WTF a 1.50 rear end ratio???

[Suicide-D]

Ok i got to wondering why my speedo reads 83 miles an hour and my gps handheld says 65... I just put stock height tires on my truck thinking the tire height was throwing it off...well that didnt solve the problem.

so I jacked up the truck and did the spin the tire thing and count the revolutions of the driveshaft.... and it only turns 1 and a half times with 1 revolution of the tire. whats up with that??

I have never heard of a gear under 2.73 I'm not saying they don't exist just I havent seen them. Is it possible I have such a low gear? The rear end acts fine doesn't pop or anything with no slack...I get outstanding mileage and the truck could get outrun by a bicycle off the line...but once it's up to speed it rolls fine


any thoughts?

[Suicide-D]

btw I checked 3 other cars with known ratios today and my tire vs driveshaft was dead on with all the others...the glove box says 3.07 on the rear axle?

[jmanz69]

Sounds like you are getting the diff ratio in there. If you have an open diff, then you have to spin both tires at the same time. If not, the differential kicks in and you get the spider gear ratio. Turn the driveshaft by hand, not the tires and try that...

[jmanz69]

If the glovebox says 3.07 then you it's right. You got 1.5 from the differential gears, not the ring and pinion. with an open rear and a 3.07 ratio, you would only see about 1.5 revolutions. If you turn both tires at the same time, in the same direction, it will come out right.

[Suicide-D]

ok then what gives on the speedo?

[Rooster's 67]

drive or driven gear in the trans is incorrect someone has changed one or the other some time back

[Suicide-D]

well it had no speedo gear in the tranny when I got it. I put a brown gear out of my 72 LWB TH350 in it.

[cableguy0]

i bet the truck has 3.07's and at some point the transmission was rebuilt and they put new speedo gear in it and didnt use the right ones so now your speedo is off by 18 mph. you just gotta change the plastic gears inside the transmission

[dinnut]

maybe your speedo is setup for bigger/smaller tires already!

[Cat Fuzz]

I had a 77 Monte Carlo that had 2.56 gears. That thing got 22 MPG with the same 327 thats now in my truck. Only get 11-12 with it in my truck.

[Suicide-D]

well it had no speedo gear in the tranny when I got it. I put a brown gear out of my 72 LWB TH350 in it.

what color gear is right?

[dinnut]

wow cat fuzz thats a big difference

[jmanz69]

I just went to the dealer and bought a bunch of the gears, they're cheap. I think I had 3 different ones. I ended up using the white one I think. If I remember the gears were like $5 each or something like that.

[Hooter]

It's hard to say whcih gear is right, best to play with them, or find a shop that can calibrate speedo's

[jimfulco]

http://www.tciauto.com/components/speedometer_gears.htm
www.novaresource.org/speedo.htm
www.novaresource.org/speedo2.htm

When checking gear ratio on an open rear end, jack up ONE wheel & do the rev counting thing, then multiply the result by two. Better yet turn the wheel TWO full turns & count the driveshaft turns for an exact answer. With both wheels up, the one opposite you can turn backwards & throw your count off.

My '79 Suburban with 3.07 gear, Turbo 350, & 255/70-15's (same diameter as 235/75's) had a green (9-tooth) drive gear on the transmission output shaft and a blue (20-tooth) driven gear on the speedo cable. The speedo read the same as the radar trailer at 50mph.

To slow down a speedo, use either a drive gear with less teeth, or a driven gear with more teeth, or whatever combination of the two gets you the best reading.

[Suicide-D]

I 'll put the drive gear out of the tranny I got the driven gear out of... hopefully that'll clear it all up.

I'm bookmarking those links... Thanks