Speedometer error and GPS

[Turbofiat124]

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.

I mean that you could swap an 85 mph speedometer out for a 140 mph and technically it should read the same no matter what the speedometer gear drive it. According to a guy I know.

But if you swap axles you need the speedometer gear that is matched for it. There is a chart somewhere that tells the ratios based on what spot of paint was placed on them (red, yellow, white). Probably long since worn off.

What's weird is I have an axle from a 1981 model (speedometer from the same car I am using) that should be a 3.9:1 but the paint marks on it says it's something else (like 4.3:1). They look factory like it's been painted using a stencil, not from one of those yellow paint markers hand painted by a junkyard.

I've also heard you cannot use a 3.5:1 speedometer gear drive from an automatic if you install a 3.5:1 axle in a Spider because the gear drive turns backwards. On the other hand it's a totally different transmission (GM) so I wouldn't think it would fit a SEAT/FIAT transmission anyway.

I've got a 3.5:1 Brava rear axle I've love to drop in my Spider but the trailing arm mounts are diagonally which would require some cutting and welding and I don't know if the drive shafts are correct length. Someone told me you had to use the rear end shaft from an automatic Spider which I don't have.

Also the Brava has rear drums so I don't know if the disc hardware could be swapped over. Sounds like allot of trouble to me but would be much nicer at highway speeds.

[Turbofiat124]

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.

I mean that you could swap an 85 mph speedometer out for a 140 mph and technically it should read the same no matter what the speedometer gear drive it. According to a guy I know.

But if you swap axles you need the speedometer gear that is matched for it. There is a chart somewhere that tells the ratios based on what spot of paint was placed on them (red, yellow, white). Probably long since worn off.

What's weird is I have an axle from a 1981 model (speedometer from the same car I am using) that should be a 3.9:1 but the paint marks on it says it's something else (like 4.3:1). They look factory like it's been painted using a stencil, not from one of those yellow paint markers hand painted by a junkyard.

I've also heard you cannot use a 3.5:1 speedometer gear drive from an automatic if you install a 3.5:1 axle in a Spider because the gear drive turns backwards. On the other hand it's a totally different transmission (GM) so I wouldn't think it would fit a SEAT/FIAT transmission anyway.

I've got a 3.5:1 Brava rear axle I've love to drop in my Spider but the trailing arm mounts are diagonally which would require some cutting and welding and I don't know if the drive shafts are correct length. Someone told me you had to use the rear end shaft from an automatic Spider which I don't have.

Also the Brava has rear drums so I don't know if the disc hardware could be swapped over. Sounds like allot of trouble to me but would be much nicer at highway speeds.

[PhillySpider]

my speedo on my 1980 2000 pinninfarina only goes to 80mph. Having your speedo run faster than actual makes your odometer climb faster than it should as well. I think Hyundai was slammed with a class action in the early 2000's for this (8% off i believe). They want you to get out of warranty faster and it devalued your trade. The 5% tolerance is necessary. You'll notice a difference of a few MPH at highway speeds on new vs old tires.

[Turbofiat124]

The other day I went to the flea market and also picked up some 8 track tapes from a guy who answered my Craigslist request.

I had to drive out in the sticks to get them so I used an old Garmin GPS my wife had bought.

I was in my 2006 Ford Explorer and decided to compare the GPS to the Explorer's speedometer to see how close they were. Both read exactly the same or was within 1 mile of each other.

So yesterday I hooked my GPS up to my Spider to compare readings since I thought my phone's speedometer APP may not be accurate.

Well come to find out my phone's speedometer app seems accurate enough. At speeds below 40 mph, the actual speed is about 5 miles per hour less than what the speedometer is showing.

When the GPS says I am going 45 mph, my speedometer is showing 55 mph.

I know the inaccuracy is progressive. Meaning I bet when my speedometer says I am going 85 mph, the speed maybe off by as much as 15 mph.

I think the late Jim Fierst who put a 3.5:1 automatic axle in his 5 speed turbo Spider he built told me he bought a reduction gear device that went between the speedometer gear mounted on the side of the transmission and cable to correct this. Since the automatics were made by GM and the gear drives were not compatible with the FIAT/SEAT 5 speeds. But can't remember where he got it.

[baltobernie]

If you Google "mechanical speedometer correction" or "speedometer ratio adapter", you will find companies that make a tiny gearbox to correct your speedo/odo. You tell them the deviation of your setup, provide a new speedo cable, and tell them where along the cable you want the gearbox installed. They are not cheap, so most Spider owners in your predicament use a "cheat sheet" or mark the face of the speedo at popular speeds.

[Curly]

I'd be thankful that you live in a country that still measures speed and distance in miles.
My ex-Californian Spider has the usual built-in speedo error, but as well as compensating for that, there's the matter of mental conversions of Kilometre per hour speed limits to the mph speedo. ...... and the cops aren't particularly sympathetic when you try to explain .

[Dawgme85]

My speedometer is nearly dead on, both for RPM vs MPH (based on calculated speeds) and actual MPH. Because I run 185/70R13 tires, the tire diameter is slightly smaller than the original 165R13 tires (~23.2 vs ~23.6 in.). This works out to a little less than 2%, which means I'm running about 49 MPH when the speedo reads 50 MPH. At 100 I'm doing about 98. I set up a spreadsheet to calculate speed vs RPM in all gears.

To be honest, I don't believe anybody can accurately determine when they are going any speed except at 10 MPH increments, given the resolution of these speedos . In any case, as long as the tire diameter isn't too far from original, and the original transmission and rear end are in place, the speedo should be well within 5%.

[Nanonevol]

Loaded a speedometer app to my Blackberry and find the Speedo in my '77 with 185/70/13 tires is fairly accurate. I'm trying to get some performance benchmarks to compare for an upcoming carb upgrade. I need an app that records 0-60 times and that sort of thing but I can't find one for the Blackberry so I'm using a stopwatch. I don't have a heavy foot normally so it was interesting to really stand on the pedal for a change! Still I was no where near the 11.9 second 0-60 that this car is supposed to achieve.

[Turbofiat124]

I'd be thankful that you live in a country that still measures speed and distance in miles.
My ex-Californian Spider has the usual built-in speedo error, but as well as compensating for that, there's the matter of mental conversions of Kilometre per hour speed limits to the mph speedo. ...... and the cops aren't particularly sympathetic when you try to explain .

I used to own a 1982 Canadian spec Spider. The only difference between it and a US spec spider was the speedometer was in kilometers. It was either 220 or 240. People would glance at it and think, "WOW this car will do 240 mph"!

The speedometer in my Trabant is kilometers only. I was going to fabricate a template in miles per hour to go over top of it but since I've gotten to where I make a mental note to multiply by .6