does this sound right?

[gadget73]

Installed the tach tonight, and it makes it seem like my speedo is really off. It seems to read fast. Tag says 3L55, and it has the white speedo gear, which is supposed to be correct for 3.55 rear. Tires are stock 205/70/15. Here is what I see driving down the road in OD

RPM speedo readout
1400 53
1600 62
1800 70
1900 75
2000 78
2200 87

I cruise keeping up with running traffic on I-95 showing 87 on the speedo, but normally when I drive my truck I see about 75 mph normally on that stretch of road. Any thoughts on why this thing is so whacked out? To my knowlege the car was 100% original when I got it. Trans has been replaced, but I kept the same gear, and it was off before the engine/trans swap. I don't think the rear has ever been apart.



Scott: that trans def has the low speed governor in it. 1-2 shift at 4150 rpm, 2-3 at 3800. Must change that one of these days.

[Lincolnmania]

the correct speedo gear is orange
215/70/15 is stock tire size
there is no spoon

[Blaze86Vic]

Here is a cool calculator for just that. But you need to know how many teeth the speedometer cable drive gear has.

http://www.sccoa.com/faq/speedgr.html

Yeah that white gear has 23 teeth, that is usually used for people with 4.10 gears.

[gadget73]

Hm, interesting. Some charts match the white one to 3.55, others match it to something else. I asked my friend if he knew if anything had been done to the rear or trans on the car, and he said to his knowlege nothing had ever been done to it while the original owners had it.

Looking around some more, it seems the 23 tooth gear is correct for 3.55 rears with a newer 8 tooth driven gear transmssion. Seems thats the standard from 90 onward, but both my old and current trans are 1986 units with a 7 tooth driven gear. I should have a 20 tooth gear on my car. Wonder why/when that was changed, unless its either always been wrong, or parts had been replaced on that car and I don't know about it.