Correct Speedometer Gear??

[Guest]

I am a somewhat recent owner of a 1970 Stage 1. The original 4 speed transmission must have the wrong speedo gear. It's a Skylark GS 455 with a 3.64 rear running 225 x 70 15 tires. At 2500 rpm the speedometer reads 62 MPH! That's 124 at 5000 and that can't be correct. I don't race the car so I have not time slip reference. Does anyone know the correct color gear or the correct number of teeth the speedo gear should have?

[Guest]

First off, you got one nice Buick there. <P>70 GS Convert gave us the below info on my Tire Circumference thread, I still don't get it as I didn't change the rear gears, just tire size. If anyone can modifiy this to work with a tire change, much ablige.<P>Quote: "(new axle ratio/old axle ratio) x (old speedometer gear) = new speedometer gear<P>The gears come from 34 to 45 tooth. Simply pull the old gear out of the tranny by the speedo cable and count the teeth. Hope this helped."<P>Hope it helps, and maybe someone can tranlate. :-)<P>CB/S<BR><P>------------------<BR>1971 Skylark Custom<BR>[ <A HREF="http://www.minimall.net/buick" TARGET=_blank>http://www.minimall.net/buick</A> ]

[The Old Guy]

If you can find an interstate with mile markings,the simple thing is to check your speedo against them. Find out what speed on your speedo give you 1 mile a minute and that will give you your speedo error. if you run a mile a mn at 40 MPH that will give you the percentage you need to figure what gear you will need.

[Guest]

According to the Buick Assembly Manual, The 4 speed should be a KA, with 14's (G78's,G70's, H78's) you should have a 17 tooth purple gear, PN 3870351. With G60 - 15's you should have a brown, PN 3860341. Hope this helps.<P>My 70 is a TH400 and the RPM's tend to be around (and this is from memory now)28 to 2900 at 62 MPH><P>------------------<BR>George C. Thomas<BR>70 GS 455 12.teens<BR>86 GN Low 13's/ high 12's

[Guest]

WOuld you happen to know amount of teeth brown gear should. Thanks for excellent information.<P> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Thundrr:<BR><B>According to the Buick Assembly Manual, The 4 speed should be a KA, with 14's (G78's,G70's, H78's) you should have a 17 tooth purple gear, PN 3870351. With G60 - 15's you should have a brown, PN 3860341. Hope this helps.<P>My 70 is a TH400 and the RPM's tend to be around (and this is from memory now)28 to 2900 at 62 MPH><P></B><HR></BLOCKQUOTE><P>

[Guest]

Sorry bout that, the brown should have 18 teeth.<BR><P>------------------<BR>George C. Thomas<BR>70 GS 455 12.teens<BR>86 GN Low 13's/ high 12's

[Guest]

I have run into this before after swapping ratios on rear ends, as well as on tire size swaps. I simply went out to the interstate, and watched the ODOMETER, not the speedometer, as passed the mile marker posts. Do this for a 10 mile stretch, your accuracy will be much better than just one mile. If your odometer says you have only gone 9.5 miles when the posts say 10, you have a 5% error, 9.8 = 2% and so on. AFTER you have done this, you can pull your speedometer gear and count teeth. If your odometer showed LESS than actual miles, you want a gear with the corresponding percentage LESS teeth,so that it will turn the speedometer faster. If it showed MORE than actual miles, you want one with the corresponding percentage MORE teeth, so that it will turn slower. Now bear in mind that some combinations of tire and rear gear may not come up exact, but you should be able to get within a half a mile an hour at least I would think with as many gears available as posted in previous replies.<P>Boyd Shuler<BR>Orangeburg<BR>South Carolina<P>------------------<BR>Boyd Shuler<BR>Orangeburg, S.C.<BR>65 Riviera