Speedometer off, or am I nuts?
 

Today, returning from Currys DE inspection I was tooling along at 80mph down 66 when a great-grandmother type passed in the right lane driving a 90's Buick Roadmaster. Is my speedometer optimistic or is travel on the interstate always 75-85 mph now. My tach shows @3000 70mph, 3100 75mph, 3200 80mph. 5th gear in a portly 79SC. Is this about right?
ARF
:roll:

Need the tire size to make calculation. Final drive effects also, but generally SCs only had a couple of choices.

Front are new Kuhmo 205/55 ZR16 [91W?] ; Rear Dunlop SP Sport D40 MS P225/50 ZR16 with a few miles. I noticed (like I should have been smart enough to look there first) that 80mph is 3500 in the owners manual.

Re: Speedometer off, or am I nuts?
 

I have read that due to a law in Germany that no speedometer can register "less" than the true speed, all German cars are optimistic. However, I have heard BMWs are the worst. I don't know if this law was in effect when ARF was born though.

My car seems strange sometimes as well.
David, mine is a euro that had a US spedo put in it when it came here.
Is there a chance the two don't match? I think I herd the gear rations are a little different on the euro cars. Is that true? I guess that would effect the odometer as well.

Can you point me anywhere for some info? Is this in the little spec books?

OK, since no one answered the question, I can tell you that my 83 indicates exactly the same speeds, at exactly the same RPMs (independantly verified by CHP. . .). No ticket, just a "strong" talking to, as he had to go chase a lawbreaker on I-5 that was in hyperspeed. :P

It is not necessarily an either/or question. Your speedometer could be off AND you could be nuts.

I found my little spec book and sort of answered some of my own questions. Sort of.

The difference between the 82-83 US and ROW gears are only in 2nd and 5th.

Type 915.62 (R.o.W.) 2nd 18/33 , 5th 28/22
Type 915.63 (USA) 2nd 18/32 , 5th 28/23

Does this indicate the # of teeth? Is the USA taller/shorter in 2nd and 5th?

I couldn't find any info on the final drive ratio. Is it the same?
Isn't the spedometer pickup on the final drive anyway? If so the transmission gearing wouldn't effect the spedometer and I guess the speedometers would be interchangeble.

For what it's worth. I did a test last weekend and at 3000 RPM in 5th I read 75mph. I knew it read high but I expected more (like +10).

I guess people are really running at 75-80 on a regular basis.


Brian