stuckinthe70s

Recently changed from Th350 to a 2004r and now the speedometer is reading 10 mph less than actual road speed when measured by gps. Would I be right in thinking that the odometer would not be recording milage correctly also? Thanks

cdrod

Here's a link to a speedo calculator on TCI's website. Plug in values for your final gear ratio, tire diameter and drive gear teeth, and it will calculate the 2 speedo gears for you. The drive gears were available in 10, 11, 12, & 13 tooth versions. I've been told that the most common drive gear for a TH2004r was the 11-tooth, orange gear. HTH.

Rodney

http://www.tciauto.com/tc/speedometer-gear-calculator

midnightleadfoot

yes,


What the 2004r come out of??
That narrow what the original car has for gears and tire dia.
example.. a 86 monte SS had 3.73 gears and a 26" tire
a buick g.n. 3.42 gears and 26" tire.


and what gear in your car? and tire dia?


problem is the drive gear on a 2004r if it needs to be changed you have to pull the whole trans apart.. the driven gear is an easy swap.. but if the drive gear needs to be changed. might as well rebuild the trans while it is apart..

Firewalker

Get someone to get on the highway with their car, or any place 50 mph or more won't get you a speeding ticket and tell them to go exactly 50 mph, and mirror their speed and look at your speedometer says. Now go back home and double your speed and pretend they were going 100 instead of 50, and you have the percentage off up or down you are going, at any speed. Now with a calculator, if you can't easily do it in your head, and that that percentage you can figure out what the true speed is, for whatever the false reading is on your speedometer.

I have a car that reads 10% less than what it actually is going because of a gear change, for example and so when it was showing 30 I am going 33. At 35 it is 3.5 more, or 38.5, at a reading of 70 it is actually going 77.

This is not the first car I have had that reads speed wrong because of a gear change or different rear tire sizes, and I can do the math and know my true speed off my incorrect speedometer readings on the fly, and I feel no great need to get them right on the speedometer, as only my brain needs to know the correct speed.

joe_padavano

Well, there's only one speedo cable from the trans to the speedo, so yeah, the speedo and odometer are seeing the same revs per mile on the speedo cable. If one is off, the other is off too.

The speedo gear in a 200-4R is part of the governor. You'll need to know rear tire diameter and rear axle ratio to determine the correct speedo drive and driven gears.

bccan

Note on speedo gear calibrating - not that much disassembly involved but pan does have to come off. If you don't have a drain plug in pan, do yourself a favor & install one when pan is on bench, will make life cleaner & easier next removal. You can even capture fresh fluid w/ funnel & bottle rather than dumping it in a "big" drain pan. It tends to be a trial & error process calibrating governor & speedo gears but worth it once you get it right. If shift points are acceptable a little careful homework & procurement might allow you to get speedo correct with only one drop of the pan.

olds 307 and 403

The TCI gear calculator has the tire height calculator. I have found most 2004R's have the black 27 tooth drive gear. Easier to change the driven gear attached to the governor.

Firewalker

200r4 Speedo Gears Calculator

http://highperformanceolds.com/phpbb2/viewtopic.php?f=7&t=5517

Vistabrat72

If the shifting points are fine, and happening at decent speeds depending upon throttle position, then what I did when I put the 200-4R into my vehicle AND 3.73 final drive, things were a real mess in the speedometer area, i did just as suggested above for exact speed calculations. With this I purchased a simple adapter which in my case, slowed down the output speed from the speedometer outlet on the transmission to the proper RPM for the speedometer. I have pictures somewhere of this gadget. My speedometer is now right on, as is the odometer. These devices can be found on E-bay. Not saying this is the exact part, but until I find my pictures, here is an E-bay example. http://www.ebay.com/itm/Stewart-Warn...lZV6EA&vxp=mtr

JohnnyBs68S

Why do all that when you have a GPS as the OP said. GPS is way more accurate.


Keep in mind that even if you get the "correct" speedo-gears for your tire size and final drive ratio, the speedo could read incorrectly if the needle is skewed resulting in an "offset". In my case, my speedo has a 4 MPH offset: Relative to GPS, it reads 4 MPH faster regardless if I'm going 20, 30, 40, 50, 60 or 70 MPH. So while the "slope" of my speedo (change in road speed vs. change in reading) is correct, it has a 4 MPH "offset". I haven't checked the ODO over a long enough distance to tell if its recording mileage correctly, but its probably as close as I can get w/ the speedo-gear selection.

Oldcoyote

I changed from 2.41 to 3.73 on my 1988 Cutlass Took it to a speedometer shop and they installed an adapter. Also did this on a 1989 Olds wagon. Cost was quite reasonable.