Hello all!

Maby a starnge question... but im trying to figure out what gear ratio a E-body car has.. its coded for 4.10 super track pack and it revs around 3500rpm in 60mph.. could it be 4.10 then ? (auto car)


Thanks
Nicklas

tire size matters

JACK up car rotate rear wheel one turn , driveshaft should turn 4.1 times

F60-15 Polyglas tires. Im not close to the car..

Assuming zero slippage, 60 mph with 4.10 gears and F60-15's works out to 3191 rpm.

Got the tire diameter here - https://turbinecar.com/tires.htm

Rom calculator here - https://purperformance.com/p-29669-rpm-calculator.html

Thanks!

I'd check the tach for accuracy too. Some are surprisingly far off.

A little tire wear along with your auto transmission will easily put you above 3400 rpm.

Especially factory ones

Absolutely can affect the RPM, especially as you go down in circumference
I use to see from 1/2 inch to 3.0 inch differences in the same size tire between different brands
put a set of L60x15 on it and see how much the RPM goes down at the same indicated MPH

For reference, I just lowered my gear ratio to 4.30's with 28" tall rear tires. It spins 2500 rpm at 75 mph with the torque converter locked up in 4th (.67 OD). There are plenty of online calculators to plug your numbers into and get a reasonably accurate result.

Hi 71 , my buddy has a 70 challenger with a 440 , 4 speed , Dana with 4.10s . Rear tires are 255-60-15 or G60-15 . Spins exactly 4,000 RPM at 70 mph . He gets passed by ricer cars and mini vans and HATES driving it on the highway !

becarefull with RPMs calculators... they don't consider the slippage AND the inertia of the car while riding, so just can take the rpms with engine pulling completelly the car.

there is a nice XLS table on the tech archives too.

and sometime ago somebody made an animated RPM calculator based on a 68/70 B body Rallye cluster where you could add a slippage % for the torque converter, as an estimate. Dunno if is still working.

as mentioned, with wheels on the air ( being SG, need to get both on the air ), a full turn of the axle/wheel will rotate the drive shaft the gear ratio. If single leg ( no SG ) you can raise just oen wheel andwill rotate half of the gear ratio

Inertia, in a steady state, is irrelevant.

With a stick car, slippage is irrelevant.

With an automatic, slippage will depend on the torque converter. A lock up TC will have no slippage, or it shouldn't.

I wouldn't say inertia is irrelevant. I have driven my Charger at 130 kmh ( 80 mph or so ) on highway just keeping the acc pedal to hold the speed and I was at 3200 rpms, while calculators give me 3800. and all that diff is not slippage, becauise slippage will give you the number backwards, with more RPMs than the calculators give

If he would install a 5 or 6 speed overdrive he would be doing the passing and love it.

Hi mg , his brother has a 70 cuda 440+6 , 4.10 Dana with a 5 speed conversion and is it sweet ! I have driven it and Highly agree , especially if you want to keep those deep 4.10 gears . But his brother will NOT change or do anything not stock , Original only .

Inertia is irrelevant. Sniper is correct.

It has a minor component on an automatic as it'll change the converter slippage. The slip percent will change going up or down a hill, as an example. On a manual it doesn't matter at all though.

Another thing that will affect it is tire pressure and the tire itself. The tire squishes and loses diameter as you apply load to it, so the rpm to vehicle speed relationship changes.

I met a guy this morning that just recently bought a 440 sixpack 4spd Super track pack 1970 Challenger. And he says the 4.10s will be the very 1st thing to go away to be replaced by a 3.54 Dana

He didnt mention RPMS but he said the car is screaming on hiway speeds