liquid_helix136

I agree I think the previous owner had gears and was using a speedo calibrator and sold the gears but forgot to adjust the calibrator. Check by the ECU on the left side of the driver side footwell, see if there's a little yellow box thing with two black dials on it.

Mistermann

Delimiter....speedo convertor....if you've had one installed and its a cheap one it'll do this to your car...small box behind the speedo.

hirev

Xviper, very glad to see you back!!!!

liquid_helix136

BTW, I tested this in my AP1 with 4.77s and at 60mph in 6th gear, Im at 4k rpms just about, maybe 3900 or so. Sounds like you have 4.77s in the rear my friend.

iDashX

If u read above, hes going 70 when his speedo is reading 60. Doesnt mean he has a 4.77 fd

iDashX

According to your information provided, we don't know if you have a yb/yj. Im gonna assume you don't because i had a similar issue with no yj/yb.

Background info: The reason for your tach shows 4000rpm is correct and means your vss is working. Your using an ap2 motor and an ap1 tranny like me. The reason why your speedo is off is because the ecu is calculating all the sensors on the motor and tranny with the ap1 math equation. To the ecu, it thinks its right and sends the electrical signal to the speedo to display XX mph. So some truth above why it is so off is because of your rim size. The calculations of your ecu is set for 16in rims, which is why if anyone goes larger or smaller their speedo is off. So with your ecu calculating all this information, it generates a certain signal for each small fraction of change that when it hits your cluster, the cluster then interprets that signal and display the anticipated reading.

Solution:
1. Live with it: $0
2. Install a yb/yj: $55 + installation ($0 if you do it yourself)
3. Ap2 ecu with ap2 cluster: $150 + $300 with pigtails + installation

I have an my00. I originally had an ap1 motor, tranny, ecu, and cluster. Blew the motor. Ran ap2 motor, ap1 tranny, ecu, and cluster. Miles were off like yours but about 4 low and 15 high speeds(annoying and got a speeding ticket). Changed to an ap2 ecu with ap1 cluster, tach and speedo was off. Finally found an ap2 cluster with pigtails with close miles to what displayed on my ap1 cluster. My speedo is showing that i am slower then my actual speed by 0 at low and 2 at high speeds. Im going to guess my tach may be off by a small margin as well but im hitting redline at the right spot. This may be due to resistance in the wiring from splicing. I can live with that. I don't know how aftermarket ecu/ems will fix this but i will find out later this year (which is why i did not list it as a solution).

bdo

This makes no sense, or else I am majorly misunderstanding how the speedo works.

The speedo gets its signal directly from the VSS -- or does it? (now you have me 2nd-guessing this based on your experience) -- so there should be no calculating for the ECU to do, and even if there was, changing the motor shouldn't affect this because 4000 RPM is 4000RPM regardless of which motor it is, as long as the trans and diff are the same. What other sensors could the ECU possibly be using to calculate vehicle speed?? Does the cluster base its speed display on anything other than the VSS signal?

xviper

You are correct. That post doesn't make any sense at all. The speedometer reading is taken via the signal from the VSS only. The signal goes through the ECU to be translated into MPH or KPH but it also uses the signal to determine other system operation. The tachometer gets its signal from the engine sensor. It doesn't use rpm to calculate anything to do with vehicle speed, but again, the ECU uses this signal to determine operational parameters for other systems. The odometer reading is held within the cluster itself and its numerical display is based on the VSS signal. That's why when you buy an instrument cluster from a salvaged S2000, it will display the last odo reading of the car it came out of. It can be set to whatever your car's last mileage was but that seems to be a closely guarded secret by those in the business.
You can cut the VSS signal and the tach will still work (and vica versa) as will most everything else, except that you will get a CEL indicating loss of VSS signal.
The ECU does use the VSS signal for such things as determining whether or not to allow the soft top to operate and of course, the cruise control. The ECU also uses the rpm signal and VSS signal to determine operational parameters for the EPS. If you've ever sat there and held your rpm at 3000 or above for more that 3 minutes, you will see the EPS light come on in the dash. Although the ECU does use these 2 signals, it doesn't correlate them with each other for the purpose of calculating the other. The ECU takes the signal from the sensor in terms of volts and transposes that into the numerical value you see in the dash. Interrupt or change the volts and you change the reading. That's how Yellow Jacket and Yellow Box works. If this wasn't the case, something like the YJ or YB would really screw with the engine rpm reading and it doesn't.

LSUWALT

I'm having the same problem - accurate tach and a speedo that reads 18 mph too low.....4000rpm, 70mph, speedo reads 52 mph. 2000 model year with transplanted 2002 engine by previous owner....replaced VSS, no change....assuming original ECU.

previous owner replaced clutch and radio too...lots of re-wiring for the radio dash controls ...I assume wiring to be the problem but I guess the cluster could be bad as well...been living with the problem for 3 years.

xviper

A VSS signal wire that is pinched or bare or corroded in one spot won't transmit the full voltage that is required to give an accurate display. Major work like an engine transplant can damage or stretch the signal wire.