Jeremy Martin

hi all. i'm pretty sure my speedo is off by 5 mhp (high)... the tires are the original size, and it's constant throughout the range, so i think my speedo just needs to be calibrated. any suggestions on a place to send it out to? and how much does a calibration usually cost? has anyone here done this so far? i really want to get this done sometime soon as 5mph is about 8% off on the highway and 20% off in city (which means my odometer is probably reading about 3000-4000 high right now).

SimonK

Impossible, 951 speedo runs of your gearbox.

If tyres are original spec, speedo can't be wrong

SK

ninefiveone

Just curious, why does running the speedo off the gearbox make it impossible for the speedo to be incorrect?

My speedo is also off by 5mph according to GPS but I notice that my odometer appears to be spot on.

-Hugh
'86 951

booster

It's NOT IMPOSSIBLE.... <img src="graemlins/nono.gif" border="0" alt="[nono]" /> <img src="graemlins/nono.gif" border="0" alt="[nono]" /> I had mine checked and with spec. tires it is off by 6 at 65 showing 71. <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

Bri Bro

My speedo is 5% high also.

Jeremy Martin

SimonK: if it's impossible then the previous owners got taken to the cleaners because as far as i can tell the speedo has been calibrated twice. my mechanic also said that i could have it shipped out to have it calibrated, finding a good place to do it is the trick...

SimonK

OK I might have been wrong as to impossible goes but this is what I was told when asking the same question here in the UK...

944 speedo runs of the gearbox measuring gears revolutions, which never change. So in theory speedo as Porsche was stating in their sales specs is 100% accurate.

In off the wheel speedo set-up of course tyre diameter matters and due to inherent system rotational properties speedo tend to show upwards. 160MPH is showing 178MPH! I drove an Audi RS2 at 178MPH (ha, ha) and friend of mine dusted me in my 944 Turbo S. We than compared notes and my car driven by my friend was showing 160MPH at the time.

As pushing my car absolutely to the max on German autobahn it magically stops at 162MPH and wouldn't go a mph faster as per Porsche book.

So don’t know guys…The truth is out there….

SK
<img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" />

Danno

Depends upon how the calibration was done. Most mechanics 'calibrate' the speedo by twisting the needle, changing the spring-preload on it. This is a surefire way to get worse errors.

I think the error in the speedo was intentionally put in there to save us from speeding tickets. At least it's only a 5mph error that's constant rather than cummulative like a mechanical speedo (off 5mph@50, 10mph@100, 15mph@150). I've done strafing runs at a radar booth at 50/100/150mph and the speedo was optimistic by 5mph. However going up to a 275/40-17 rear tire almost makes up this difference

To truly calibrate an electronic speedo which reads pulses from the output-shaft in the tranny and measures the frequency, you need one of these ratio adaptors:

<a href="http://www.altavista.com/r?ck_sm=d8c6cb3e&ref=20080&uid=729ba2adab23385c&r=http%3A%2F%2Fwww.abbot t-tach.com%2Fera.htm" target="_blank">Electronic Ratio Adapter</a>

<a href="http://www.atrol.com/cx_era.htm" target="_blank">Abbott Enterprizes</a>

Jeremy Martin

thanks for all the help guys. i'm less worried about it now that i know it's pretty common... still pisses me off since i do alot of city driving and thats usually around 25mph, which means my odometer is going to be 20% high for those trips

ninefiveone

Jeremy,

I really doubt your odometer will be off. Mine is optimistic by 5mph at any given speed but on long drives (300miles and up), the odometer has matched the GPS distance exactly. i.e. A 3 hr drive at an indicated 65 would be 195 miles but my odometer has always faithfully shown 180.

-Hugh
'86 951

Modena 395

Jeremy Martin:

Could you share some of your math? How do you get to 20% off in city and 8% on the highway? Why would it differ?

Curious... <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

Robby

I believe he's taking into consideration that the city speed limit is lower than the freeway, like in the area of 30mph & 60mph. For instance, if he's off by 5mph, then that's 10% of 50mph, but at 25mph, he would be off by 20%...

At any rate, I've seen many pics of the speedo's of these cars at high speeds- one of the best is where a guy is a hair closer to 180 than 170, so ~175. He still had ~400rpm left to redline! He would have to have some pretty tall tires to accomplish this, as each 1000rpm in our cars (in 5th) is worth ~26mph. For some reason, my Turbo S is off (by comparison to the tach) in the 3000 range in 5th, but seems to be right on everywhere else. At 2000rpm, I'm a hair above 50mph, at 4000- ~105mph, at 6000- a hair above 150mph, but at 3000, I'm slightly below 75mph. At 80mph, I should be barely over 3000rpm, but I'm almost to 3400rpms. Oh well, nothing's perfect I guess...

BTW, why would the tire size not have anything to do w/it? The RD of a tire directly affects the gearing, which is why a shorter RD helps to accelarate faster, but causes the car to rev higher at a given speed. If the speedo is measured at the gears, then it would still seem (to me) that a taller (or shorter) tire, would effect the axle and the rate at which it turns- am I missing something here?

Jeremy Martin

yup, thats exactly the math i did out. a constant 5mph isnt alot until you figure that in town you're doing 25mph. oh, and the only reason the tires dont factor in here is because i'm using stock sizes with stock rims. if that were at all different, i suppose i'd be off by 5mph plus a percentage based on my outter wheel diameter.

Danno

"one of the best is where a guy is a hair closer to 180 than 170, so ~175. He still had ~400rpm left to redline! He would have to have some pretty tall tires to accomplish this, as each 1000rpm in our cars (in 5th) is worth ~26mph.

Almost... The relationship between the tach and speedo can never change. It will always be 26.2mph on a 951 because it's hard-wired into the DME. The only way you can change it is to doctor it with Photoshop like that guy did.

Now what you can change is the relationship between the speedo and ACTUAL speeds. For example with stock sized 24.87" tires going an indicated 80mph@3100rpm, you are actually going 75mph. Going up to 1" taller tires like 275/40-17, the same 80mph@3100rpm will have you going 79mph actual speed. Downsizing your tires to say... 225/50-15... at 80mph@3100rpm you're really going 70mph.

Simple, eh?

Robby

Actually Danno-
if you are doing an actual 75, then 3100rpm would NOT be possible- at 75mph, you would HAVE to be doing LESS than 3000 if the speedo and tach were together, b/c, as you also stated, the speed is 26.2mph/1000rpm in 5th w/stock RD. You are using 225/50/16- I've always used 26.1/1000 b/c my Turbo S has 245/45 which is 24.68 as opposed to 24.86. Anyway, you said an INDICATED 80mph @ 3100rpm would really be ~75- did you mean it the other way around? B/C 3100rpm should be 81mph, or were you just giving a hypothetical example?

At any rate, the relationship b/t the tach and the speedo obviously IS off on my car, otherwise, I wouldn't be running almost 3400rpm @ 80mph- as I had stated before, my 5th is always right on, but, it IS MOST DEFINATELY OFF in this range- I don't know how, or why, but it is. Also, there are numerous other 951's w/speedo/tach glitches- not sure about the 175mph one- do you know for FACT that it was edited? It was on a guy's website- "Spooky's 951 page" or something. I have seen others, showing higher than possible speeds, and even e-mailed them, as their speedos did NOT match their tach's, and I've been told by them that they were running stock RD's- I have seen this (where I've e-mailed the person) at least twice. Also, a European Car article "Powermath" (I can't find my copy, but it was a summer issue from around '96) where they took 4 Porsche's to the dessert (2 modded 911 Turbo's & a 951, Powerhaus cars, and a stock 993TT) trying to hit 200mph. The 951 had a 968 crank, K27 #8, MAF w/almost 400HP, and they backed off at 175mph saying they had a couple hundred rpms left till red. When I asked David Raine's about it a long time ago, he wouldn't comment. Anyway, there are many other's that claim to have hit well above 170, which you and I know is impossible w/stock gearing w/out going past the rev limiter- it shouldn't break 170mph, as the redline is 167mph. Doesn't really matter- these are just examples of how the tach and speedo definately do NOT always line up- I don't know how OR why of course <img src="confused.gif" border="0">