Which Diff ratio is best?

[Johann65]

I have a 3.55 and a 3.36 diff to choose from. I have a HO Camaro engine and 700 4r auto box. Engine and box is as original with no modifications. Comes from a 1984 model. No racing planned just cruising. It appears that the 4spd +OD is going to be reasonably economical.

Can the ratio be changed without changing the whole diff?
I have the 3.55 as original in the Kommando. The 3.36 is in the el Camino that I am using for spares.

[drgfinger]

Johan
It won't make much difference what you use , both ratios are very close to each other , it wont make any sense to change from whats in the vehicle right now if its working right.
I have a 2.6 diff + 700r4 in my vette , 250 hp FI motor , I get 10km/litre on long runs at 120-140kph. Im going to 3.55 for a little more oomph from the robots.
If you drive "nicely" ,you will most likely get between 6-8 km/litre in town type driving...economy shouldnt be a real issue unless you plan to use the vehicle as a hyour daily driver.

[Johann65]

Thanks DR,
I think it should then be fine to stick with the 3.55 diff and see where it takes me. The Kommando is heavier than the Corvette and will need the higher power to weight ratio. My Malibu with its Powerglide gives me 9.4 Kml at 100Kmh on the road. I hope the Kommando gives the same.

Any Idea what the 3.55 top speed at 1:1 ratio will be with standard 195x14" tires as fitted on the Kommando? My knowledge is restricted to High temp Furnaces.

[drgfinger]

Johan , I have a nice program I can send you that I wrote many years ago , will work out all maner of things among em kph/1000 rpm based on diff and tire specs. PM me your mail addy and I will send it to you - called et1.exe

Assuming a 195/14 with a 60 profile - you will be geared at 31kph/1000 , in od it will be 44.5/1000
Assuming a 70 profile - 33/1000 and 48/1000

If for eg , you went to a different tyre and rim , lets say 225/60/15's , you will actually gain a bit , 35/1000 and 50/1000, better traction , nicer looks , better handling etc etc.

The figures aren't quite accurate as there is the fact that the TQ wont always be locked and some slippage occurs across all gears.

The 700r4 actually has a pretty short 1st , at 3:1 vs, lets say, a th350/400 which has a 1st gr ratio of 2.48 so its pretty easy to get the car moving , my vette comes in at 1550kg without me , and im about 95kg ... not so light.

Top speed will depend on RPM - generally with the OD boxes , you cant rev to the max rpm of the motor - wind resistance overcomes HP - if you could rev to 5500 in 1:1 , it's unlikely you would get there in OD unless you had a lot of power
Even so , I would be a little scared of taking the car to speeds in excess of 200kph

[Robert_F100]

The formula that you can use to calculate top speed is as follows:

Engine revs/diff ratio*Circumference of tyre*60/1000= KM per hour

The diameter of the tyre is:

195/60R14 = approxomatly 23 inches
195/75R14 = approxomatly 25 inches

circumference of type = pi * 23 = 74
=pi * 25 = 78

Convert to centimeters 74 *2.54 = 1.87m
78 * 2.54 = 1.98m

So if top revs are 5000

5000/3.55*1.87*60/1000 = 158

I hope this gives you an idea of what your top speed will be.

[Johann65]

Hullo all, My tires are 195R70 X 14" Therefore:
@ 3000 Rpm= 100Kmh (Average Standard)
@ 3000 Rpm = 144Kmh (Overdrive)
@ 120 Kmh = 3630 Rpm (Average Standard)
@ 120 Kmh = 2541 Rpm (Overdrive)

Those are fine figures for my "Normal Driving"
Looks like the 3.55 is just fine and I should not be concerned.

Busy with the wiring, and the OD set-up that locks out of fourth gear when applying the brakes and back in fourth when applying the throttle.
Will Post some pics over the weekend!
There is a dual switch set-up coming on the brake pedal and a B&M switch on the console. I believe the 84 Corvette has a similar set-up.
The 84 Canadian Camaro has the same set-up

Thanks DR and F100 for your input