# Speedo Question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by bada55_tx_k5, Jun 3, 2003.

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The speedometer on my 80 /forums/images/graemlins/k5.gif is non-functional and I'm trying to figure out if the guage is broken, the cable is warped or stretched, or if I'm way off base. What are some things to check in the process of diagnosing the problem. Also, until I figure out the problem, is there a way to tell my speed from my RPMs... I have 38" tires, 4.10 gears, and a 350/th400/np205 in rwd most of the time. I'd like to know when I'm doing 40-45ish as most of the roads in this area are that speed. Thanks guys

2. ### Skigirl1/2 ton status

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mph = rpm x tire diameter / gear ratio x 336

/forums/images/graemlins/thumb.gif

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so 45 = (38x)/(4.10*336) right?

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Well according to my calcs that at 1631rpms I should be doing 45... I'm assuming thats in 4th but how would it differ btw a 3spd trans and a 6spd trans?

5. ### Skigirl1/2 ton status

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Yep, 45 mph is right for 1631 rpm.

Don't know how anything differs between a 3 and 6 speed.

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Well say my car is in 3rd gear then it would be at a higher rpm at 45mph then if it was in 4th gear... right? So just curious if I had say a 3spd tranny instead of a 4spd if it would make a difference. Thanks for the help though... should help when I'm cruising along without my speedo to be able to keep an eye on my rpms

7. ### dyeager5351 ton statusPremium Member

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Final drive is all you are concerned with if you are measuring the "top" gear. Pretty easy with no OD, ratio is 1:1. You'll be turning (esentially) the same RPM with any transmission with 1:1 top gear at the same speed.

Overdrive is typically where you see the non 1:1 "top" ratio, .70 in the 700R4 for instance.