Hey all,

First a short story to explain the subject.

Just got back from a weekend at the cottage.
I drove down w/ my 91 and my friend took her 91 Civic. We had 2way radios to talk to each other. She was constantly whining that I was going "too fast". I thought oh well... women driver and she'* driving a Civic, so whatever right.

Well anyways we were over 1/2 way to the cottage and she comes on the radio, "slow down, I'm doing 130kph and I'm not even catching you".
I look at my speedometer... I'm doing 120kph on the mark... maybe even a little under.
I said to her, "your speedometer must be wrong, I'm doing 120". He car has loads of problems so we just naturally thought it was hers.

I let her pull up beside me and we measured our speeds.
When I read 100, she was reading 110.

So anyways next day (with the 2way radios again) I'm driving and my friend Andrew has his dads truck. We try the same thing, only same results. When I was at 100, the Ram was reading 110. My speedometer is off by like 10kph @ around 100kph. :?

Might explain why a few years ago I got pulled over for doing 117kph and I could have sworn the cop tagged me just as I was dropping below 110kph. :?
Noticed something on the drive down.


Now onto the real question....

Any ideas what could cause my speedometer to be off?
I'm guessing it'* probably off by 10-12% at any given speed.
I'm using the stock rims, proper size tires. There is no error codes in the system.

well... the pcm counts pulses from the VSS, 4000 pulses per mile.

the speedo itself gets ahold of that 4Kppm signal and decodes it solid state style..

So... here'* one, Does the car shift at the right speeds the speedo shows, or do you think it shifts by the actual speed you're going?

I have no idea.... I thought it was shifting by rpm to be honest.

I usually find a stretch of highway with mile markers and run for 10 miles with a stopwatch. At 60 mph (97.5 km/hr ?) it should take exactly 10 minutes and the odo should read 10 miles. If the odo is correct and the speed is off, the speedo needs calibrating. If both are off, it is the ECM.

I do know there is a PROM calibration byte for MPH. Would sound like you have a speedo for a 3.33 axle and a 2.94 actual gear but the VSS (vehicle speed sensor) is supposed to be fixed at 4000 pulses per mile reguardless of ratio (sensor driven by output shaft ?).

Could also check the speedo against the ECM with a scan tool, one of the parameters output is actual MPH and you can check that against the rpm in lockup to get the actual gear ratio.

Yes, try the scan tool. Your mechanical speedometer could be the culprit.

Weird. Last week, cruising anywhere between 110-130km/h (130k bursts were rare - most time was spent at 110-120, the "real" speed limit on the highways here) and an Isuzu Rodeo on the other end of a 2-way radio and all other families in our convoy (we were lead car) were complaining about our speed

I'm running the stocker 215/60R16'*. Hows about you, Merlin?

(the general perception among family and friends now is that the SSE is one fast-*** car, even though it ain't ahaha)

I'll need to double check the tires.... I'm 99% sure they are proper.
The SSE is a fast car, it just takes sometime to get upto speed compared to the SSEi.

Remember the SSEi'* are stock limited at like 210kph.... there is no gov on the 87-91 SSEs..... so technically out of the box, they could goto a faster speed.

Back on topic....
I talked to my mechanic.... he'* going to run a scan tool on it and we'll see what the problem is.

Oh yeah, it can definately get the top end easy, but acceleration-wise, it lags.

16 or 17 second quarter miles?

Nine-something 0-60'*?

Sounds about right....

I'll be going on another long trip with other family cars in a convoy at the end of the month, I'll see then how wrong the speed may be. 110-120 isn't fast :?