Help! 4000 RPM's at 65mph . NORMAL?

[Tech1224]

My uncle would like to drive his 74 corvette over 100 mph but at 65 mph the rpm's are at 4000.
He has a 4 speed manual transmission .

 

[Toms007]

First off, Welcome to CAC Tech1224. This is a friendly community with lots of great people with answers.
Although the C3s are not my speciality....if I have one , I will throw this your way. 4k at 65 mph tells me that he must be running a very low gear ratio in that Vette. Something that low would indicate to me the car had been used for drag racing or at least ordered for that purpose. He should have great acceleration off the line, but as you and he have noticed, not a great top end. What size tires does he run in the rear? This might be one place where he could help without going into the rear end and changing the ring and pinion. A taller tire will reduce the effective ratio. There will be a limit on how far you go though, and I'm not sure what size a tire will fit without modifying the wheelwell.

Again, welcome to CAC and hopefully someone else will come along here soon and give more insight.

 

[Tuna]

How long has he owned the Vette?

Way back when, I had a '55 Chevy with 4.10 gears and it would show 3500 RPM at 70 MPH. So, 4000 at 65 sounds like a 4.56 ratio - great for a quick sprint to an 1/8 mile but not very good for cruising or gas mileage.

Check the ratio of the rear gears and consider changing it out.

 

[Toms007]

Tuna, 4.56 is what came to my mind when I first read the post. My old 64 had 3.07 and it was turning about 2000 @ 65 (If I remember correctly, and that was a looooooong time ago).

 

[catbert]

Old Unc was a drag racer, for sure.

 

[JohnZ]

My uncle would like to drive his 74 corvette over 100 mph but at 65 mph the rpm's are at 4000.
He has a 4 speed manual transmission .

The calculator comes back with a 4.96:1 axle ratio, so it's most likely a 4.56 axle with some speedo error. There are no 100-mph blasts in your uncle's future until he changes the axle ratio to something more street-friendly.

 

[Tech1224]

Thanks for the input

My uncle has been dumping money into this corvette for a year.
rebuilt rear end, suspension,motor, and he still cant drive it on
freeway. where can we find spec data for gear ratio?

 

[catbert]

Who replaced the rear end? Whoever did it should know what was put in.

 

[Tuna]

Back then, they used to put a metal tag on the diff to show the ratio installed but if the diff has been changed, the tag isn't any good and probably isn't there.

Jack up the car and count drive shaft rotations and partial rotations per tire rotation (make sure both tires turn the same amount). That will give you an idea of the diff ratio.

 

[Yoda]

Tech1224 Welcome to our Corvette Community

If you are looking for the original numbers you can look up in our "Specs" drop down, or I made it a bit easier

Here you just need to arrow down to the "1974 Information"

and for an overall view of The 1974 Specs

Welcome

Bud

 

[KANE]

Another option is to leave the rear alone and consider a 6 speed tranny.

Yeah- I know a 6 speed is more expensive than swapping out a rear gear- but it certainly would get down the street quick and hit 100 with a double overdrive.

 

[Vettehead Mikey]

where can we find spec data for gear ratio?

There's a code stamped into the bottom of the diff housing that will denote what the original ratio was, but that's obviously long gone.

 

[MIKE80]

Another option is to leave the rear alone and consider a 6 speed tranny.

Yeah- I know a 6 speed is more expensive than swapping out a rear gear- but it certainly would get down the street quick and hit 100 with a double overdrive.

I removed my 4 speed and clutch linkages and sold them. I bought a used T56 6 speed, and installed it with a hydraulic clutch. It really wasn't that expensive after selling the old parts. Trust me, you'll love the 6 speed!

 

[Wegone]

You can use this method to check the ratio. You may have an issue with the tach.

HOW TO: Find out your Gear Ratio Easy in less than 5 minutes - YouTube

 

[TimAT]

I have 4:11 gears, 25" tall tires and turn 3500-4000 @ 70. Ol' Red will pull to 7000 no problem and that's kissing the triple digits pretty hard.

 

[rwpeders]

Final Gear Ratio

My uncle would like to drive his 74 corvette over 100 mph but at 65 mph the rpm's are at 4000.
He has a 4 speed manual transmission .

I have changed the rear end gears and have had 4:11, 4:56 and 4:88. I think the car may have 4:88. A good all around final ratio is a good topic for discussion. I would say he should change it to something in the 3s, like 3:36. Another factor is the type of manual transmission in the car - close ratio or normal ( don't know what they call that). The normal transmission has a much lower first gear so that cars with 3:36 have an easier start.

 

[targatom]

4.56 gears

Hi, I have 4.56 gears in a c4, 85, it also has a richmond six speed, and 26.5 tall tires. It turns 2500 rpm at an indicated seventy mph, which is prolly reading about five mph low. I want taller gears, and am considering a punkin swap or maybe changing out sixth gear to the smallest possible, a .50 ratio.

I have changed the rear end gears and have had 4:11, 4:56 and 4:88. I think the car may have 4:88. A good all around final ratio is a good topic for discussion. I would say he should change it to something in the 3s, like 3:36. Another factor is the type of manual transmission in the car - close ratio or normal ( don't know what they call that). The normal transmission has a much lower first gear so that cars with 3:36 have an easier start.

 

[rtpilot]

Or you could check the speedometer gear.

 

[RPO469]

'74 gearing/tires

As you can see from factory available gearing 3.70 was the most available so your uncle's '74 has either had a ratio swap, lower profile tires, or both--or major speedo and tach problems! I have a '77 with a 3.90 I installed and lower profile tires for autocrossing. Measuring the tire height and considering my gearing I run the same as about a 4.10.

 

[63his77hers]

High RPM

My uncle would like to drive his 74 corvette over 100 mph but at 65 mph the rpm's are at 4000.
He has a 4 speed manual transmission .

I have a C3 and if I recall correctly it has 3:96 gears (stock) and runs about 2800rpm at 65mph. Sounds like someone put 4:11's or 3:55's in the rear end.

My C2 used to have 4:11's and it ran 4000rpm at 65mph, but it had a short ratio tranny.

Anyway you look at it, someone changed the gear ratio. When I swapped out the 4:11's for the 3:55's on the '63 it dang near doubled my gas mileage.

Good luck!