ANORXIC'51'

Good evening, everyone. I was wondering if anyone has a rough diagnosis for what's happening with my '86 951.

I'd always suspected the speedo might be off a little as I'm seemingly always traveling a bit faster than it should be going, but just chocked it up to the low ride height of the car.

I was following my g/f northbound on I-35 from Fairbault, MN up to the twin cities last night. We were already on the phone(both of us using bluetooth headsets ..as she works for T-Mobile) and I ask her why we are "only" going 62 mph on a 70 mph interstate.

She pauses for a couple seconds before stating we are traveling at 75 mph. (She has an '03 Mazda 6...and I can vouch that her speedo is accurate) The same went for when we hit the 55mph zone in St. Paul. She follows the speed limit in this section religiously, though my speedometer only reads 40/42 mph.

This kind of scared me because on the trip down (Jessi was already there since she had the day off)...I tapped 110 indicated a couple of times. I know the speedos on our cars are inherantly inaccurate for the most part, but shouldn't it be reading on the high side as opposed to 12-15 mph too low?

So.....does this seem like a speedometer calibration issue or a speed/reference sensor problem?

On the down side, I wonder if I look like a jackass on my morning/evening commute since I'm thinking everyones only going 45/50....

On the up side....IF my car was actually doing 120+, the 951 is REDICULOUSLY poised and stable at those speeds. And the fact that it was still pulling like a Bullet Train on a 400 shot of nitrous when I let off at what I thought was 110 is a testament to what these cars are really capable of.

Oh, and I apologize for traveing that quickly as US roadways are NOT acceptable for those speeds, BUT, the interstate was pretty empty and her mom was waiting on me so she could start serving her killer lasagne(I murdered that spelling) for dinner.

Thanks for any help and suggested remedies as to what the speedometer problem may be

Christian

The DareDevil

Do you have the correct sized tires on the car? Also, where does the speedo needle sit when the car is stopped? Does it sit below 0mph?

michaelathome

Your owners manual also should have a page dedicated to speed in gear at given RPM. If you have your manual you may give it a try and find some known good point, 3200RPM in 5th and see what the book and the speedo say. There will always be some variance but I don't think that 10mph is right.

Michael

The DareDevil

Unless the RPM gauge is off!

75ohm

3200 in 5th on a dyno perhaps will be accurate but depending on drag, etc. will change the actual MPH reading.

LL951, did you recently fix your odo gear and not put the needle back on at zero?

Change your tiresize as DareDevil suggested?

ANORXIC'51'

Hmmm, just went out to check.Speedo needle sits under 0 at stop, by a considerable margin..say 10-15 mph hash marks below 0.

Tire sizes are spot on at 205/55 in front and 225/50 out back.

75ohm

Well damn dood, there you have it. Unless your speedo is broke, the needle should be on 0!

ANORXIC'51'

Havn't done any work behind the gauge pod...suprisingly the odometer is still chugging away as it should. But that sounds like that should fix it..if its as simple as reattaching the needle to 0

Thank you for the quick responses

ANORXIC'51'

Well hot damn...that was unbelievably simple.

Thanks to everyone: Daredevil, Micheal, and 75ohm

Chris

michaelathome

Gearing is gearing. Your speed should be constant in a specific gear at a specific RPM. The ratio will never change? Am I just missing something?

Your ability to reach that specific speed/rpm will depend on conditions though like if you are traveling up a hill or down a hill, drag and resistance.

Michael

Charlie

Be careful twisting the needle and don't break it. If moving the needle does not correct the problem then the calibration could be off. Mine was off around 8 mph and I sent it to North Hollywood speedometer for calibration.

Izzy

Be an idiot like me and break the damn needle.

jpk

There's a pretty simple way to check the accuracy of the speedo. Get on the highway, hold a constant speed (as indicated on the speedo), and time how long it takes you to go between the mile markers at the side of the road. Divide 3600 by the time in seconds and you get mph.
For example, if it takes you 58 seconds to do the mile, you're traveling at 3600/58 or 62 mph.
Take a couple of samples and different indicated speed to average out for error and you should get a pretty good idea of what kind of error you have in your speedo.

75ohm

m@home... wtf was I saying? You are absolutely right, gearing is gearing, and it doesn't change. What I said made no sense. Thanks for pointing it out and giving me an opportunity to gracefully retract.

For those who are mathematically challenged:

http://www.web-cars.com/math/speedometer.html

mudbuddha

luxuriouslyloud951- mine is also below zero...how do you fix this again? do you need to pull out the needle or just turn on the ignition and twist it back to zero? Thanks for your help.