How fast does it go?

[jody]

You could always check the odometer against a mile marker or 10 mile markers for a better idea as to how far it is off. You could run the car up to 60mph and use a stop watch. The car should pass a mile marker every minute, or 120mph and a marker every 30 seconds.

[Pilot_951S]

1.2, Jody,

Thank you.

I am however still curious as to Mr Coppari's evidence that all 951 speedos read high.

In any case it doesn't really matter, I was cruising along at an indicated 163, I'don't care if there is speedo error I was going damn fast. Besides what am I going to do buy a radar gun and try it again?

Still 7% is a large error and I want to see some evidence. Besides which 1.2 you say the speedo was pessimistic by 10% at 100?

[Lawrence Coppari]

I only get to read this at night; hence, the delay in the reply. Where I live there is a measured 2 miles that agrees with I-81 markers. I use a stop watch and keep the car as close to 60 as possible. That is how I determined the error. As an aside, I have another sports car, an 87 of different make but still European. Coincidentally, it is off by the same amount. May car is a 1986 944T with a considerable amount spent on engine upgrades. Upon first completing the first of the upgrades, I tested the acceleration from 70-100, both directions on flat ground and noting temperature because that is significant with turbos. The result was unbelievable acceleration. The thing is fast but not that fast - I was comparing it to C&D testing of fast cars from 70-100 and looking at hp/torque ratings, rpm range, and weight. Furthermore, I drive it at a lot of PCA driver school events with friends who have similar cars. Their turbos are all off just like mine with stock size tires even though they are racing tires. That is why I made the statement. I did refer to the 944T when I wrote what I wrote.

[Pilot_951S]

Thank you

Now did you happen to check your odometer reading against the measured mile? The odometer works off the speedo so it will also be 7% out of whack with the measured mile if your speedo is 7% off. This is more accurate than hand timing the mile and attempting to hold 60 mph, and is repeatable at any speed.

Just intuitivly it seems rather odd that every european car would have a 7% speedometer error as standard equipment.

Lastly anybody know if the 86' 951 had a speedo cable to the front wheel or is it electronic and in the transaxle like my 88'?

[Lawrence Coppari]

Of course I checked both. Both cars' odometers and speedos are off about the same amount to the high side. I think sports car manufactures used to cheat on their speedos or they changed tire sizes without correcting the speedos and odometers. I'm not saying yours does not go that fast; I'm suggesting one cannot believe what one sees on a speedo. Do you know how to determine circumference or diameter of a tire from the tire nomenclature?

[Lawrence Coppari]

I have two other German cars besides the 944T. One reads about 3% low at 60 while the sedan is pretty much dead on. The former has a cable; the latter is electronic. My Italian sports car reads high like the 944T being only slightly off when cold but reaching about 7% off when used for 20 miles or so. Both are electronic.

[Pilot_951S]

You can not accurately determine tire circumference or diameter from the nomenclature, however I do know how. Anyway it is irrelevent becuase tire manufacturers are allowed a 5-7% leeway in the acutal dimensions of a tire. (oddly enough I spoke to a Michelin engineer last week about a different topic so I've got more tire knowledge than I know what to do with right now)

Anyway it wouldn't do you any good to calculate tire dimensions, you are better off with a tape measure.

[Lawrence Coppari]

Well you have gotten me curious about how close I can compute diameter(circumference) from the nomenclature. I have 4 vehicles with tires from BFG, Michilin, Goodyear in several sizes. Some are even R1's and Yokahama A032's for track use. I'll get to it this weekend if things get a little boring. Actually, I have oil changes on two vehicles to do so the things will be off the ground anyway. I will let you know what I learn not to prove anyone right or wrong but just for the sake of information. Probably will come up with some sort of table.

[jody]

Man what a subject. My tire's are said to have the factory size. At least thats what the guy at http://www.tirerack.com told me.
I think I'll check the speed this weekend, just for fun!

[sqgabriel]

being in the uk i think i can answer the 7% error part, unofficialy the police in the uk allow you to be 7% over the speed limit before issuing a ticket, wonder if this helps
(policeman 83 944na)

[Mac]

Driving my 86' 951 ,I to thought the speedometer was incorrect , so I compared it against a 96' Vette that I was following to a track event using portable radios.
My findings was the 7% high readings. Here is the puzzling part,when I tested against the mile markers, the odometer was right on ?
Finally using the construction signs that also read your speed it confirmed the 7% error. The speedo on my car is electronic driven off the trany.

[Pilot_951S]

Oh well, looks like I'll bhave to gut my car and replace the gauges.

[thamlin000]

1.2, sorry I'm so late to reply. With my luck, the one time I do a top speed run, I'd be caught. Last year I got a 90 mph speeding ticket and the cop told me that if I'd been doing 95, that would be grounds for wreckless driving. Luckily the ticket was dropped after I attended a defensive driving course. Hopefully the courts will be sympathetic to your case. Good luck with the courts.

[1.2gees]

Thanks, well, I can only hope it won't cost me more than a couple of $100...

[Lawrence Coppari]

Tire nomenclature and circumference. I did not do all the tires I intended, but what I have done gives me confidence that the circumference (diameter) can be computed from the nomenclature.

If there are two numbers in Measured Column, percent diff correspond in order.

Size Computed Measured Circ Pct Diff
225/60-16 83.66 83 ¼ 83 ½ -0.49 -0.19
245/45-16 77.54 76 ½ * -1.3
225/50-16 78.09 78 ½ 79 +0.53 +1.2
195/65-15 78.48 77 ¾ 78 -0.93 -0.61
* Tire has no tread remaining

[Robert MccRiley]

I have a GPS in my 944 witch dispays my speed using sats, and I've lernd that the speedo is not acurit at all, below 40mph the speedo reads slow, around 45-50mph its about on, at 55mph the speedo reads 60mph, at 65mph it reads 75mph, at 70 it reads 85mph, thats about as fast as I go these days "getting to old fore a pocket full of tickets"

[jwn]

I believe the advertised speed was 137mph with the headlights up, 143 with the headlights down. I too have a GPS and have done some testing, has everyone found with a 5 speed at 3000rpm in 5th gear to be doing 80mph? (My 83 944na speedo, GPS and tach all seem to agree here)

[p0r2chby]

I have an 86 na. This morning I did the 5th gear, 3000 rpm test. My speedo showed 83 mph. Am running 235/60 15's in back.

[redp944]

I just got a speeding ticket...thank god the cop didn't see me when it was really wound up! My girlfriend wanted to "go fast"...the car can do that, so I did. We took a quiet country road into town at night with hills and bends. Somewhere along I passed a cop without noticing - he claimed he clocked me at 70. Fine, I said because I didn't remember going that slow. All I remember is I passed a car, then rounded a corner and hammered the accelerator to top a hill at 95 (giving a great negative G effect), then round a sharp corner near 75 to head into town. According to the cop he couldn't keep up in his Crown Victoria around the curves and up/down. I completely lost him. He caught me because I didn't realize he was back there at all. A mile after the hills and curves end is town. I saw the lights coming, so I just pulled over and waited. He was particulary cool about the whole incident, and it only set me back $80.00. I would have hated for him to see me top that hill though.

[Lawrence Coppari]

The speedo reading is independent of tire size. It just measures how fast some output shaft is turning. It doesn't care how fast the car is going.