Speedometer Stamp Number?

[Conserv]

Use Beaner's formula given above and calculate the correct number for your car. Use his original wheel size for this calculation. Basically the calculation determines the number of driveshaft turns per mile then divides by the spedometer gear ration that the transmission has. All the thransmissions use the same ratio so it will be 2.5 DS turns to 1 speedo cable turns. There are only a set number of speedo ratios made so one cannot exptect to get one that exactly matches your tire size.

Thank you, but that's not my question; I've obviously been unclear (or the fact that my inquiry is #14 in a 7-year-old thread, has simply confused matters). Curt Ingraham's chart shows a W=1.39 speedometer. Does it exist? If so, what is its date of manufacture?

If my April '76 car came with a W=1.393 speedometer, manufactured March '76, then which cars came with W=1.39 speedometers (which would appear to be the most accurate for a 3.90 diff)? If the W=1.39 existed, did it come before or after my W=1.393 unit? Or were some W=1.393 speedometers simply marked W=1.39?

Thanks,

Steve

Edited July 28, 2013 by Conserv

[bnam]

As a late-ish '76 car, and since the 320is were already in production with the 1.393 perhaps they also switched the late 76 2002s to the 1.393.

[Conserv]

As a late-ish '76 car, and since the 320is were already in production with the 1.393 perhaps they also switched the late 76 2002s to the 1.393.

I think that's a really credible explanation! Now I'd just like an owner of, say, a January '76 example to step up and say "Yeah, I've got a speedometer marked W=1.39"!

Thanks and regards,

Steve

[M3M3]

but.....

even if you are using the matching diff and stock sized tires and wheels,  there is no guarantee the 40 yo speedo will be accurate.  in fact, it won't be.   best bet is to have it rehab'd and calibrated somewhere like north hollywood speedometer.  then you can have it cal'd to your set up of diff/tire/wheel combo.

[Conserv]

but.....

even if you are using the matching diff and stock sized tires and wheels, there is no guarantee the 40 yo speedo will be accurate. in fact, it won't be. best bet is to have it rehab'd and calibrated somewhere like north hollywood speedometer. then you can have it cal'd to your set up of diff/tire/wheel combo.

Marshall,

You're right and that's what I will do: have it calibrated. Nonetheless, my "Rochester-to-Syracuse at 115 mph" run, ca. 1978, certainly suggests a speedometer error consistent with today's 8% error! (At the time, of course, I just figured the factory was ultra-conservative, I was a better driver than those factory "hacks," and/or my '76's motor was one of those special motors that, by chance, was perfectly balanced from new!)

And, of course, after 40 years with countless sets of 165HR-13 XAs's (my '76 came with XAs's, my '70 came with XAs's, and, although my '67 was on Continentals by the time I purchased it, I quickly converted it to XAs's) it is seriously doubtful I could ever switch -- at least not while the Michelin-Passion division, with Coker Tire as its U.S. retailer, feeds my addiction!

Thanks and regards,

Steve

Edited July 29, 2013 by Conserv

[M3M3]

^{pretty cool that you have stuck with the stock tire...and brand Steve!}

^{i wish the speedo on my E30 was as accurate as yours...consistently reads 10% high.  ODO reads accurate, but speedo is whacked!}
 

[Conserv]

^{pretty cool that you have stuck with the stock tire...and brand Steve!}

^{i wish the speedo on my E30 was as accurate as yours...consistently reads 10% high.  ODO reads accurate, but speedo is whacked!}

Thanks, Marshall!

And now I have to figure out if my odometer is also 8% overstated!

Regards,

Steve

[Beaner7102]

Marshall I believe all E30s are over by 10% out of the factory. I have learnt to drive mine looking at the tacho. But it is a pain, as I just swapped tires around from the E30 to the 02 and I need to tea learn the revs again. 

Beaner7102