Hey all, I got absolutely nailed by a state trooper about a month ago, and after getting a speedo calibration I realized that my speedometer was way way off. Probably the bigger tires, that’s my fault for assuming the PO re-geared the speedometer for them.

Some context on my transmission: it’s a 700r4 from 1987, and has never been rebuilt or majorly repaired. It has a little under 100k miles on it. It runs great, the only problem is that it bangs into gear from P or N into D or R. New U-joints, new trans mount (rubber GM genuine). I’m going to re-check the idle speed soon. When I changed the fluid last month, it looked great and there wasn’t a concerning amount of clutch deposits on the magnet. I’m not worried about its health apart from the age.

So for my actual question: I understand that you have to remove some parts of the trans, such as the tail shaft, in order to get to the drive gear. What would you guys recommend I replace while I’m in there? The car is a daily driver and isn’t going to be ridden hard often. What’s some common wear items that should be taken care of?

So far I’m thinking of replacing the transmission mount, the speedometer cable, the speedo gear, the output shaft seal, and the torque arm bushing. Might do the U-joints again just to be safe, especially since they’ll be so much easier than last time (screw that injected plastic!).

Thanks in advance!

And if you have any tips, pictures, parts and gaskets needed, or videos on speedometer gear replacement for a 1987 Firebird with the 700r4 auto, drop em here

Before taking stuff apart, determine what speedo gears you need, and have them both on hand before you start. Beats the crap out of disabling the car, then figuring that out, then ordering and waiting, or worse yet discovering that whatever you need is unavailable.

Replacing the speedo cable outright is probably unnecessary. Wouldn't hurt to clean and lube it though. That needs to be done from the end behind the dash though.

Might want a new speedo "bullet" if there's any signs of leakage around it.

Def the TA bushing and trans mount. Again, determine which TA bushing you need before starting. There are 2 types; one with the "lips" of the TA pointing toward the transmission (looks like "E" from behind), the other with them pointed away (looks like "3" from behind). Saves you the trouble of ordering both and returning the one you don't need. No you can't "look up for your car" and get the right one for sure; they're seemingly totally interchangeable in what cars they were installed in, and the ONLY way to know is to LOOK.

Change the drive shaft yoke bushing as well as its seal.

Can't speak to a need for U-joints, but it rarely hurts to replace them, except in the wallet. Noises and whatnot when shifting into gear can also be pinion bearings but that's a bit outside the scope of the project at hand.

A certain amount of fluid may come out when you pull the extension housing, so be ready for that, and have a quart or 2 on hand.

I agree with Sofa. Taller tires shouldn't affect it that much. How much are we talking?

Hey sofa, thanks for the response. Based on the RPOs and the 86 GM F car speedo gear sheet, my drive gear should be the grey 15 tooth and the speedo gear I need is the 39 tooth. You’re right though about checking it first, I’ll dig into it as soon as I get it back from the body shop.

I also agree replacing the speedo cable is most likely unnecessary. I replaced it over a year ago with a cheap pioneer brand one just so it would stop the GOD AWFUL squealing. My thought about replacing it again was that this time I could get the (assumably) higher quality GM genuine part.


Hadn’t thought about the speedo bullet. Do you have a preferred brand/source for them?

Thankfully already figured out my torque arm bushing Forget the direction but I know I need energy suspension 31111G, NOT 31112G.

I’ll put the driveshaft yoke and bushing on my list. Probably will do the U-joints again anyway unless the other more important parts go over budget. Removing the factory u-joints went terribly and took way longer than I expected. Wouldn’t be surprised if I broke, loosened, or otherwise messed something up during the installation of the new u-joints. And I’ll grab some more trans fluid. Thanks!

I’m at work right now so I don’t have the exact specs, but off the top of my head, the car left the factory with 195/r15 tires. Unsure of the sidewall. The car had 235/60/r15 Winston winner G/T tires on it when I got it, and I have an under a year old set of Mastercraft avenger G/T 245/60/r15 tires on it currently. So roughly 50mm over the stock size. I assumed that the PO re-geared the speedometer when he changed tire size (wishful thinking) but given the amount of factory original parts I’ve had to replace, I’m willing to bet it’s an original gear.

The speedo calibration sheet says that the speedo is under by 3 at 25 and 35, under by 4 at 45 and 55, and under by 5 at 65 and 75. Calibration was done on a cold-ish morning so there was a bit of needle float. In my experience, the speedometer is more or less accurate until you hit 55, then it gets off by 5 mph too slow almost exactly, and then above 75 it gets off by random higher amounts.

Also not that this matters that much, but for extra context the gear ratio is 3.42:1

I'd suggest MEASURING the tires for their ACTUAL dimensions, not guessing based on the trade size. Might make a difference, might not. Only way to know is to do it.

Verify that the tires are filled with air to the correct pressure. Park the car on a level surface. Pull a tape measure from the ground to the EXACT CENTER of a rear wheel. That will give you the ACTUAL radius of the rear tires when loaded, which of course is what matters, NOT whatever it is whenever it's... doing something else. If your "calculator" asks for tire dia (bald-faced stuuuupid to do that, butt hay, that's what most of those things do), then double that number to get the correct dia to enter.

I did the math using the sticky on this forum and also the calculator on Jegs’ website, both gave me a 39 tooth gear. I used the official diameter of the mastercraft tires I’m using, 26.5’. I hear you though, I’ll re-do my math tomorrow when I have the car with me and it’s ready to be measured like how you laid out.

Yup, "the math" depends on having THE RIGHT inputs.

Garbage in, garbage out.

Measure. All that "calculate" crap is not reliable AT ALL. If you ever go to work around tires you'll see what I mean. The amount of variability in them is STAGGERING. From one lot to another, let alone from one mfr or model to another. And of course the pressure you run them at. And how much tread is on them (ceteris paribus, a typical street tire's diameter changes by nearly ¾" from new to wore-out). SO MANY variables.

I can type "2 + 2" into "calculators" all day long, and EVERY ONE I ever use will tell me it's 4, butt if the quantity in question isn't really 2, then NO "CALCULATOR" IN THE WORLD is gonna give me the right answer, whatever it is.

All very true. That said, what’s the best way to figure out which speedo gear I need? Use the equation Atilla provided in the sticky once I have my own radius/diameter measurement? Or is there a better way?

I'm a bit confused. I thought 1987 Firebird used an electronic speedo and electronic VSS, not cable drive.

So every Firebird from I think 86 onwards did except for the base model, which still used the cable driven speedo. I forget the RPO but mine has the code for a cable speedo, so it wasn’t a weird swap by my car’s po. Guess it was because GM didn’t want to update the base model’s gauge cluster? Who knows. My bad though, should’ve added that it was a base model in my op.

Once you have the valid input data, any "calculator" you use will give the same result, if it's written right. Or, you can just do the arithmetic yourself. It's WWWWWWWAAAAAAAAAAYYYYYYY too eeeeeeeeezzzzzy to rely on some dumb "calculator" that you can't see the workings behind the front end.

The tire rolls 2 x pi x the MEASURED radius, per revolution, in inches. There are 5280 x 12 inches per mile. Divide 5280 x 12 by the tire rollout. That's how many times per mile the tires turn. You'll probably come out with something in the 700-800ish range. Take a look at your tires; if they're brand new, then over time, they'll get smaller, and whenever you have roundoff error, such as in gear tooth counts which obviously have to be whole numbers, you can choose whether to round up or down, and maybe even by about how much.

Multiply that by the gear ratio. That's how many times the drive shaft turns per mile. You should come out with something in the 2800 - 3500 neighborhood. You can make it a bit more accurate by using the gear tooth counts rather than the ratio; for example 3.42 is actually 41 & 12, so, multiply by 41 and divide by 12, instead of using the 3.42 roundoff.

Since the speedo needs to turn exactly 1000 times per mile (NOT 1001 as is often claimed: a brief look at gear trains that can indicate tenths of a mile like in older mech speedos will instantly show you why, as there's no possible gear train you can build that will indicate tenths of a mile and then divide by 100.1 to get whole miles), you will need gears in a ratio of somewhere near 2.8 to 3.5, in a combination that you can actually buy. Not all possible gear tooth counts are made, and among those that are, not all can actually be bought at any given moment. If some "calculator" comes up with 39 & 13, that's believable, as far as it goes; and those AFAIK are gears you can buy. If you come up with something like 13 and 39.4 or 38.6, you might want to select 40 or 38 instead of 39, based on your tire wear estimate; like, perhaps if the tread is new, the speedo will read 1% low today but will gradually drift through being exactly correct and eventually read 1% high as the tires wear. Each step of driven gear (the higher #) is about a 2½% change obviously, 1 ÷ 40 being .025, so that's the "coarseness" of the choice you're working with.

Another technique you can use is to get on the freeway and drive with the speedo as it currently sits reading exactly 60 mph (exactly 1 mile per minute) for like 5 miles, and time it with a stopwatch. So if for example you do this, and you end up driving 5 miles in let's say 5 minutes and 15 seconds, then you know that the gears you have NOW are 15 ÷ 315 in error, making the speedo read low. That example would be 4.76% error, so based on that, you could LOOK AT the gears you have, and buy new gears in a ratio that differs by that much. So for example you might discover that your current gears are 13 and 40, and in that case you'd be able to just buy the 38 that goes with your existing 13 and not have to mess with that one. They have to be matched that way; a 40 driven for a 13 drive may be different from a 40 driven for a 14 drive, because the angle the teeth are cut at may be different.

As you can see, you can be MUCH more intelligent about this than "calculator" if you just use your brain. I wish I could do that, butt I don't have a brain, so I'm kinda left out in the cold.

Here's a place you can go that usually has most of those things in stock. https://transmissioncenter.net/?prod...edometer-gears No doubt The Google knows of others and will be glad to tell you if you ask it.

Yes your car probably has an electronic speedo; butt the sensor is gear-driven. So all the same rules as for mech speedos apply to yours. The driven gear is different from one for a mech speedo (drive gear is the same) butt the whole tooth count deal works the same.

to sum up:
Pop the driven gear / bullet out. Find a way to safely count drive gear teeth. It's 17 or 15. Neutral and a sharpie may be involved. Don't order an unneeded gear. And don't pull a torque arm, etc. when the drive gear is already correct.

Then, spend your time looking at the GM speedo gear charts in the sticky threads at the top on this forum.

I have owned and serviced 700R4 and 200-4R and T5 and other transmissions with validated original speedo gears that read accurately according to roadside mile markers and radar readings. Every time, the drive and driven gears matched the GM tables.

NONE matched dozens of online speedo gear calibration websites.

Just go by the tables. If your speedo, and odometer clusters, and cable, and gears are in good shape, you'll be all set.

Ideally, you may already have the right drive gear and can swap out the driven with one or two that you order, and be all set.

And yes, there are service books that go into more detail with math to be done. But your car is close enough to stock, you can streamline and get done.

The stock tables work right IFF (mathematical language for "if and ONLY if") the tires have the same rolling RADIUS (NOT diameter) as the stock ones.

Butt yes, the factory's speedo gears are virtually always within 2½% or so (the granularity inherent in whole numbers of teeth near 40) for any STOCK combination of rear gears and tire sizes that they offered.

If your setup is NOT stock, then the factory choices can't really help much.

Drive gears generally cover a range of rear gears, very much like the "series" of rear axle carriers. The greater # of teeth on the drive gear goes with lower rear axle ratios. So if your rear gears are stock, as seems likely, then odds are the drive gear you have won't need to be changed. Still, best to check before "assume", and you get the result we all know THAT produces.

Hi KITT87, I'm left wondering, how exactly did you get a "speedo calibration"?

My favorite method of checking is a Speedometer app that uses GPS in my phone, I've found that to be very accurate when checked against those roadside radar units that display your speed as you drive past. My totally stock unmolested car is dead accurate up till around 70 MPH. Going faster than 70 results in the car's speedometer indicating higher than actual speed, by the time the GPS says 90 the speedo is reading around 94 to 95 MPH.

Hey Sofa and JMD, thanks so much for the detailed responses. I just got done putting in a new heater core but the speedo fix is next on my list. I’ll continue to update and respond to this thread in the meantime

Couple things. For me personally, I checked the speedo vs the Waze GPS app, and it showed the speedometer was very close everywhere except 55 and above. Above 55mph it was off by between 2-5, and 5+ (seemingly random) above 70. In my experience, Waze is +-2 from the speedometer speed. One other thing that may or may not affect things is that my car has the 85mph speedometer. I haven’t gone fast enough long enough to get a good reading on Waze at 85mph on the speedo.

Now the actual “official” calibration was done by a local company who specializes in speedometer calibrations. I got it done to help plead my case to the court, as I live in Virginia and they don’t mess around with speeding. My attorney has the document with the calibration results right now, including how they were obtained, but from my memory they did it with a radar gun. I did not witness the actual calibration, but the sheet listed the calibration of the radar gun, the certificate it was used correctly, the stamp that showed it was notarized, etc. This was the calibration that returned that the speedo was 3 under at 25 and 35, 4 under at 45 and 55, and 5 under at 65 and 75, etc.