caymandive

iTrader: (2)

So for the longest time I have suspected the speedometer to be slightly off and I'm wondering if it's common for us 2IS owners to have it read 2-3 mph faster than what the car is actually doing. First time I suspected this was at the 1/4 mile track. Every time I crossed the 1/4 mile line I tended to look down at the speedometer only to find that my time slips were showing slightly slower trap speeds. So today I drove past a few of those radar speed checking signs and I set the cruise for exactly 40 mph. The result was consistent 37-38 mph speeds.

My question is whether or not new cars are purposely showing a slightly quicker speed to error on the side of safety?

9520G

Yes they all show slightly higer speeds for safety reasons.

sykocus

iTrader: (1)

I tested mine against my hand held gps receiver and it was pretty close to dead on at 35mph. Though I have heard for a long time that they manufactures make their speedometers read faster then the car has going. I've also heard that the amount of error increases with speed.

caymandive

iTrader: (2)

I'm curious as to what it would take to re-calibrate our speedometer, especially if we change the wheel/tire sizes from stock. I was planning to switch to a 255/35/18 tire in the rear once my tires wear out, but now I'm rethinking the idea. With the new tires sizes, my speedometer would be reading about 6mph faster than what the car is actually doing. I know Gernby runs this rear tire size..I wonder what thoughts/opinions he has with the difference.

Here is the calculator that I was using. http://www.miata.net/garage/tirecalc.html

jimmyjazz

Exactly the same here. But if I try to drive the speed limit, which is hard because everyone goes 5-10 mph faster, I can usually fine someone going 40 mph.

slowandlow

they can be off by +/- 10% it is a fact. and that is why cops usaly dont write tics for 5 over.

Bichon

My speedo reads about 5% high. I hope the odo isn't 5% fast too, since that would cost me real money at resale time.

caymandive

iTrader: (2)

Thats a good thought Bichon. I'm sure someone on this forum can give us some technical details on how this stuff works and whether or not they can be adjusted.

Gernby

iTrader: (1)

One of the major car manufacturers lost a class action suit about this because it cheated the owners out of their warranties. I think it was Honda, but I'm not certain.

I have 255/35-18's in the rear, and my speedo reads about 5% above what I'm doing acording to my Escort 9500i. However, I think it was reading high before as well. My theory is that the speedo and odometer is looking at all four wheel speed sensors and averaging them together. If I'm right, then it would take 4 speed correction devices ($90 each) to dial it in "correctly" (for VDIM), but you could probably get away with 2.

Bass Mech

i have never heard of a speed doing this, more than likely like every other car on the road there is a speed sensor at the tail end of the transmission that reads the speed from the rpm of the driveshaft.

Evitzee

In the old, mechanical days the speedometer/odometer was cable driven off the front wheels. They are almost always fast by about 5%. Best way to check is to go out on the interstate and run a 10 mile constant speed check using the small, green mile markers. They are as close to actual mile intervals as you will find. And if you can keep a constant speed you can calculate the speedometer accuracy as well as the odometer accuracy. I haven't done it on the IS yet, but think I'll do that in the next couple of weeks. You can't do this in suburban locations with constantly varying traffic conditions, but if you are in Southern California take a trip out I10 towards Palm Springs, you'll find plenty of open space in that direction. Makes for a good project on a weekend afternoon.

Gernby

iTrader: (1)

Only on front wheel drive cars. The cable was from the tranny, not the wheels.

Letz250

For those who have Nav, you can verify onboard. Go to the diagnosis menu (set headlight switch to "off" ,then hold "Info" button and switch headlight on twice, the menu screen should appear) under: Navigation Check\ Vehicle Sensors you can see what the GPS is measuring. That's up to 10 kph less than the speedo.

lobuxracer

iTrader: (2)

Click here for accuracy tolerance from Toyota. FWIW, it's -0.0/+5.0%

Speed has never been measured directly from the wheels before the most late model cars. It has always been measured at the transmission output which could not account for tire slip.

The current measuring system uses inputs from all four wheels. All of this has been previously discussed here.

caymandive

iTrader: (2)

Thanks I'll give that a try later. Btw..what do you mean "up to 10kph less than speedo"?