Hi all.
my Chrysler currently has a 2.92 open wheeler diff.
at 100kmh or 62mph it’s doing 2,600rpm!
that’s with a 727 trans behind my 318.

now I’m hoping someone will have an idea on this as google hasn’t come up with much I’m afraid.
I’m wanting to change my diff ratio to either a 3.23 or a 3.5 but can anyone tell me just what revs I’ll be pulling at 100kmh or 62mph with either of these two please.
????

if it helps my tyres are a 235/60/15

 

Look up Automotive Calculator, Wallace Racing, and DIY.

 

Shano -

Actually, there are a bunch of gear.rpm calculators on the net, including Summit, JEGS, and most gear manufacturers.
You'll supply any three of the following, RPM, MPH, gear ratio and tire diameter and it will calculate the third.
https://www.summitracing.com/newsand...tio-calculator

Now, assuming a 28" tire diameter, you already have a 3.5 in there.
With a 2.92, you should be doing under 2200 rpm @ 62 mph.

Archer

 

Thanks guys. I’m now thinking that my tacho is out!
like I stated previously at 100 kmph I’m doing 2600rpm and according to these calculators it should be around 2400rpm.
I need to find a dyno to get it up to speed so I can adjust my tacho as it has an adjustment screw on the rear. A potentiometer I believe it’s called.

 

Shano -

Finding a hand held tach or multi gauge would be a lot easier, and cheaper, than finding a dyno

Archer

 

hello,yes I did this already. Had my friends new digital tacho/timing light hooked up and calibrated my tacho according to this,of course while stationary in my shed but I thought it was accurate?!

I'm not so sure now after checking out some online calculators although it may only be out by about 200 rpm so not a great deal. However I did find out that with a 3.23 ratio I would be about 2800rpm at 100kmph so not bad no.
I will save,slowly (sigh) and get one.

 

Shano -

What size tires are you running?????

I've used 3.23s for years and with 28" tires, @ 60 mph, I was holding 2300-2400.
Now, with 3.91s 60mph = 2800

Archer

 

yeah, tire size and all that is necessary to calculate anything properly.

 

oh, i reread lol, he said... if it helps my tyres are a 235/60/15

 

yes,^^^^this

 

I’m guessing the final drive in the transmission plays a role in this too??
Dunno
its a 727

 

It's 1to1 but converter slip can make a difference.

 

well if it helps any my trans has a 2300rpm stall converter in it.

 

Shano -

You have a 26" tire, that's a factor; the 727, has a 1:1 final ratio and the stall on the converter doesn't factor into the calculation, since you're above the the stall speed.
Which raises the question, ... why are you using a 26" tire?.

Archer

 

Stall speed doesn't but converter slip does unless you have a lockup converter. A converter with a higher than stock stall can be less efficient at highway speeds especially if the stall speed is close the cruze rpm.
If you ran a car with a stock converter for top speed and swaped one with even a mild stall increase the car would go faster with the stock converter.

 

I suspect the numbers in the calculator are for a manual transmission.

 

I am running these tyres as this is what was advised by the tyre shop and my speedo is accurate so I’m guessing it’s the right tyre size?

 

Shano -

Just curious, what car?
They might be fine, just curious.

Archer

 

hello.
my car is a 1980 CM CHRYSLER VALIANT 318/727

 

Shano -

Nice ride!
Use the calculator and determine what rpm range you want to be in at what speed.
That will tell you the best gear ratio.

Assuming the motor is fairly stock, if the car sees a lot of highway driving with those tires, I'd be a little hard pressed to change anything.
2400 - 2600 @ 62 mph is a pretty good compromise.

Archer

 

Thank you Archer.
I’m only wanting to change diff ratios to give it a bit more pep from take off. A bit sluggish now. So I am considering a 3.23 or a 3.5 ratio.
got to make the most of a mild 318. Oh and the odd burnout if the boy in me comes out 🤣

 

Shano -

Yeah, it will do that, but put the numbers through the calculator and see if the extra pep of the line is worth the higher highway rpms and gas usage.

I'm not telling you not to do it, just making sure you understand all the consequences.
As I said above, I basically did the same thing, and I love it, but the car sees limited use (shows, club meets and just "fun").

Technical stuff: you have a higher stall converter, and want steeper gearing, so do you know the power band on the motor?
Like I said, plug in all the number and see.

Archer

 

This is why I think I will go with a 3.23!

better response and still fine at cruising.
a happy medium really.