Anyone with 4.11 gears mind telling me your top speed?

[Vamisk]

So it's always been a little bit of a chore driving my Corvair outside of town on the sole fact that it'll do like 60 mph tops and the engine is screaming. It has 13 inch wheels and i'm assuming 4.11s in the back.

I've been doing some calculations and what doesn't make sense is that Clark's catalog says the 4.11s will do tops like 100 mph and some top speed calculators corroborate this.

This is either where i find out i really have the only 5.12 Corvair or my 4th gear is lower then 1:1.

[Jerry Whitt]

The word "assume" gets lots of us into trouble. Cutting the word apart to "ass" "u" "me" is what happens. Suggest you do some basic math.

With a tape measure device, check the circumference of one tire. My 13 inch is 77 inches. That is about 6.4 feet. One mile is 5280 feet. That means the tire must turn 825 rotations in each mile.

60 mile per hour would be 825 tire rotations in one minute. If the gear ratio was 4.11, then the engine rpm would have to be 3390.75 rpm. This rpm is well within reason.

100 mph would mean 528,000 feed in one hour. Divide by 60 to get feet in a minute, that would be 8800 feet. Divide that by 6.4 feet and the speed of the tire would be 1375 rpm. If a ration of 4.11, that would put the engine speed at 5651 rpm. The tachometer on my Corsa tops out at 6000 rpm with a yellow mark at 5500.

Sounds like this might work, but do not feel at all comfortable driving at this rpm.

[Vamisk]

Believe me, i don't feel comfortable at 60 let alone 100. I would have had to have been coasting in neutral down a mountain to hit 100.
Did GM make any 4 speeds with a 4th gear that has a ratio lower than 1:1?
Tomorrow i'm going to go out and rotate the tire in 4th to count how many times the crank spins. I can't say i'm looking forward to it.

[Jerry Whitt]

Special note! My speedometer is not especially accurate. It usually shows at least 10 mph slow. I suspect others are not too accurate either.

[MtnVairMike]

Vanish

Jerry’s math checks out. Most of us are so use to overdrives in newer cars these days that the older 3-4K rpms are not a sound we are familiar with anymore.

Drive safe

Mike



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[Vamisk]

I'm just trying to understand how some people (even Clark's) are claiming the ability to hit 100 with 4.11s when i clearly cannot. While i don't have a tachometer in the car it's blatantly obvious the engine has topped out at about 65-67 mph. If the gearing wasn't the issue and i simply didn't have the horsepower it surely wouldn't be revving so high. Using a GPS speedometers, radar traps, and other traffic as a reference i'm getting consistent "speed-to-revs".
Are there any gear reductions that i don't know about aside from the final transmission gear, final drive gear, and wheel diameter?

[MtnVairMike]

By topping out are you saying that the gas is to the floor on flat ground and it won’t go any faster than 65?


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[Vamisk]

Yes, the pedal is flat on the floor and it sounds like it's about to explode. As soon as i can find a cell phone holder i can make a video. I don't believe this is a horsepower issue, my motorcycle will run out of power in 6th gear and this feels like a different situation.

[66vairguy]

O.K. lets deal with the real world ---- Yes the LM speedometers are calibrated for 825 revolutions per mile, but THAT IS NOT what the stock tires produced. Why? If you go look at any of the old road tests of Corvairs (and most 60's cars) You'll see the speedometers typically read 1-4 MPH fast at 60 MPH against a calibrated test speedometer. The error towards reading fast was done deliberately to allow for tolerance variations so a NEW speedometer would never read slow as this would be considered a safety issue.

A stock 6.50 tire is about 866 revolutions per mile and a 7.00 tire is about 839 revolutions per mile. If your modern tire is even smaller (I see this a lot) then that means even more engine RPM at 60MPH.

My calculations for an IDEAL 825 (my car uses 245/60X14 which is almost 825 revolutions per mile) with a 3:55 rear gear common in Corsa's is 102 MPH at 5,000 RPM (the beginning limit of the yellow warning arc is 5,200 which ends at 5,500RPM, then goes red). Unless you have high tension valve springs operation above 5,000 RPM is not advised.

The Covair engine in good shape should be able to run continuously up to 4,000 RPM. With a 4.11 and a 7.00X13 tire that is about 69 MPH based on some rough calculations, or O.K. for freeway driving, but the engine will sound busy.

Note that the EM cars have a speedometer that is 1,000 revolutions per mile and a speedometer cable gear was installed in the transaxle to match the ring and pinion ratio so the speedometer read correctly. If you did not change this then I'd run my smartphone app (GPS) to see what the real speed vs. indicated speedometer.
Good luck

[Jerry Whitt]

A note that the "pedal is flat on the floor" gives thought that although noisy, maybe the throttle linkage is not actually opening the throttles completely.

With a friends help, pull the air cleaners off, have a friend push the gas pedal to the floor. Then with a flash light, view the throttle plates.
Are the plates completely open? If yes, good, go no further. If the plates are not open completely, throttle linkage inspection and repair would be the next step.

Let us know what you find.

[64powerglide]

I have a 110 powerglide with 3:55's & 175 X 70 x 13 tires cruise at 65 with 2 inches of pedal left. Sound like you're running on 5 cylinders. My speedometer reads 70 but it's 5 MPH fast, checked by radar.

[Vamisk]

I can assure you i'm still buzzin' half a dozen. It doesn't lack power it lacks the gears. Unfortunately it's snowing right now so i can't do anything with it.

[bbodie52]

I can assure you i'm still buzzin' half a dozen. It doesn't lack power it lacks the gears. Unfortunately it's snowing right now so i can't do anything with it.

What??? You're not up or a high speed snow run in a 4.11 Corvair???

Install a tachometer and go for it!!

[Vamisk]

While that would be fun and this thing likes to slide on dry roads even, any chance of snow is met with piles of road salt.

Anyway i was able to make a video since the snow melted before it even hit the ground. It doesn't sound like it in the vid but this thing was screaming. I'd reckon i hit 70 mph and it was getting pretty squirrelly. I doubt there's any room to go but i'm going to check the throttles on Wednesday like Jerry Whitt suggested.


I was thinking about getting cheap 15 inch steelies to help. Unfortunately the only larger tires for my rim size are trailer tires.

[terribleted]

Video sound like it is still accelerating fairly well.

[bbodie52]

The closest to stock original tire size (6.50-13 bias ply) is 185/80R13. This size has an overall diameter of approximately 24.7". (Revs per Mile: 842)
Calculated circumference (24.7 x π) = 77.6"
With a 14" rim P195/70 R14 provides an overall diameter of approximately 24.8" (Revs per Mile: 839)
Calculated circumference: (24.8 x π) = 77.9"

Source: https://murfy.us/doku.php?id=timtechcor ... xle:ratios

So... where did the 4.11 Corvair gears come from?

More food for thought...


What gear ratio? 4.11 or 3.89

http://corvaircenter.com/phorum/read.php?1,772493