Thread: How do I figure which speedometer gear drive I need after I change rear end ratios???

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....I just switched from 3:00 rear end gears to 4:11's. So I need to switch the gear on my speedometer cable. I went to a transmission site & it said I need the number of revolutions it takes to register 1 mile. I have Ford Classic Gauges & I'll call tomorrow & get this. Then it said I needed the drive or driven gear drive. Is this the number of teeth on the shaft in the transmission that turns the gear that I need to get??? I'm confused!!! I have a Turbo 400, 4:11 rear gears, 30" tall rear tires. Please Help!!! THANKS Bill

20168/30=672

672x4.11x14/1001=38.6

devide 20168 by tire hieght then multiply ratio by that number then multiply drive gear by that number devided by 1001 gives you driven gear 34 to 45 are your choices

Assuming your speedo was right with the old gears, it's just a percentage thing. Count the teeth on your present driven gear and get one that's got correspondingly more teeth. Like, 3.00 to 4.00, if the old one has 12 teeth, you need one with 4/3 of 12=16 teeth.
Never had much to do with 400's. Can you just change the driven gear, or do they come as a set?
Oops! Had to edit . I went the wrong way! Lower gears need the speedo cable to spin slower, not faster. Duh!

Sometimes you have to change both gears to get the proper pitch so they mesh. A good speedo shop should be able to help you,as most dealerships haven't stocked parts for turbo 400's in quite some time,but they did have charts to show which gears worked with the other,they were color coded. Hope this helps you,Hank

I just change my [ 350 trans. ] chevy , gears to 373 . I had to change both drive and driven gears. Drive gear teeth = 7 and Driven gear teeth was 20 .

Lots of calculators here.

http://www.wallaceracing.com/speedo-calculator.php

Jud

Originally Posted by billlsbird

....I just switched from 3:00 rear end gears to 4:11's. So I need to switch the gear on my speedometer cable. I went to a transmission site & it said I need the number of revolutions it takes to register 1 mile. I have Ford Classic Gauges & I'll call tomorrow & get this. Then it said I needed the drive or driven gear drive. Is this the number of teeth on the shaft in the transmission that turns the gear that I need to get??? I'm confused!!! I have a Turbo 400, 4:11 rear gears, 30" tall rear tires. Please Help!!! THANKS Bill

Well, it won't help ya know... But I cheat and use the electronic speedo's, easy to calibrate with the push of a button..... Probably cuz I'm a geezer and hate laying on the creeper and changing speedo gears!!! Also, have ya ever got your ponytail rolled up in a creeper wheel??? Hurts like heck!!! To me this justifies the extra cost of the electronic speedo's!!!!!!!