedo gear to change based on the weight it's carrying.....

A speedo works by knowing how many turns the tailshaft of the transmission turns, that gear is sized to take into account the ratio of the differential and the the stock tire size...

Technically you can use any size rim on the car so long as the outside tire diameter doesn't change.

My speedometer gets exceedingly off after I pass about 90mph, we're talking about 5% after 100mph, and that's with the stock wheels and stock sized tires (205/60/15)

 

When compared to GPS mine is about 10% off all the way.
So at speeds less than 55mph it's not a big deal, but the faster I go the more optimistic it gets.

 

Is it a convertible? Reason I asked is that someone could have put a 4.1 rear in the car.

 

Mine is off too and gets worst after about 60MPH. At 80 MPH it is off by about 5 MPH so I'm really only doing 75 MPH and it seems to get worst the faster I go. Stock tires stock rims, bought the car brand new, but I now wonder if my mileage is correct or it shows more mileage then I really have.

 

ha i gave up on my speedo over 85.... i know in 5th gear over 6500 it hadnt cracked 140....mine jumps around a li bit. i wish i had gps running all the time.

 

In general, speedometers on almost all stock cars (from all manufactures, not just Mazda) tend to read a little high. It's a percentage offset so it gets more off the faster you go (eg. off by 2mph at 30 is off by 4mph at 60). Different tire brands with the same numerical size may also have slightly different diameters, which will impact the readings. Finally, under- or over-inflated tires can impact the readings significantly.

-bill

 

Is it possible that the speedo cable is too far into the tranny case. Cuz ever since i put in my new speedo cable i hear a CLUNK sound when im taking off in first gear or reverse. It makes me think that my speedo is so far in there that the spinning thing inside that acts a passes the speedo cable for the speed signal is rubbing. Or am i just imaganine these things. Sometimes i feel like my car is going faster than what the speedo says, is that just cuase its a sexy rex and not a honda


Jason NYC