[Bill]

I have an indicated RPM of 4,300 at an indicated speed of 70mph in my '78 Spider. Is that possible with a 4:11 rear end? I feel it may be a 5:11 but have no knowledge of that coming from the factory. Anybody have an idea?

 

[Bruce Colby]

Both the tach and speedo readings are most likely inaccurate. A 10-15% error is not unusual for the speedo. A decent electronic tach should good give you an accurate rpm reading for comparision. A stopwatch and a section of road with a measured mile will help with the speedo. Also, if you have access to a gps you might try getting a speed reading from it while you are underway. It helps to have a passenger actually take the reading and it woeks best on a straight piece of road.

 

[144]

Sounds about the same as my 74. Do you have standard size tires? I doubt seriously if your axle has a non-standard ratio.

Check out:

http://www.veloce.nu/

Click on "Mech Index," then "Speedometer" to see how different tire sizes affect speedometer accuracy.

 

[jamieandthemagi]

Put a multi meter to the negative on your coil, this will give you an accurate rpm.

So get someone to hold the cars revs at 3000 rpm on the dash Tacho, & get the multimeter on the coil to see if the tacho is giving the correct reading.

The Alfa handbook normally tells you what rev in what gear gives what speed, so once you've confirmed or fixed the tacho, you will be able to see if the speedo is right.