speedometer issue

[matthufham]

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?

[Longhorn Man]

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.

[matthufham]

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

[jacobs]

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

[Longhorn Man]

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

[Longhorn Man]

Since you haven't mentioned your rear gear ratio, I'll assume your truck was originally a small block with automatic...

the formula for determining the proper gears for any application consists of multiplying the rear axle by 20.2, and dividing that number by the tire diameter. (rear axle x 20.2 / tire diameter)

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.

[matthufham]

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

[Longhorn Man]

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.

[matthufham]

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.