Speedometer WAY Off

[4WDVETTE]

Need help with getting the speedometer within reason. I have a NOM 427 and a variable pitch turbo 400 in my 66 coupe. Finally got a speedometer cable that would fit AND work, however it is reading about 25% too high. My son was driving my T/A on cruise control at 50 mph, I was reading about 78 mph. How do you go about deciding which color gear to get or do you just buy a bunch and go for the trial and error method. Help please.

Steve

 

[magicv8]

What color gear is in the trans, and what year is the trans?

When I had a TH400 in my car, it rubbed holes in the fiberglass floor. How did you mount it?

 

[WayneC]

Your math is off... reading 78 when you should be reading 50 means your speedo is reading 56% too high. Wow, that's a huge difference!

You need to find out what color (or number of teeth) gear your TH400 transmission now uses for it's speedo driven gear, and choose a new driven gear with about 56% more teeth. The gear must be chosen from a selection that mates with one of the two possible driving gears now in your tranny; selection of driven gears for one of the two possible driving gears ranges from 18 teeth to 22 teeth (I doubt your transmission's gear is in this range), while the selection for the second (probable) driving gear ranges from 22 teeth to 45 teeth. The complete selection is shown in the Dr Rebuild catalog:
http://www.docrebuild.com/dr-r-web/ASH-TRAY.PDF

Buy a gear with close to 1.56 times as many teeth as your current gear. So for example, if your current gear has 25 teeth, you want a gear with 25 x 1.56 teeth, or about 39 teeth, so you'd buy the brown 39-tooth gear. At least I think that's the right calculation, perhaps others can double-check my reasoning.

Dr Rebuild's narrative would lead you to buy a 31-tooth gear (1 tooth per 5mph too fast at 78mph = 6 more teeth), but I suspect the narrative is incorrect.

 

[4WDVETTE]

Magicv8 - thanks for the reply. Don't know which gear is in there yet. Hope to find out one noght this week. I utilized an additional 3/8" spacer at all body mount points. Was looking for some additional tire clearance and also received extra space for the trans.

WayneC - great info - Thanks. Will hit Dr. Rebuild site today and when I get the gear out, I will make the choice. I will also make sure of the actual speedometer variance. Your discussion regarding the percentage off calculation makes sense. Thanks again.

Steve

 

[lonestarvette64]

One sure way to verify your speed is find the nearest state trooper hiding behind a tree and blow by him as fast as you can.... the radar never lies!!