Have any of you ECU gurus found the speedometer routine? Is there a variable we can maniuplate to compensate the speedo for different size tires. I know it's a minor thing, but it'd sure be a nice feature, for me at least.

 

Isn't the speedometer mechanical?

 

pretty sure its mechanical coming off a sensor on the trans my friend. I'll pull up the service manual pic in a sec.

edit: here ya go. ...not as detailed as I would have liked, but heres the mechanical sensor. The gear ratio is what gives the fixed rate. BTW... the manual also has a table for speed tolerances. It seems that the speedo is allowed to run +/- 3mph from the FACTORY. I don't think you are going to run wheels/tires that are so different that they affect it more than that. (meaning there is no reason to adjust your speedo if you still fall within the factory tolerance, because it could already be that much off)




Double edit: even if you go 255/45/18 (you would normally get a much smaller aspect ratio) you would be just over 3mph difference. ...lol and I'm just going to guess that you haven't gone SMALLER than stock. Here's a super easy way to figure out the difference a tire size makes: http://www.miata.net/garage/tirecalc.html
So long as its within +/- 3mph, I wouldn't worry about it.

 

Better (and very likely correct) answer above!

 

-Quote removed-

The wheel speed sensors are for ABS, and go to the ABS ECU. That info may also be passed on to the main ECU... but that's not where the speed signal is coming from. It is from the speedo sensor that I posted above (mounted to the trans).

 

The sensor is mechanical, like every sensor. The ecu just gets a signal. The question is can we edit the equation the ecu uses to interpret the signal in the output for the speedometer. I am also curious if we can edit this.

 

I believe the correct terminology is the VSS - Vehicle Speed Sensor, if memory serves me correctly.

 

Well, if it makes any difference, I have a VSD Concept head up display that you can adjust depending on the stock speed signal from the ECU so that it can be moved from vehicle to vehicle. It even has a "free setting" so that if it can't calculate the speed from the stock signal, it can be set from a certain speed using the speedometer and then it will use that to calculate all speeds.

I'd imagine it's fairly basic in terms of electronics, so hopefully the stock ECU will have a similar ability.

 

Any more info on this guys. it'd be nice to get the speedo corrected if need be. I realize the difference isn't much, but if my speedo is already off 3% and I change tire sizes, then it can get up to 5% off. just a nice option if it was known.

 

i do know on my trans am when i hook up my power programmer it has a option for speed correction for different tire size and or gear ratio's. the speed senser is located on the trans just like my evo. so there has to be a way to flash the ecu for it.

 

interesting topic, thought I'd give it a bump to see if anyone's found anything out. I definitely don't want to incrementally increase the +/-3 margin by going with a smaller diameter. I'm already off by 2 or 3 mph with the snow tires and 14s I have on. That miata calc is nice

 

I've been wondering the same question here. Cuz I know it does make differences in speed/mileage when switched to bigger or smaller diameter wheels and tires.

 

The question is, what should be corrected?
The sensor signal goes to the ecu, to the diagnosis connector and direct to the dash panel. So if you want to correct the dash display, you must change it inside the dash.
And this cannot be done with ecuflash.

 

I guess to restate the original question, is there a way to alter the signal going to the dash. And with your response i assume the answer is no.

 

I am not known much about the speedometer, sorry.