onofre442

hi all,

trying to find out the answer to this....how many revolution per mile do the speedometer cable turns? trying to get the right speedometer gear... its a 1968 442 thank you

zeeke

There is a lot more to know than that. Rear differential ratio, tire size, trans type are just a few. There is a science to it to which one rarely gets it right the first few times. The first place to start is the differential. There is a formula out there for all this. sooner or later someone will respond with that info.

onofre442

Yeah, I've got all the info except the revolution per mile of the speedometer All I need is this answer

joe_padavano

No, there really isn't. The OP asked the revs per mile of the speedo cable that the speedometer is designed to accept. The common GM speedo is designed for 1000 revs per mile on the odometer. Given that, you can do the math for a specific tire diameter, rear axle ratio, and speedo drive/driven gear set to achieve 1000 revs/mile at the cable.

onofre442

so this number would be most common? I had replaced the old transmission and installed a new tci#211300. Problem is I sold old transmission and I did not take out old speedometer gear and housing. I ask buyer to send pics of old speedo gear so I could count the tooth, so he sent me pics. I counted tooth of speedo gear and I come out with 43 tooth. Color of old speedo gear is light brown, new one with 43 tooth is blue....have they changed colors? Any thoughts out there?

joe_padavano

The 1000 rev/mile is designed into the speedo head. Trans and gears are irrelevant. Pick the speedo drive and driven gears based on your current rear axle and tire diameter. Be sure to understand the difference between theoretical tire diameter and actual rolling radius. Radial tires are worse than bias ply tires due to the more flexible sidewalls. Some tire vendors actually list their tire revs/mile, which accounts for this sidewall flexure.

onofre442

Here is the picture of the old speedo gear. I counted 43 tooth, maybe light brown? New speedo gear with 43 tooth is blue!!!!

onofre442

thank you joe, ive got the numbers of the speedo drive and driven gears. based on the rear axle and tire diameter. my problem is speedometer end cable (square) is not mating properly when inserted into the shaft of the new speedometer driven gear. existing speedometer end cable (square) is too small...maybe there is an adapter that's available to correct this problem, or maybe get a new speedometer cable with the correct cable end...