speedometer error much more than it should be

[zachmac]

So, running 205/50 15 on a 35mm offset 7 inch rim versus original 165/R13 on a 5 inch rim with 25mm offset all the size calculators say I should only be seeing a few percent speedo error. BUT, I'm actually seeing more like 15%!!! At 60 indicated I am doing 70 per GPS. That says something other than tire size is at play.

I don't know if the rear end is original or not and haven't checked the ratio yet. So, couple of questions:
1. What rear should a 78 have stock (manual tranny)? My phone GPS says I am seeing 20 mph per 1000 rpm in 5th. I believe that means I have a 3.90 rear gear (assuming .881 fifth gear tranny ratio). The online calculators peg that combo right at 20 mph per 1000 rpm for the tire height I am running. My factory repair book says a 78 manual should be a 4.30 gear?
2. Did Fiat offer different drive gears or speedometers for the different rear end ratios? A quick calculation says changing from a 4.3 to a 3.9 rear would introduce about a 10% error in speed everything else the same. I think the answer is yes, there were 14 and 15 tooth drive gears dependent on the year, at least Midwest Bayless is showing two choices with the 15 tooth being for later cars with the 3.9. Guess I'll pull mine and count teeth?
3. New cable, are these speedometers just that bad? OR are there speedometers specific to the 3.9 versus 4.3 rear ends? (I think maybe speedos are same but drive gears at tranny changed?
4. Does anyone offer a speedo repair and calibration service? I doubt I'll pony up the bucks as the phone app works fine.

[baltobernie]

!. Unless you're the original owner of the car, don't assume anything. During their lives, many (most?) Spiders have had organ transplants.
2. Yes. The speedo gear assembly is external to the transmission, but unfortunately cannot be changed in situ. They were color-coded, and you can find that info somewhere in the archives.
3. No, the speedos are pretty accurate if the correct gear is in place and the tires are OE diameter. All speedos have the same internal gearing, its the assembly on the tranny that's different.
4. I'm sure they're out there, but it would be a lot easier to post in Parts Wanted. Many owners of later Spiders swap to the earlier 140 mph style.

[zachmac]

[quote="baltobernie"]!.
2. Yes. The speedo gear assembly is external to the transmission, but unfortunately cannot be changed in situ. They were color-coded, and you can find that info somewhere in the archives. [quote]

Why can't they be changed in situ? The external cable drive is easy to remove from the transmission with it in place. Does an internal gear in the tranny extension need to be replaced as well or just the external drive?

[baltobernie]

It's a clearance issue.

[spider2081]

I'd be curious what the tach reads when the speed on the GPS is 70 MPH

[zachmac]

I'd be curious what the tach reads when the speed on the GPS is 70 MPH

In fifth gear I'm seeing 20mph per 1000 rpm. So, 70 on the GPS is of course 3500 rpm. I believe that means I have a 3.90 rear gear assuming 0.881 fifth gear ratio.

[bobplyler]

The stock 3.90 rear end used a different panhard rod than the earlier ones. The earlier ones used a straight
one, while the later ones have a kink.