Speedometer gear

[buckethead]

Does anybody know a part number or how I can determine what gear to change to for a '77 with 350 turbo and 4:11 gears? The speedometer is off by about 15mph as near as I can tell.

Thanks,

Doug

 

[Bob Chadwick]

Doug,

Do a search. There are posts about the number of teeth and the mph difference created by purchasing gears with fewer and greater number of teeth.

This is one of many posts that I found searching for speedometer gear that might help you. http://www.corvetteactioncenter.com...?s=&threadid=19902&highlight=speedometer+gear

Bob

 

[VetteAtv]

Slightly off subject, but how many RPMs are you pulling at 60 MPH?

 

[Stallion]

Slightly off subject, but how many RPMs are you pulling at 60 MPH?

Well it depends what gear you are in. You can run 60 in 3rd gear high, or go into 4th gear and run it middle. So it all depends.

 

[Stallion]

And it also depends on the gear ratios of the gears and differential, too.

 

[buckethead]

Thanks, Bob that gives me some place to start. I'll have to pull out the gear and see what I have. VetteAtv, figure it's running around 3 grand at 60 but I haven't really checked that cause I don't know how far off the speedo is. The only reason I know it's off is because when I was bringing the car home everybody was passing me on the highway and my wife who was following me home asked me why I was going so slow. I had seen a cop on the way out so I didn't want to exceed the speed limit in my red Corvette and according to my wife must have been going 55 in a 65 zone.

 

[VetteAtv]

I went through the same thing when I brought my car home with a 3.73 rear end.
My concern is the tach says I am pushing 4 grand at 60 mph. I know something is wrong, not sure what yet. Right now I am just looking for what it should be reading.

Thanks for the info

 

[Bob Chadwick]

I understand that 3:08 was the standard gear for a 77. I do about 3000 rpm at 70 mph.

 

[Stallion]

I went through the same thing when I brought my car home with a 3.73 rear end.
My concern is the tach says I am pushing 4 grand at 60 mph. I know something is wrong, not sure what yet. Right now I am just looking for what it should be reading.

Thanks for the info

Well that doesn't sound unbelievable. You have a pretty hefty ratio for your differential. You'll be a lot more stressing on the engine and it'll run faster because of that. Do you think this is a problem? So if you are doing 4k at 60, are you talking about 4th gear? Or 3rd gear?

TR

 

[VetteAtv]

I bought a '79 with a L81 (not orginal engine, but stock), a th350 three speed auto driving the 3.73 rear end. From what I have seen so far other simalr setups are pulling a lot less RPMs. I am wondering why mine is runner higher.

 

[Dave L.]

Doug - one tooth on a driven gear is about 4 mph. Add teeth to slow up the speedo,subtract to increase the indicated speed.
How many teeth do you have on your driven or pencil gear now?
I'll let you know if GM makes a gear to rectify your problem or of you'll have to change the drive gear also.

VetteATV - With that set up you have(TH350/3.73) you are going to be turning some pretty high rpms. For comparison,my 81 with 4spd/2.72 turns about 2500 rpms at 80 mph.

Dave

 

[Stallion]

Doug - one tooth on a driven gear is about 4 mph. Add teeth to slow up the speedo,subtract to increase the indicated speed.
How many teeth do you have on your driven or pencil gear now?
I'll let you know if GM makes a gear to rectify your problem or of you'll have to change the drive gear also.

VetteATV - With that set up you have(TH350/3.73) you are going to be turning some pretty high rpms. For comparison,my 81 with 4spd/2.72 turns about 2500 rpms at 80 mph.

Dave

Wow, Dave, you have a close ratio in the back of your Vette. Is that stock or did you put that in there? I have a 3.36 and she runs great.

TR

 

[Dave L.]

2.72 stock

TR - All 81 four speeds cars came with 2.72 stock. the automatic trans cars had 2.87 stock. I like the low rpm's at highway speeds since I do most of my cruising on the highway. When driving back roads and around town I rarely shift out of 3rd gear. It's sort of like having a 3 speed manual with overdrive!

Dave

 

[Stallion]

Re: 2.72 stock

TR - All 81 four speeds cars came with 2.72 stock. the automatic trans cars had 2.87 stock. I like the low rpm's at highway speeds since I do most of my cruising on the highway. When driving back roads and around town I rarely shift out of 3rd gear. It's sort of like having a 3 speed manual with overdrive!

Dave

Oh, I didn't know they brought the gear ratio so close for the last C3s. I think my 3.36 is perfect. Good acceleration, but keeps revs nicely and efficient. Happy medium.

TR

 

[Dave L.]

TR - I agree. Good overall ratio. Gives you some good power off the line and a fairly good cruising gear. BTW did you get your intake manifold problem (water leak) remedied?

Dave

 

[Stallion]

TR - I agree. Good overall ratio. Gives you some good power off the line and a fairly good cruising gear. BTW did you get your intake manifold problem (water leak) remedied?

Dave

No I haven't done that yet. A couple of more weeks of school and I'll have mucho free time. Until then, I'm pretty swamped. But I'm ordering the '78 Service Manual tomorrow so that should be in time to start working on it.

Can't wait until the summer!!

TR