speedometer issue
 

i have bought a new speedo cable and housing, new speedo (the odometer messed up my last one) and i have messed around with the driven gear a few times.

the tires are smaller than original, they're 60 series on 15 inch rims. the transmission is a th350. the speedo drive gear in the transmission was yellow-ish so that means it's a 19 tooth. the driven gear i had was brown-ish and it should be a 18 tooth, i proved it was an 18 tooth because i counted. i bought a natural colored driven gear that is a 19 tooth.

with the 18 tooth driven gear my speedo read 10-12 mph too slow
with the 19 tooth driven gear my speedo reads 10-12 mph too fast




can anyone help me?

Re: speedometer issue
 

there's calculators on line .. I'll see if I can find a link, you'll need to know the diameter of your tire. 60 series tells you nothing without the cross section width too.

Re: speedometer issue
 

275s on the rear, i found one at tci but i couldn't get it to work.. it told me my tire height however is 27.99999 or 28 inches tall

http://www.tciauto.com/tech_info/speedo_gears.htm

Re: speedometer issue
 

Use a speedometer adapter. They are available from many sources including eBay.

Re: speedometer issue
 

here's some reading and guidence for gears;
http://www.novaresource.org/speedo.htm

Re: speedometer issue
 

Since you haven't mentioned your rear gear ratio, I'll assume your truck was originally a small block with automatic...
Taking that formula...and my assumptions of your present gearing... we'll take 3.07 X 20.2 which comes out to 62.014. Now divide that by your 28 inch tall tires, and we get 2.21485714. Lets just call that 2.21.
Looking at the chart, there's a few ways to get the 2.21.

Re: speedometer issue
 

with my 3.73's i get an answer of 2.69, and the closest match is a 2.71. that calls for a 7 tooth drive gear and a 19 tooth driven gear (which i have). i looked for a 7 tooth drive gear and couldn't find one on tci's website, it went as low as 8 for a gm transmission. what's the deal

Re: speedometer issue
 

take a look again, they list it there.
You can go to the dealer for these parts still, most places will have them on hand or get them pretty quick.

Re: speedometer issue
 

it's on their table that lists the colors designated to their teeth count, but wasn't on the table where you could buy them. i found a 7 tooth drive gear on ebay for a th350 for $8 bucks. i think i'm gonna "buy it now" since it's not too expensive. thanks for the help, i'll keep you updated.