Hey guys. I have an 84 Trans Am. According to the RPO codes, the original tires were 215/65/15. Now, there is a set of 245/50/16s on him. And no, I didnt put these tires on, they were on when I got the car and deffinently too wide, on the front atleast. So, two questions.

1. My spedo is off. I've done some rough tests and know about how far its off. For instance, when spedo reads 55, I am really doing 40. When it reads 80, I'm really doing 60. How do I fix this?

2. What size tires should I put on here? I would deffinently like to keep the 16" rims, but I need something that wont rub the inner fenders every time I turn.

Also, I have the 700-R4 out of an 87 Trans Am, and 3.73 posi rear end. Thanks for any help guys.

Brian

 

[QUOTE]Originally posted by Slepewesel
Hey guys. I have an 84 Trans Am. According to the RPO codes, the original tires were 215/65/15. Now, there is a set of 245/50/16s on him. And no, I didnt put these tires on, they were on when I got the car and deffinently too wide, on the front atleast. So, two questions.

1. My spedo is off. I've done some rough tests and know about how far its off. For instance, when spedo reads 55, I am really doing 40. When it reads 80, I'm really doing 60. How do I fix this?

2. What size tires should I put on here? I would deffinently like to keep the 16" rims, but I need something that wont rub the inner fenders every time I turn.

Also, I have the 700-R4 out of an 87 Trans Am, and 3.73 posi rear end. Thanks for any help guys.

Brian [/QUOTE

It seems to me you have two issues...your tire size and your 3.73 gears. Neither one are stock. Your speedo is way off mostly due to the 3.73 gear ratio. You will have to change speedo gears based on your preferred tire size and 3.73 gear ratio. Sorry, but I dont have access to the speedo gear part numbers you need to change to. Maybe they are listed here in a tech article.

 

Hi Capn,
Thanks for the info. Also, according to my RPOs (and the decoder on this website), the 3.73 posi IS factory. And any ideas what tires I SHOULD have on here? Thanks

Brian

 

Check the tech boards here and see which tires it came with and switch back to those if you want. You should be able to find out what tires it came with. I didnt know it came with stock posi 3.73's. Thats interesting.

 

Your tire size is not an issue. Those 2 tire sizes are within a half inch of the same diameter. Tread wear will cause more error than that. It makes less difference than the change from one speedo gear to the one that's one tooth more or one tooth less. Forget about it.

Sounds to me like you have 3.73 gears in your rear, but speedo gears for a 3.08 or 3.23 or some such. Which makes perfect since since the low-perf LG4 cars in 84 didn't come with 3.73s, they usually came with 3.23s.

All you need to do is to get the right speedo gears for your transmission.

There's some tech info on this site's tech database. Alternatively, do a search for my userID and the phrase "SMUC", you'll learn all you need to know about how to calculate what speedo gear set you need.

 

[QUOTE][i]Originally posted by RB83L69 [

Sounds to me like you have 3.73 gears in your rear, but speedo gears for a 3.08 or 3.23 or some such. Which makes perfect since since the low-perf LG4 cars in 84 didn't come with 3.73s, they usually came with 3.23s.

All you need to do is to get the right speedo gears for your transmission.


Exactly as I said earlier except he was convinced his car came with stock 3.73 gears. I had never heard of that.

 

Capn and RB8,
Ok, so I understand 84 Low Perf LG4s usually didnt come with 3.73. Fine. But, according to MY RPO codes for this car, it did. And just for fun, this was also an L69 "HO" car. Now, why in the hell would I have spedo gears for a 3.08 or a 3.23 when the 3.73 DID come factory in this car??

Brian

 

Probably because you don't have the HO drive train in the car; your trans probably came from the same car as the motor did.

But that's all beside the point; we're not here to argue about how it got that way, only to help you get it right. You can either argue about stuff that won't set your car straight, or you can listen to advice about stuff that will; it's your choice. Which do you think will make your speedo read right?

 

Ok, the LG4 and Trans both came out of my previous Trans Am (may he rest in peace....), an 87. So, if I just use the SMUC Formula, I will be able to figure out what gears I need and go. And the different tire size has nothing at all to do with the spedo being off?

Brian

 

Oh yea, the 87 T/A my trans came from was also a HO, but only had 2.73 gears (I think), does this make a difference at all, since thats where the Trans came form?

 

Yes, the rear gears absolutely do make a differemce.

Yes, the formula will work. It has to. It's numbers. They have no choice but to work. Plug in the right numbers, do the pencil-pushing correctly, the right answer will come out. That's why I always liked math and science when I was in school; because the answer always comes out the same no matter how may times you work the problem. Not like psychology or political science or something where things aren't quite so clear-cut. In college I was a math and physics major.

There's no way your 87 T/A was a HO car; the last year of the L69 in these cars was 86, and there were only a handful. More likely it was a TPI car. If it wasn't a LG4, then that's what it was, a LB9, not a HO. If the trans came from a car with 2.73 gears in it, and you didn't change the speedo gears out, then that's what you've got.... speedo gears for 2.73s.

No the tires do not significantly alter the speedo accuracy, as the 2 sizes in question are almost exactly the same size, as I posted above.

Work out the formula, and see what you need. Then just for the fun of it, go take a look at what's in the car now, and work the formula backwards against the gears that are there, and see what gear they're for.

 

What RB83L69 said...

 

Ok, lets throw another wrench into this whole mess ....

I just got to thinking about this, since the 84 Originally came with the 3.73 gears, wouldn't it make sense that that spedo gears FROM the 84, would be geared for the 3.73? Yes, the tranny in it now is from an 87, which had an electric spedo. 84 spedo is cable driven, so I swapped the 84 speed sensor on the side of the tranny, INTO the 87 tranny. So, since thats the case, my spedo should be right, right? Or am I completely missing something else here ....

Brian

 

Are you not reading these threads? CHANGE THE SPEEDO GEARS!!!!! You can swap the dam sensor all day long it dont matter. The gears are in the *** end of the tranny.

 

Ok, thats where I was getting confused then. I didnt know the spedo gears were in the *** end of the tranny, I thought they were all part of the VSS. So now, where are they and how do I change them??

 

Now were getting somewhere... I havent done mine yet but I believe they are in the cone part. Try doing a search here on the boards, it will probably be faster than someone trying to explain it all. First you should find out what gears to use. I believe they are color coded. Someone may have part numbers for you on the gears. Good luck.

 

Hey guys,
Thanks for your patience and help. I found a "gear calculator" at www.hanlonmotorsports.com, and according to it, I need a 6 tooth drive gear, and an 18 tooth driven gear. So, I think I kinda am starting to understand this .... I always hated math, still do. Is there a way to work those 2 numbers to come up with 3.73, or close to it?? Is that how I will know I have the right combo? Sorry for being a moron ....

 

Look up the SMUC.

You can't go by gear alone; tire size enters into the picture.

AFAIK you can't buy a 6-tooth drive gear. The correct one for a T-5 with 3.73s and a typical tire for these cars of 26" gives a 7 and 19 combo.

But you don't have a T-5, so you also don't have a 7-tooth drive gear. You have a 700, and the drive gears for those are in the neighborhood of 15 to 18, and drivens are around 38 to 50.

 

Your not a moron...the site you listed doesnt work. Did you spell it right? I will soon need to do what your doing because I am swapping to Moser Ford 9" soon with 3.70 gears.

 

Hi Capn,
Sorry, I got a coma stuck in the website. Its www.hanlonmotorsports.com


I went thru the SMUC thing and here's what I got:

25.65" (tires) * pi = 80.58
(12" * 5280 ft) / 80.58 = 786.30
786.30 * 3.73 = 2932.89 (Time driveshaft spins per mile??)

So, do I want a spedo gear combo that will give me 2.932 (or as close as possible)?

If so, I came up with a drive gear with 7 teeth and a driven with 20.

20/7 = 2.857. Does that all seem about right?

 

Hi RB83,
Please see above post to see if I have the right idea. Also, if the gears are different for the 700, you wouldnt happen to have part #'s for them, or know where I could find them do you? I cant seem to find spedo gears of any kind on gmpartsdirect.com. If I did everything right, I figured I need a 15 and 44, does this all sound about right? Again, thanks for all the help guys.

 

Ok, found the part numbers, but how do my numbers look?

 

That would seem right, if you had a T-5. But you don't. You have a 700, and those T-5 gears won't fit it.

You need to find auto trans gears in a combo that gives that same ratio.

 

Wow, I got it, finally. I worked thru the equation and came up with needing a ratio of 2.931. With a 44 tooth driven and a 15 tooth drive, that comes out to 2.933. Now ..... how do I change these little bastards?

Brian

 

Anyone....? Please keep in mind, I'm an idiot on this sujbect, and have never changed spedo gears. Thanks

 

Have you tried doing a search here on the boards? I have never done it but I will be soon. I think its mostly time consuming though. Try the search.