308 Speedo Accurate?

Don't know about 308. When I went for a drive with my 328, a friend brought his GPS. On the motorway it indicated exactly the same speed as my speedo. This surprised me since most speedos are a bit "optimistic".

 

Ferrari speedos reporting faster than reality is common. After all, how many cars are doing 10mph while seemingly standing still?
I sent mine out for calibration to Palo Alto last year. Since it's back, it seems worse. But they swear it's spot-on.
below 25 (as checked against GPS & local fixed 3M speed sign) sometimes it's spot-on, sometimes a couple mph low or sometimes high.
it tracks fairly well from 30-60.
above 60 the GPS says it is off by up to 8mph or so.

 

Get lots of opinions first but ........consider checking your speedo sender unit . It's on the right hand side underneath of the diff / gearbox , you'll need to buy a big socket and cut part of the side out to allow the wire / connector to go somewhere. Lots of Ferrari places carry the sender units and they aren't too badly priced.

I have a spare speedo that reads exactly double -- Quite impressive ......... I gotta get it fixed one day.

 

Hi
The sender won't help with accuracy issues, it either produces pulses or it doesn't. You can find threads from the past where RJay and I describe the inner workings of the older style speedos. They are difficult to make linear, it takes fiddling around with several mechanical and 1 electrical adjustment to get it somewhat close. Getting it close at low speed doesn't mean it will be close at higher speeds :-( And some units just don't want to cooperate at all.
When I fixed my own speedo I ensured that it read a bit high at most common speeds. This is in line with general settings as bang-on will lead you into speed trap issues.

Re speedo reading double, it could be a mismatch between the sender type and the speedo head. Old and new style are not interchangeable. Usually the symptom of new style head and old sender is erratic operation, I haven't tried new sender, old head which sounds like Paul308gtsi situation since he mentions using a big socket for the sender (aka new-style).

Gerrit
http://dino308gt4.com

 

By the way, my speedo suddenly got a bit jumpy at very low speeds before it should read anything, below 20 kph.
It jumps up every two seconds up to 40kph. Running above 20kph it acts normal again.
Is this common?

And the accurate reading I had was at high speed. After reading the post above it would mean that at normal speeds it would read a bit high since it's not lineair?

 

I use a GPS and my 83 308 is dead on as well! I guess I got lucky or the GPS is junk! I have however tested it on two other cars, both new, one with stock wheels and tires, dead on, one with after market wheels and tires, it's off by 3 miles per hour. So I think the GPS is right.

 

I had mine dialed in by a shop and they advised me that they can dial it in spot-on at 60mph but it will read slightly higher than that the higher you go, and slightly lower below 60mph. The early speedos cannot be dialed in perfectly at all speeds, I do not know about the later speedos. The best way to know your exact speed at rpm according is to know your gear ratios and speeds per 1k rpm in specific gears given one has the correct size tires. I know the '76-'77 US 308 in 5th gear yield 20.8mph per 1k rpm which makes 100mph come up at 4807rpm. My speedo reads about 107mph when I know I am going 100 and then it reads a bit higher than that the higher I go.

 

There are two issues here. One is related to accuracy, and it depends on the calibration, frequency, etc. We all know the accuracy will not be linear. One can make it accurate at the lower 1/3 of the speed range, or on the middle 1/3 or at the upper 1/3 of the speed range, but not on all 3/3 of the speed range the car will be driven.

The other issue, a far more common issue, is defective reading of the speed. It's either jumping all over, or not reading at all, or reading twice the real speed or reading 1/2 the real speed. This issue is caused by problems in the sensor, electrical issues, or calibration errors on the speedo itself.

My experience in this department has been that my 82 GTSi always showed jumpy speedo readings, untill I cleaned all electrical contacts, and specially the two ground straps. Then , the problem became the speedo showing approximately 1/2 of the real speed, and till today I still have not been able to correct it. I suspect it is a fault in the "mechanically driven" speedo pick up mounted in the gear box. I am waiting our "Ferda" here in Houston to come up with another sender similar to mine, from another car, to mount it and run the gears while on a lift to see if the speed reads correctly.
Cheers.

 

Had my speedo overhauled last year by palto and it indicates about 5mph faster than actual at highway speeds. Im also running 16" tires so I don't know if that makes a difference? I have a 80' 308. joe

 

Yes, tires make a difference. Had custom rims and with Mich Pilot Spot PS2 the speed was off a bit, yet with Blizzak snow tires the speed was correct.

 

Joe, I'd bet you are running faster than your speedo but I am lazy and not going to do the math.

The location of your sender has not changed. So the electrical input to your speedo has not changed. PA Speedo probably dialed in your speedo for the TRX 14's.

You are now running 16's. Much bigger tire.

You are physically moving down the Super Slab, faster.

 

Hi
The pulses coming out of the sender are not related to gear ratios. The sender rotates a fixed number of turns per wheel revolution, namely 2. Each sender rev generates 4 pulses.(old style)
You can determine the frequency using freq = pulses * (value * FINALDRIVE) / (TireCircumference * 3600) where Value is desired speed to indicate (e.g. 30mph), FINALDRIVE is always 2, pulses is 4 for old style, 8 for new style sender, Tirecircumference is "TireCircumference = (3.14 * Convert.ToSingle(TireDiaTextBox.Text)) * ININMI" where ININMI is 0.0000158. (this last constant is mi per in). Use a free frequency/sound generator on your PC and connect the audio out to your speedo head. Voila, speedo tester and calibrator.

Note the difference in old and new style, 4 vs. 8 pulses per rev of the sender. This is why a mismatch in sensor to head will show half or double speed. I don't know of a reliable way to differentiate the head style from external labels, you have to test the head. Opening it up will make the style obvious, the old style have larger pcb with a trim pot, the new style has no such adjustment, it is a soldered in resistor that you have to change. Linearity adjustments are the same for both styles.

Tire diameter has some effect on speed indication but not large. Changing from 14" rim to 16" doesn't make a lot of difference to the overall diameter either, there are some sources on the net that will show the effect. The rim diameter change is offset by the sidewall height change.


Gerrit

 

What kind of GPS will give mph? This thread comes up every two years in June. Must be the odd years. Last several times we agreed that even American cars were fudging on speed indicating about 10% higher at highway speeds. Some thought a taciturn government scheme since everyone drives about 10% over the limit anyway, and some thought since government intrusion into mpg, this would give back some of that lost by emissions but I don't think the odo is affected, so that's unlikely.

Bottom line, 85mph speedos, reading 70 at true 60 is common. I'd sure like to have a large LED gizmo for my dash with speed. GPS...tell me more.

 

THere's a radar speed reader on the east side of the westbound lane across the CHesapeake Bay bridge (Hwy 50). My wife's 2002 MB S500 speedo and the radar readout are identical at speeds between 50-70MPH. The 328 shows 60; the radar shows 53. The 328 shows 70, the radar shows 66. Seems odd that it would get closer to the correct speed as it goes faster but that's what the readout shows. I haven't gone any faster since the speedlimit at that point is 45.

I have a computer GPS program than I use for marine nav; it will do speeds to 1000Knots so maybe I should take it out and see what is REALLY going on.

 

I have an old Garmin nüvi 200W that will display speed IF I am not navigating to a programmed destination. I was using a Navigon 7100 for a while and it too would display speed. BTW, the speedos in the Porsche 996 as well as the 328GTS and F355 were always a bit optimistic, in my Benz it was spot on.

 

I've got a Falcon handheld radar unit (I was a cop for 25 years so we have toys too lol) so that's how I checked mine. The Falcon is a real L.E. traffic radar unit and not one of those baseball radars. If I recall correctly the speedo was off only 2-3 mph at speeds below 70mph. Between 70-90 it's about 5mph off. At 90+ I wouldn't want to have the radar in my hand! lol Incidently, I have another radar unit "covertly" mounted in my Porsche. Its fun to play with people who have radar detectors on I-10. Use your imagination? hahah Joe
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GPS will be spot on (far better than ANY speedo) as long as you have a good number of satellites in view (i.e., try to check your speed on a wide open road and you should be okay). Pretty much any consumer grade GPS unit I've ever seen will allow you to set the unit to either metric or imperial readout (i.e., km/h or mph).

 

Hey , quite interesting stuff , , sooooooo do old and new sender types both have the same thread diameter and pitch where they screw into the gearbox ? I was told by dealers and F/chat people that they do not ... If the thread is the same does the electrical connection for the sender match ? My car is 36629, fuel injected , I am told it should be the later type .. Thanks for any help , Paul

 

Paul,
I'm with you....same issues here. Would like to know if my pick up (see photo) is the newer or older. My 82 gtsi is #38773. My speedo shows 1/2 real speed.
cheers.
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My non-working radar gun is for sale if anyone wants to scare their neighbors. The microwave tube/front end was borrowed for another unit. Electronics are ok. PM if interested
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LOL , , we should swap speedos ! I'm going to video my double reading before I fix it , , could come in handy one day for some sneaky editing ... a 2V GTSi doing 200mph !

That sender is the old type as far as I know , , and to my surprise it comes in from the top !! Mine goes in from underneath !! What is going on ??

I have added a photo of my sender , , I wasn't going to get under the car to show where is goes in.
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THAT is just Oh so WROOOOOOOOOOOOOOOOOOOOOOOOG !!!!!! So many ways !!!!!!!


But I like it.