Speedometer error and GPS

[Turbofiat124]

How accurate is your speedometer? Are these things spot on, or are they inaccurate on every Spider? Mine has never read correct since day one.

This is an '80 model and came with a 3.9:1 final drive. The pinion gear broke so I had a 4.1:1 axle and driveshaft from a 1970 Spider laying around so I used it instead of sourcing another 3.9:1 axle.

But before this happened, the speedometer would peg out at 85 mph doing 60 to 65 mph. The only thing I had changed was the wheels and tires (14" 60 series). Would going from 13" 85 (80?) series tires to 14" 60 series change the ratio that much?

The 140 mph speedometer I installed some 20 years ago alone with a custom "grafted" speedometer cable started to doing weird things one day then eventually crapped out.

This was what it was doing at around 55 mph:



So I put either the original 85 mph speedometer back in the car.

Like the 140 mph unit, this one is just as inaccurate as well but not so much.

Using the GPS speedometer APP on my phone, I determined when the speedometer is up to 40 mph, my phone says my actual speed is around 5 mph slower. But when my speedometer says I'm going between 55 and 65 mph, I'm actually going 10 mph slower.

Actually the error ratio increases the faster I go. The speeds mentioned above are a rough estimate.

I have used these GPS apps in the past on other cars and it seems to be the opposite. My phone always shows I am going 5 mph faster than what my speedometer says. I have not tested it against my Subaru or Chevy van since I'd suspect the speedometer would probably be more accurate being newer vehicles and having a speed sensor instead of a cable/mechanical speedometer.

Also seems like the speed varies depending upon how straight the road is. If I'm going around a curve, the speed on my phone changes despite going the same speed!

Even though I have swapped transmissions I know for a fact I do not have a 4.1:1 speedometer gear so I'd think if I had one, the speedometer might be a bit closer.

So the question is, are these GPS apps actually that accurate? Anyone know where I can get a 4.1:1 speedometer drive ? And how would you remove it without dropping the transmission or cutting a hole in the driveshaft tunnel to remove and replace?

I tried once and it hit the inside of the tunnel. Seems like someone told me you could disconnect the transmission mount which would gain just enough clearance to pull it out and put the new one back in.

[RRoller123]

CORRECTION:

My 80FI 85mph speedo reads about 5% consistently HIGH across all speeds.

This is a handy little tool regarding tires size and speedo error:

http://www.circuits.dk/calculator_wheel_tire.htm

[GeorgeT]

Two things: 1. you are dealing with 30-40 year old instrument in which the accuracy has certainly deteriorated unless it has been rebuilt and 2. By Federal law the speedometer has to read 0% to no more than 5% faster than actual speed, they are not allowed to read less than actual speed.

Now if you went from 13" rims to 14" rims and the total diameter of the wheel/tire package increased, the speedometer will read less than before the change when traveling at the same rate.

[joelittel]

My '80 reads about 5% high. I have always assumed it was because of the tires/wheels not being stock

[baltobernie]

So the question is, are these GPS apps actually that accurate? Anyone know where I can get a 4.1:1 speedometer drive ? And how would you remove it without dropping the transmission or cutting a hole in the driveshaft tunnel to remove and replace?

I suppose every GPS unit accuracy depends on the sophistication of the device. Maybe some phones are more accurate than others? My Garmin seems to be quite accurate if I'm driving a steady speed.

I'd post in Parts Wanted for the drive. I don't know of any way to replace the drive without dropping the g'box. Beware of cutting the tunnel; it is a critical unibody strength member. If you don't need a new clutch any time soon, I'd consider borrowing a real GPS unit and noting speed vs. RPM. Commit to memory, make a placard, or Sharpie the speedo at a few points.

My speedo and odo are dead on with stock size tires.

[RRoller123]

My '80 reads about 5% high. I have always assumed it was because of the tires/wheels not being stock

I corrected my earlier post, mine also reads consistently 5% HIGH, not low, (brain cramp). And it did this with stock rims and tire sizes before I began changing the running gear.

[FiatMac]

Anyone know where I can get a 4.1:1 speedometer drive ? And how would you remove it without dropping the transmission or cutting a hole in the driveshaft tunnel to remove and replace?

Please note that if you are talking about changing out the speedometer drive to one for a different rear axle ratio you have to change not only gear that installs from the outside, but the internal gear as well. They are a matched set.

[KevAndAndi]

I guess I'm lucky: My speedo reads only about 2 MPH faster than the GPS, which is fine by me.

[Turbofiat124]

Thanks for the feedback guys. I'll have to check my speed based using an actual GPS. So my actual speed may not be that much off if my phone is off by +5 mph.

I did check my odometer's accuracy. I know the distance between my house and my job is exactly 8 miles. My Spider says I have traveled 9 miles.

Out of all my "classics" my Trabant's speedometer seems the most accurate. I drove to the county courthouse one day and the distance traveled (~25 miles) was spot on compared to Google maps. But that is the only car that I'm running the correct size tires on.

[azruss]

of all the fiats i have had over the years, they all read high. As others have said, 5% seems the norm. I had a speedo that did the same as yours. got worse with faster speed. Over a short period of time it got way off, like reading 80 at 55. I went into my instrument stash and found another "early" speedo. It is very accurate when just starting out, but develops a little error as i drive. Like reading 75 when i am really doing 72. This is probably the most accurate speedo i have had on a fiat. If you changed the rear end and kept that tranny, you will have a hard time getting a speedo to read right. As others have said the drive gear for the speedo cable needs to match the rear end. The internal gear in the tranny needs to match the speedo drive gear. I've never been inside the speedo, so i don't know if there is something in there that could be messed with.

[Turbofiat124]

Yes the original speedometer would peg at 85 mph when I was really doing 55 to 65 mph. This was with the original differential and I think the original transmission. The only mod at the time was 14" 60 series tires. So I wondered if the speedometer was actually bad.

That's the reason I did not put the original speedometer back in. I used one from a parts car thinking it might be more accurate.

It was my understanding that speedo drives were only matched to the final drives. That's the first I ever heard that there is a different gear inside the transmission that the external speedometer drive runs against. I thought it was driven off a gear made onto the main shaft which were all the same ratio.

Also I've heard that the speedometer doesn't care. Whether it be

an 85 mph from a 3.9:1, late 70s/early 80s
140 mph from a mid 70s 4.3:1
or a 140 mph from an early 70s 4.1:1
Or even the elusive 1978 140 mph clip on type cable.

[azruss]

the speedo is not clairvoyant. It only responds to how fast the cable is spinning. By this logic, the speedo drive would have to be matched to the rear end ratio.

[Zaffer]

My speedometer is about 3-5% high compared to actual speed.

I know BMW does this on purpose in their vehicles as we used to get a lot of complaints from customers at the dealer that their GPS read lower speeds than their speedometer. Maybe Fiat did the same?

[klweimer]

Wait a minute. Trabant?

[Turbofiat124]

Wait a minute. Trabant?

Yes!





I know one guy in Longmont CO that has one.