Question: 3000 RPM at 55 mph

[Brownie]

Hello. I purchased my first Corvette a couple months ago and I am new to this site. I have an 81 with, I think, the stock 3 speed automatic. It seems to run and shift really good, but when I get it out on the road and open it up it is cranking a lot of RPM's. At 55mph I am cranking about 3000 RPM. I took it to my mechanic and he said he thinks that is normal for a vette. To me it seems like it should have a higher gear. BTW I only get about 11mpg. Would appreciate input on whether this is normal or not. Thanks.

 

[rewind]

sounds like you've got 4-11 gear ratio way low for hiway driving,surprised youre getting that good of mileage.:ugh

 

[bill81vette]

I agree.I have factory gears and average 16-18 mpg highway driving at 70-75 mph with the same tranny.
2500 rpms at this speed
Btw...welcome to CAC!!!
Besure to register your '81
In the L-'81 Registry!!!!

 

[6880 Mike]

I'm wondering whether the torque converter is locking up correctly. 3K at 55 is high.

 

[Tim81]

I agree with Mike, 3000 at 55mph is high. I would have the TCC checked to make sure the torque converter is locking up. Sometimes if you pay close attention you can feel the converter lock up, it feels like the trans shifted to another gear. I can feel mine at about 40 mph after the tranny shifts to third gear.

Welcome to the club Tim

 

[XLR8]

Welcome to the Brownie!!

As you can see, we've got some great folks here that are knowledgeable AND willing to help and advise those new to the Corvette World!

Enjoy your 'new' 81 and the forums!
Jane Ann

 

[Brownie]

Thanks for the replys and thanks for welcoming me to the forum.

 

[Tommy Almond]

I checked mine today while out on the road and was doing 2200 RPM at 55 mph and as far as I know I have a stock rearend. I have not notice a shift feel when my converter locks up but assume it is working as should. I get about 14-15 on the road but that has been doing some mountain roads, Blue Ridge Parkway etc. Not great on fuel but with new 350 GM crate motor 195hp I am running 89 octane gas and no spark knock......thinking of testing some 87 octane if price keeps going higher. But still driving a classic 1981 Corvette.....nothing finer here in the Carolinas.

 

[wallyknoch]

sounds like you've got 4-11 gear ratio way low for hiway driving,surprised youre getting that good of mileage.:ugh

or even a 4.56.....4.1`s you can double the speed from the RPM`s....say 3000, 60 MPH, but a 4.56, 4000 will give you a 70 MPH...but that said, no reason for a 81 to have that or those ratios....maybe because the lack of horsepower in 81`s it was the only way to move it...just joking.....

 

[Brownie]

I think you all are right about the 4.11 or 4.56 gears. I don't have any top end, but she will make it to 60 at the drop of a hat. That's nice but I would like to be able to drive at regular road speeds without feeling like I may blow it up turning all those rpm's. Would it be expensive or difficult to change to higher gears?

 

[Peer81]

The gears aren't that expensive. I got a nice set from a forummember (3.07 I think) but the rebuild will give you a higher price then the gears alone.

Greetings Peter

 

[LannyL81]

Costs

Like said above, it is not the gears that are expensive, but the rebuild of the differential. You can get complete diff swaps online that will run you about $1000 with all shipping involved. You can take the one out of your car and take it to a shop and probably end-up spending $700, depending upon the shape of the diff.

Or I guess you might find someone that is willing to do an even swap with you...but if shipping is involved, then it is still going to cost $100.

No special Corvette costs here...pretty much the same for any diff.

From what you wrote, your driving style is more highway or cruise and it would be best to get a higher set of gears in that diff. Stock was 2.73...so you can see there is quite a difference here.

Good luck.

 

[Peer81]

If you're not into a factory correct 81 Corvette, you can also look into the option to swap the Th350C for a 200R4 automatic. The extra 4th gear will also let you cruise at lower rpm's but you'll still have the ratio for a quick lanch (maybe the diff ratio with the 200R4's 1st ratio could also cause the rpm to take off to quickly so you have to look that one up before you choose an option).

Greetings Peter

 

[GerryLP]

Brownie,

First verify your speed. Use a GPS to compare the vehicle's speed. Without this, you can end up chasing your own tail on this problem. It could be speedometer error, although it would have to be grossly out-of-wack, and perhaps you would have gotten a speeding ticket already.

Once you verify vehicle's speed, then it will be easy to find a calculator on line to help you figure out what gears are in the rear end.

For comparison purposes, my Vette's rear end has 3.31:1 gears, and with the TH350C, it was spinning at approximately 3,100 - 3,200 RPM at highway speeds. It now has a 200-4r, and it spins at around 2,000 rpm. I did figure out that the speedometer was off by almost 9 MPH too low (the Vette is going 9 MPH faster than indicated).

GerryLP:cool

 

[Brownie]

Gerry,
I checked the speedometer today. It was a little off. According to my Chevy pickup, my vette is 3mph slower than what it should be. I will try and GPS it to get it more accurate.
I have been checking around locally trying to find someone that may have a vette that wants to trade rearends. Thanks for the replies.

 

[GerryLP]

Gerry,
I checked the speedometer today. It was a little off. According to my Chevy pickup, my vette is 3mph slower than what it should be. I will try and GPS it to get it more accurate.
I have been checking around locally trying to find someone that may have a vette that wants to trade rearends. Thanks for the replies.

Here is a trick to get you in the,"ballpark".

Raise the Vette's rear end off the ground, and support the Vette on jack stands. Then, mark the passenger side rear tire with a bright paint marker, crayon, or chalk, so that you can see it (perhaps the inner tire wall) from under the Vette. Mark the driveshaft in a way to enable you to count how many driveshaft rotations happen to each tire rotation. If it is 3 turns and 3/4, the you likely have a 3.73 in the differential. Four turns and change would hint of a 4.11 rear end.

GerryLP:cool