I just swapped my stock 3:08 to a 3:23 rear end. I took it out fot the first time yesterday and didn't notice very much if any RPM change. I did the math and the numbers do match. My car seems to run a little faster mid 3000+ RPM's. Does this seem right to anyone? Would 3:42 's made a bigger difference? thanks.

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'85 Z28 Copper/ Gold, LG4 Auto
dual snorkel w/ ram air, Accel Cap Rotor and 8MM wires, Edelbrock Headers, Catco Cat, Flowmaster 80 series, 3:23 rear
American Racing Wheels.

 

Did you change the speedometer gears? If not, then you would have zero RPM vs. speed change as indicated on your tach/speedometer.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/LB9 block, ZZ3 cam and intake, TBD heads, Hooker headers & y-pipe, 3" Catco cat).
57 Bel Air, my 1st car. 0.030 over 396, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.

 

I thought that a small gear change like that would not require a speedo gear change? I thought I was ok unlesss more than 3:42? Please advise. Thanks.

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'85 Z28 Copper/ Gold, LG4 Auto
dual snorkel w/ ram air, Accel Cap Rotor and 8MM wires, Edelbrock Headers, Catco Cat, Flowmaster 80 series, 3:23 rear
American Racing Wheels.

 

Your speedo caculates speed by using the rpm of the output shaft on the tranny. This has a direct relationship to engine rpm. If you put in 4.10's without changing the speedo gears, your rpm at a given speed registered by the speedo will be the same. This is because the speedo gets its reading Before the rear gears. For example, if you ran 1800 at 60 with 3.08's, you would run 1800rpm at 60(according to the speedometer) with 4.10's. But actually only going 40(a guess). Putting it the correct speedo gears will correct the speedometer. 3.23's are a pretty small change.

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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

It is a small change, less than 5%. For instance, at 65 mph with 3.08's, your speed with 3.23's at the same RPMs will be 61.8 mph. That's probably not enough to get a ticket for going too slow in the fast lane, and it isn't unusual for speedometers to be out by 10% just from tire changes.

So, is it worth changing the speedo gear for your gear change? That's up to you, but I'm a stickler for that sort of thing. Use the fomulas in the tech section (you have to know your tire diameter) and see how close you are.

Another example: When I had the original 3.08 gears, the speedometer was right on with the 25.5" diameter tires that were on the car when I got it. I put in the 2.93 posi and got 24.2" diameter tires, and the speedo is still right on with the same drive gears. That gear change was exactly the same % difference as your change (just the other direction), and a tire diameter change "fixed" the speedo calibration (that wasn't my goal with the new tires - it just worked out that way).

You do understand, I presume, that changing the rear end gears did not change the relationship between the engine RPMs (tach) and indicated vehicle speed (speedometer) - that's what 82camaro is saying. You have to change the speedometer drive gears to do that.