another speedo question
Thread Starter
Advanced
Joined: Jul 2005
Posts: 86
Likes: 0
another speedo question
I have an 85 that was a 4+3, but now it has the ZF6, so now my speedo is off about 7mph. I need to know what gear to get but i don't know what gears are in the rear end. there isn't a build sheet in the consol or the rear storage compartment. I am also going to get some larger wheels (ZR1s) if that makes a diff.
thanks
thanks
Burning Brakes
Joined: May 2004
Posts: 1,230
Likes: 27
From: Athens Ohio
I don't know about the change in the transmission, but the change in wheel size should not effect the speedo. I am assuming the wheel change is from 16's to 17's. With the lower profile tires the circumfrence still comes out the same. I have an 86 with 17's and my speedo is dead on with my GPS speedo.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
You can jack up the car and turn both rear wheels exactly one revolution, while counting the drive shaft revolutions. The number drive line turns equals your rear axle ratio. I can determine what rear end ratio your old trans was set up for if you'll give me the tooth counts and/or the colors of the speedo gears in it.
RACE ON!!!
RACE ON!!!
Team Owner
Joined: Sep 2003
Posts: 25,394
Likes: 2,741
Speedo error!
Speedometer errors are a percentage of the display! A 7MPH error at what speed?
CFI-EFI's method for rear axle ratio can get you CLOSE!
3.33, 3.45, 3.54 are pretty close and if you've one of these it's tougher unless you use a pointer at the pinion and "plumb" the mark on the tire rotation.
What year is your ZF? If it's a '91 or newer I believe you need to use '90 or earlier speed sensor. All ZF's from build use a 15 tooth drive gear and there are various calculators out there to arrive at driven gear. There are calculators "OUT THERE" that are "FLAWED" !
Leave this calculator at 1000 and it's "spot on" for a known "drive gear" in nearly all instances! In your case "15"!!! If you arrive at a number greater than 45 it gets tougher. If your axle ratio is 3.75 or less you'll be fine. Complications usually only surface with 3.90 or higher gears.
http://www.bgsoflex.com/auto.html
CFI-EFI's method for rear axle ratio can get you CLOSE!
3.33, 3.45, 3.54 are pretty close and if you've one of these it's tougher unless you use a pointer at the pinion and "plumb" the mark on the tire rotation.
What year is your ZF? If it's a '91 or newer I believe you need to use '90 or earlier speed sensor. All ZF's from build use a 15 tooth drive gear and there are various calculators out there to arrive at driven gear. There are calculators "OUT THERE" that are "FLAWED" !
Leave this calculator at 1000 and it's "spot on" for a known "drive gear" in nearly all instances! In your case "15"!!! If you arrive at a number greater than 45 it gets tougher. If your axle ratio is 3.75 or less you'll be fine. Complications usually only surface with 3.90 or higher gears.
http://www.bgsoflex.com/auto.html
