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I have a 77 vette, and I'm changing the 350 trans for a 2004r. I have 355 rear gears, I've look everywhere to see what gear I need on the governor so my speedo will be correct. Hope someone else has already figured this one out. Thanks a lot for your time.
Robert Edwards
Retired Marine
30 years
First of all it's the gears in the speedometer drive that needs to be changed; not the governor that is behind it. What size of tires are you running? For the most accurate calculations you need to place a dab of grease onto the tread of your tire then roll it one revolution. Then measure the distance between the grease spots and that'll give you the tire's rolling circumference. Then divide 63,360 (inches in a mile) by that distance and you'll get the tire's revolutions per mile.
You'll most likely need a 17 tooth drive gear and a 43 or 44 tooth driven gear. Give me the tire's rolling circumference and I'll do the calculations for you.
I guess you're not familiar with the Th2004R. The governor gear drives the speedo gear and a combination of the two will make the speedo read correct. 2004R's can have a 10, 11, 12, or 13 tooth governor gear and a driven gear from 28 to 31 teeth. Here is a calculator to help choose. http://www.tciauto.com/tc/speedometer-gear-calculator
edusmc1, while you are about it is a good idea to figure out what governor is fitted to the 200R4 trans you intend to use so you will have a better idea of the trans wot upshift characteristics. The valve body that is fitted is also very important to get good part throttle shifting.
A Grand National valve body and governor are good choices but very hard to find, someone like Ck performance can supply a suitable VB and modified governor.
I have more info on the trans swap if you need it, I had to go with a green drive gear for my speedo drive to get it correct.
There is a calculator on the Bowtie Overdrives site that will help you. The challenge is figuring out the internal gear that was used inthe speedo drive. Pretty easy to pull out the final drive gear and look inside with a flashlight when it's out, but not so when the transmission is installed. I had to change mine twice before I got it right.
Originally Posted by Haggisbash
edusmc1, while you are about it is a good idea to figure out what governor is fitted to the 200R4 trans you intend to use so you will have a better idea of the trans wot upshift characteristics. The valve body that is fitted is also very important to get good part throttle shifting.
A Grand National valve body and governor are good choices but very hard to find, someone like Ck performance can supply a suitable VB and modified governor.
I have more info on the trans swap if you need it, I had to go with a green drive gear for my speedo drive to get it correct.
This is pretty interesting. I've never been thrilled with the shifts on mine though it works perfectly otherwise. My builder said he had changed the valve body when he rebuilt it but I wonder if he left the original Caddy governor in there.
Yes, the governor gear is driven by the output shaft, it has the two sets of weights, one for part throttle shifts and one set for wide open throttle (wot) shifts. The shift points are a balance between valve body spring pressures on the shift valves and the weight of the governor weights. So the shift speeds are dependent on throttle position as well as output shaft speed which will equate to different engine rpm's depending on which rear end gears you have.
I presently have a trans from a Caddy with the AA code, it gives pretty good shift points, about 4800 1-2 and around 4500 2-3 shift at wot. I have another built up ready to install from a Monte Carlo SS with the CZF code, it should give over 5000 rpm shifts. I am running a 3.55 gear.
Yes, the governor gear is driven by the output shaft, it has the two sets of weights, one for part throttle shifts and one set for wide open throttle (wot) shifts. The shift points are a balance between valve body spring pressures on the shift valves and the weight of the governor weights. So the shift speeds are dependent on throttle position as well as output shaft speed which will equate to different engine rpm's depending on which rear end gears you have.
I presently have a trans from a Caddy with the AA code, it gives pretty good shift points, about 4800 1-2 and around 4500 2-3 shift at wot. I have another built up ready to install from a Monte Carlo SS with the CZF code, it should give over 5000 rpm shifts. I am running a 3.55 gear.
Great thx 75.
That gives me 2 new questions again.
1. What governor gear do I need.
2. What governor do I need.
SBC 427, TH2004R, rear 3.70, tyre 255/60-15
Not a race car, but a fast street car.
You will need either a 10 tooth gov gear and a 27 or 28 tooth driven gear. Or you can use an 11 tooth gov gear and a 30 tooth driven gear. Looke into the speedo gear hole and see what color the gov gear is, that will tell how many teeth it has and whether it needs changed or not.
You will need either a 10 tooth gov gear and a 27 or 28 tooth driven gear. Or you can use an 11 tooth gov gear and a 30 tooth driven gear. Looke into the speedo gear hole and see what color the gov gear is, that will tell how many teeth it has and whether it needs changed or not.
The governor and valve body need to be match for the easiest results. Valve body and gov from a 86 or 87 Buick Grand National are the most sought after and the 86-87 Monte Carlo SS are running a close 2nd. Like in the picture above, the performance governors have smaller weights. Does your trans have a tag rivited to the right rear of the case? The numbers on the tag will tell what car it originally came from. The 200r4 experts hang out on Turbo Buicks
Yes, the governor gear is driven by the output shaft, it has the two sets of weights, one for part throttle shifts and one set for wide open throttle (wot) shifts. The shift points are a balance between valve body spring pressures on the shift valves and the weight of the governor weights. So the shift speeds are dependent on throttle position as well as output shaft speed which will equate to different engine rpm's depending on which rear end gears you have.
I presently have a trans from a Caddy with the AA code, it gives pretty good shift points, about 4800 1-2 and around 4500 2-3 shift at wot. I have another built up ready to install from a Monte Carlo SS with the CZF code, it should give over 5000 rpm shifts. I am running a 3.55 gear.
I have the same rear gears as you, do you recall what drive and governor gears you have ? Color and teeth. Thanks or you time!
MY 200 4r build some years ago, I did the tire/ratio/gear/site crap and found out the hard way that in fact my '72 shark was not doing ~150 on the freeway one time.....in fact my car was barely doing 120....
found out the hard way with a GPS/speedo telling reality.....
found out the gear was some ~12% over speed.....found out the tach over indicates by some 15% at higher RPM, did my own electronic measurements on THAT one....
found out the speedo was dead nutz ON when gear was fixed...up to about 80 then it was really 78, at 100 indicated it was really 94.....ran outta room.....MY thought is that GM deliberately did this to the instrumentation for safety and warranty reasons....
200r4 trans swap
