Data compilation- check it out

[boostmaster]

I'm having an issue with my 86GN. As some of you may know I've been trying to figure out why my RPM is at 3000 RPM at hwy speed (65-70).
Tranny is rebuilt (couple of days ago), 4th gear is working, lockup is working, visual/physical inspection indicates 3.42 gearing.

I have been informed that GN's should be running at about 2200RPM at these speeds and should be getting about 25 mpg! Yet I have no proof to back up that claim (unless someone can tell me where I should look)

I'd like to ask if everyone who reads this would post up a few of their numbers so that I have something to work with.

I'm looking for TR's w/ 3.42's, lock up, stock to mild engines (no racecars)

I need to know RPMs at 65-70 mph as well as general fuel economy.

TIA

[LV GN]

I'm having an issue with my 86GN. As some of you may know I've been trying to figure out why my RPM is at 3000 RPM at hwy speed (65-70).
Tranny is rebuilt (couple of days ago), 4th gear is working, lockup is working, visual/physical inspection indicates 3.42 gearing.

I have been informed that GN's should be running at about 2200RPM at these speeds and should be getting about 25 mpg! Yet I have no proof to back up that claim (unless someone can tell me where I should look)

I'd like to ask if everyone who reads this would post up a few of their numbers so that I have something to work with.

I'm looking for TR's w/ 3.42's, lock up, stock to mild engines (no racecars)

I need to know RPMs at 65-70 mph as well as general fuel economy.

TIAit seems to me if you have 3:42's and 26 inch tall tires. the converter just cant be locking up. if your cruzing at 65 and stay steady on the gas and just touch and hold the brake pedal enough to make the brake lights come on but not the brakes. the converter will unlock and the rpm's will jump up a few hundred. now still staying steady on the gas. let off the brake pedal and the rpm's should come down a few hunderd. does this happen? it is common for the switch on the brake pedal to back out a little. even tho the scanner says it ready to lock up. the switch on the brake pedal wont let it happen.

[boostmaster]

According to the shop who rebuilt the tranny and per our test drive. All of the conditions you desribed have occurred. I can tap the brake pedal and the RPM's go up slightly and you can feel lock up but I'm still at 3000 RPM.

I'm running 245/60/R15's btw...

[turbo nasty]

Physical inspection of the rear end ?
Did you count the teeth on your ring gear??

[boostmaster]

I did not count the teeth but I did jack up the car, Marked a point on the driveshaft as well as the tire I was to turn. I had 3 full revolutions of the driveshaft before I had one full turn of the tire. After which, I had an additional almost complete .5 of a driveshaft revolution before I reached my 1 full turn on the tire.

I understand that this is not entirely 100% accurate or scientific but I suppose that it gets me close enough. Or is my method incorrect? I mean, how many variations can I have in gear ratios with considering my current findings?

[84T-typeMike]

My RPM at 60-65 varies from 2000-2200 RPM and My car gets 23 MPG highway all day long.A little less in the city and WAYYYY less when I am beating on her all day, which is most of the time, so in reality I probably average 5 MPG.LOL! HTH!

[boostmaster]

84T-typeMike: Thanks for the input.


Anybody else? I see 37 views and only 1 guy posted? C'mon guys I need this info so that I have something to work with.

[boostmaster]

C'mon people.... 71 views and no other posts? I really need some info on this matter... I have nowhere else to turn.

[dlevey]

How do you know you are at 3000 rpm, if you are going by the factory tach that is your problem, If you are using a scan tool you do not have 3.42's

[boostmaster]

dlevey: Thank you for your post but I don't understand what you mean.

I'm using the factory tach because the OTC 4000 I used on it a few weeks ago showed the same RPM as the factory tach.

Secondly, Even supposing that the Factory tach and the OTC were wrong, 3000 RPM on this car FEELS like 3000 RPM. When I'm on the hwy it feels like I'm still in third. I find myself staying around 50 mph because it feels as if I'm pushing the car.

Third, what does my use of a scan tool have to do with having 3.42's?? :fragez:


Please clarify. I just want to fix my problem...

[TurboJim]

What size tires?

If my math is right:

A 245-60-15 has a sidewall of 6", x2+15=27" diameter roughly. 27" diameter has a 84.78" circumference. Theres 63360" in a mile. The tire needs to spin 754 times in a mile. At 60 you're going a mile a min. A mile a min means the tire went 754rpm's. Times 3.42 is 2579 rpm's times .60 overdive ratio puts you in the 1547 rpm's with no slip. So assuming the average 10% slip add 150rpm, you should be closer to 1800rpm with the converter locked at 60mph. Somethings wrong, or you got real small tires.

[boostmaster]

UHHH your calculations look good but I don't understand them. Nevertheless, the tires are 245/60/15's on stock rims.


I've been basing my assumptions about the vehicle speed and RPM on other GN drivers and of course the BUTT DYNO, which tells me that the car doesn't "feel right". I used to have an 86 T-Type years ago and never had this problem. I KNOW that this RPM is just too high but I can't seem to be able to find a way to prove it!!! The tranny is rebuilt, 4th works, Lock up works, everything seems to be working like it's supposed to .

Does anybody have any other ideas??? I'm desperate... :add_shit:

[dlevey]

To simplify,

The Buick factory tachs are unreliable, do not trust that to determine your rpm, a scan tool reads off the ecm and is allways corect.There is no physical way you are at 3000 rpm at 60 mph with 3.42 gears .

[TurboJim]

UHHH your calculations look good but I don't understand them. Nevertheless, the tires are 245/60/15's on stock rims.


I've been basing my assumptions about the vehicle speed and RPM on other GN drivers and of course the BUTT DYNO, which tells me that the car doesn't "feel right". I used to have an 86 T-Type years ago and never had this problem. I KNOW that this RPM is just too high but I can't seem to be able to find a way to prove it!!! The tranny is rebuilt, 4th works, Lock up works, everything seems to be working like it's supposed to .

Does anybody have any other ideas??? I'm desperate... :add_shit:

Uhhh, I proved it by telling you how many times the tire spins in a mile, how many times a minute, multiplied by the diff ratio, then by the overdrive ratio. Provided my math is right, I have proven, without a doubt, something is wrong with your car. Are you looking for someone to tel you its OK? Thats not what you seemed to want o know before....

[boostmaster]

Jim: I was not looking for someone to tell me that my car is ok. BTW, the "uhhh" is to show my confusion - I'm not very good with math...

What I'm looking for is for someone to take the information I have provided, as you have and give me a diagnosis. However, telling me that the car ain't right is like preaching to the choir. I know it ain't right. But I'm not that familair with all of the intricacies so I'm looking for people to maybe offer different suggestions. (for example: if my gears are not 3.42's then what could they be? If it's not the gears, then what else could it be?)

As I've stated before: 4th works, lock up works, I used an OTC 4000 to check the tach and it was almost exact, I jacked up the car and spun the tire and counted the turns on the drive shaft-looked like 3.42 turns to me...

What's my next step? DO I remove the rear end cover and start counting teeth? Can anyone post a pic of what I need to be looking at???


I appreciate all of the help. I'm just a bit P.O.'d that I can't find the problem ...

[Tonka]

Who rebuilt the trans? Did you get the same one back? How have you verified it is shifting into OD? Have you pulled the speedo gear out of the trans to verify is has the correct number of teeth (in reference to whether you got the same tranny)?
Take it for a test drive, leaving it in D and take it up above 50 mph so convertor should be locked, and note rpm at a steady cruise.

[boostmaster]

A local shop rebuilt the trans. I'm assuming that I got the same one back. That's what they told me they put back in... I didn't check it.

I have verified that it is shifting into OD. I have not pulled out the speedo gear. I was present, however, when the tranny was disassembled and it was an orange plastic piece. The rebuilder said it was fine so it would go back in...

I have done the road test in OD and the RPM at 50 is 2k and 55 it's more like 2500.

I'll have to go back out later on and try the 50mph in D only. I usually only drive in OD because the D seems to "push" the tranny even more than the OD.
I'll try and post up in a soon. I'm pressed for time right now so I won't be able to take her out right now.

Thanks,

[LV GN]

A local shop rebuilt the trans. I'm assuming that I got the same one back. That's what they told me they put back in... I didn't check it.

I have verified that it is shifting into OD. I have not pulled out the speedo gear. I was present, however, when the tranny was disassembled and it was an orange plastic piece. The rebuilder said it was fine so it would go back in...

I have done the road test in OD and the RPM at 50 is 2k and 55 it's more like 2500.

I'll have to go back out later on and try the 50mph in D only. I usually only drive in OD because the D seems to "push" the tranny even more than the OD.
I'll try and post up in a soon. I'm pressed for time right now so I won't be able to take her out right now.

Thanks,well i think the bottom line is. your not locking up or your in 3rd gear or your trans is slipping or your speed-o is wrong or you dont have 3:42 gears. thats it. did you pull your rear end cover off and count the number of teeth on the gears?

[boostmaster]

I haven't counted the gears yet. Soon, Very soon, just been real busy..