John Swift

Update: For those interested
First, thank you all for your comments and/or suggestions!
Swapping transmissions, while a great idea, after my investigation is not in my budget!
Tire size is 235/45/17-ZR
RPM at 40 is approximately 2000. The speedometer is off by about 10mph at that same speed.
The weather has been terrible here, so I've only driven it once and didn't take it to the highway, just because the speedometer is off.
I have had a hard time finding any transmission shop that was willing to do the work, BECAUSE it is a vette!
As usual, the last place I checked was twas the first place I should have looked. They do both trans swaps and gear changes all the time.
They told me the there was only about 120 rpm between 3:36 and 3:55, and he recommended 3:55's. So I go in in 2 weeks to have it done!
They are also aware of series of carrier etc.
Thanks again everyone, I'll let you know how it all turns out.

ezobens

You had better check what you have before you spend money on something you may not need-
If in fact you are turning 1800 RPM at 45 MPH, that would only be 2400 RPM at 60 MPH or 2800 at 70 MPH.
Something is not adding up.
If your 1800 @ 45 is accurate, you probably don't have 4:10 gears.
Just sayin'

John Swift

I'm sorry, I should have proof read before posting.
I have an almost immediate shift to 2nd from a stop!
I was able to download an app for my phone that gives me my MPH. I checked it against my other car and it is very accurate.
I will use that to help me.
Either way, it will be checked.
I appreciate that, and I WILL be checking that before it goes in!
Thank you.

OldCarBum

The easiest and most accurate way to determine your rear end ratio is to lift both rear wheels off the ground, crawl under the car, mark a line on the drive shaft and a mark on either tire, while rotating the rear tire in the forward direction one revolution, count the number of rotations of the drive shaft.

3 rotations of the drive line per 1 rotation of the tire = 3.08 rear end ratio.
3 1/3 rotations of the drive line per 1 rotation of the tire = 3.36 rear end ratio.
3 1/2 rotations of the drive line per 1 rotation of the tire = 3.55 rear end ratio.
3 3/4 rotations of the drive line per 1 rotation of the tire = 3.73 rear end ratio
4 1/10 rotations of the drive line per 1 rotation of the tire = 4.11 rear end ratio.

John Swift

Thank you OldCarBum,
I will do that tomorrow before I do anything else. I don't need good weather to accomplish that, then when the weather gets better, which should be Monday, I'll take it on the road to check my RPM's!

John Swift

I was able to get the car on the highway today.
First, I did 1 rotation of the tire and got a little over 4 turns of the drive shaft. Less than 4 1/8.
On the highway I am getting 3000 rpm at 64 mph
It doesn't agree with the chart provided above, but that's what it is!
Tires are 235/45/17 ZR 93W
Mechanic suggested 3:55 to keep better low end.
What do you think?

ezobens

Something is still off-
3000 RPM at 64 MPH with that tire (25.32" Diameter) = 3.55
So either your speedo is off, your tach is off (or both?) because the numbers don't add up compared to what you say the driveshaft turns were.
Personally, I perfer to turn the drive shaft and see how many turns it takes to get the tire to rotate once (mark the tire so you can see it). You did have both wheels off the ground, correct?
Elm

jb78L-82

3.70 gears with a 27 inch tire (255/60/15) should be 2860 RPM @ 60 MPH.


235/45-17 tire:


Diameter inches (mm) 25.33 (643.3)

A smaller tire than 255/60/15 (27 inches) would increase the numerical effective gear ratio. You may already have a 3.55 or 3.36 rear gear based on 3000 RPM at 64 MPH with such a short tire and the increased revs is coming from the 25 inch tire and NOT the gears themselves........

John Swift

Yes, both tires off the ground. The speedometer I used was an app I downloaded on my phone. I verified the accuracy of the app by driving at different speeds in my Cherokee. I also drove past a school zone that radars your car and the app was dead on. The speedo in the car is way off!
I rechecked the tire size. The size I gave was the front..duh.. But the back is 255/50/ZR17 1001W
Actually I DID turn the driveshaft and marked both shaft and tire with chalk. My wife told me when I had rotated the tire one full turn.
The app measures top speed as well, and it said top speed was 66mph.
The only real way to measure is counting the teeth which I can't do.
I'm taking it out one more time to check. Not sure how to verify RMP's
I am at a loss.

John Swift

I just spoke to the mechanic and explained what is going on with my results. He had me measure the height of the tire. It's 26 1/2 inches ground to top of tire,he also wanted me to measure the top of tire to rim He then told my calculations were off but not by much. He said he would still recommend the 3:55 but when I take it in he will take it apart and THEN decide what to put in.
I'm sorry that I am so uneducated in this matter. I understand the principles, but the specifics I'm getting lost

OldCarBum

So your tire size is slightly over 27" which results in the aspect ratio of the tire as 26.4" tall mounted with the weight of the vehicle on the tire.
At 64 mph your rpm should be 3237 with a 4:10 rear gear.
At 64 mph your rpm should be 2945 with a 3:73 rear gear.
At 64 mph your rpm should be 2804 with a 3:55 rear gear.
At 64 mph your rpm should be 2653 with a 3:36 rear gear.
If you are sure your mph is calculating correctly on the app, I would suspect your tach is slightly off.
If you saw 4 1/8 revolutions on the drive shaft that takes all other variables out of the equation which should result in having a 4:11 rear gear
The only other way to confirm it is to open up the differential and look inside.
As I showed above changing only the rear gears will only slightly reduce your rpm and the roadway and even running a 3:36 rear gear only decreases your rpm to 2653 at 64 mph.
Open it up, count the teeth and then decide what you want to put in by the rpms on the highway and anything you might loose around town driving.
I like my 3:36 gears but I have a big block and TH400.
I'm in the process of stroking my 454 to 496, swapping to a TKO-600 close ratio and installing 3:08 rear gears.

John Swift

Thank you again OldCarBum, and everyone else!
Something is definitely not right with the tach. I notice that the tach stays at my idle RPM speed when I shut off the ignition. When I turn it back on it drops to 0. When I turn it back off without starting it creeps back to about 400 and stay's there. At a minimal change I hardly think it's in my best interest to change the gears. I guess I just can't justify the cost.
Maybe some of it is in my head. As I drive it just "feels" like it needs to shift, and that's just not going to happen.

jb78L-82

To get a meaningful drop in RPM's if you have 3.70/4.11's, you will need to switch to 3.08/3.36 gears. My 425+ Gross HP/450tq 355 L-82 could use either easily with my power but if you don't have the power in the motor, you will notice such a gear change. AS others have stated, using lower numerical gears with BB engines is a non issue since the BB's have big torque........

ajrothm

Does your T400 have the stock torque converter? If not, and it has a stall speed converter, your RPMs will be higher at cruise and none of the "charts" will jive with reality.

Use a GPS app on your phone to figure out your REAL speed, then calculate rpms from there.

The ole "jack it up and turn the tire" is almost fail proof though. If you got 4 driveshaft rotations to one tire rotation, that's pretty indicative of a 4.11 gear.

If you really have 4.11s, you'll be around 57 mph at 3000 rpms on a 26" tire, depending on converter slip.

All that being said, switch to a 3.36 gear and cruise at 70 mph around 3000 rpms (on 27" tires and a tight converter). This is the setup I have and it cruises nicely at 70 mph, gets 11-12 mpg highway and works well. A 383 has plenty of torque to pull 3.36s.

OldCarBum

I am on a budget and don't have the big bucks to spend on my resto/mod all at once.
I've been driving my 73 as is for almost two years, saving my money and stuffing my shelves with all the parts to complete my 73 and to do it right.
Save your money, make a plan and do it as you can afford to do it.

John Swift

No Alan, It's not the stock converter, however I don't know what the stall rating is. The car was sold to me by the guy's nephew that had the work done. He passed before it was complete.
When I received it, it didn't even have the emblems back on from the paint job. The aunt hasn't been able to find ALL the paperwork for the restore as of yet.
Most, but not all.
Think I need to sleep on this!
Thank you for your help!

caskiguy

https://tech.corvettecentral.com/201...o-information/

Hope it helps.

caskiguy

And I'll add this for the hell of it.

https://www.corvsport.com/1977-c3-corvette/

resdoggie

What does "take to the highway" mean? If you are going to do 5000 miles a year or more on the highway, then it maybe worth the cost to switch to a 3.55. Otherwise, I'd leave the 4.11 in it for street cruising if that's where you will be spending most of the time. I just switched from a 3.55 to a 4.11. I won't be switching back and will trade off the poorer gas mileage for the better performance of the 4.11.

caskiguy

Just came across this companies site.http://www.gounitrax.com/ratio_needed.htm