68 Charger Gearing

[Roger S]

Hi all

I now finally have a working Tach. I have had the car some 6 months and have only been out a few times. I always suspected the revs seemed a bit high in relation to the actual speed.

Went out yesterday for the first time with a working Tach, I am getting 2500 rpm at 60 mph and 3100 at 70 mph. It is 440 with torque flight auto box and sure-grip rear end. Are my readings about standard for the time ?

My limited history on the car from its time before import from Texas in 2013 contains claims of regular 11.70 quarter mile times from the Mopar dealer who was selling the car. Certainly the engine has been breathed on with big valve Edelbrock heads, lumpy cam, high rise manifold, Holly double pumper etc, but I have no paperwork on the mods, it has only had one owner since import and was only used as Bullitt show car, so the previous owner can not offer any input.

So I am thinking may maybe the Diff has been altered for use on the strip and been mildly modified for that,although visually both inside and out there is nothing to suggest that,no evidence of a previously fitted Monster Tach or strobe etc, all suspension and running gear is stock.

Any thoughts would be much appreciated.

Thanks

Rog

[SJH]

Hi Roger,
Have you checked your speed with a GPS speedo ? (You can get an ap for your phone). Sixties speedos are quite variable in accuracy. This should give you a more accurate value.

Also the overall gearing will be affected by the size of the wheels and tyres you have. So putting larger (radius) wheels and tyres on will higher the gearing as long as they don't catch anywhere or spoil "the look"

From your observations, your mph/1000rpm is approx 23/24. (60/2.5) or (70/3.1)

cheers
Shayne

[Pete]

I would also test your tach against a Snap on or equivalent high quality timing gun.

I have seen these installed incorrectly and read with 100% Error - strange but true!

[Roger S]

Hi Pete & Shayne

The Tach is an integral part of a Dekota digital dash I purchased and fitted from Classic Industries, so I would hopefully expect that to be accurate.

Yes I have checked against a Sat Nav I had in the car at the time and my speedo is within 1 MPH at 70 MPH although it does seem to fluctuate at around 40 MPH seems to be around a 3 MPH difference on the low side compared to the SAT Nav. I had a good look at the diff bolts on the casing this afternoon and indeed all the mounts for the axle and the look like they have been undisturbed for years. I guess I should check the Tach against a strobe.

Thanks

Rog

[Matt]

Your car could have left the factory with anything from 2.76 to a 4.10 rear end gears , depending on its original spec. That's a huge difference in rpm per mph

As a starter you should probably measure the ratio you have, as a point of reference for any action.

Jack up one rear wheel and rotate it by hand two revolutions, with the other rear wheel on the ground.
If you have a limited slip diff (which i see you do) then you'll have to jack both wheels off the ground at the same time, and rotate only one revolution.

Count the number of turns the propshaft makes. This is your rear axle ratio, and is most likely to be one of the following


3.23
3.55
3.91
4.10

After you know your axle ratio you can better judge whether you want to change it, and whether the tach reading is about right .

[RobTwin]

Fro the revs @ mph you mentioned I would guess you have 3.55 gears - a good compromise between the better cruising 3.23s and the more drag-suitable 3.91s

[cadboy]

sounds like it could be 3:91 like mine is, 3100 RPM at about 70 miles.

[Roger S]

Your car could have left the factory with anything from 2.76 to a 4.10 rear end gears , depending on its original spec. That's a huge difference in rpm per mph

As a starter you should probably measure the ratio you have, as a point of reference for any action.

Jack up one rear wheel and rotate it by hand two revolutions, with the other rear wheel on the ground.
If you have a limited slip diff (which i see you do) then you'll have to jack both wheels off the ground at the same time, and rotate only one revolution.

Count the number of turns the propshaft makes. This is your rear axle ratio, and is most likely to be one of the following


3.23
3.55
3.91
4.10

After you know your axle ratio you can better judge whether you want to change it, and whether the tach reading is about right .

Hi Matt

Yes I have LSD,do I have to rotate both wheels or only 1 ? I tried tonight but only turned 1 wheel and that tells me I have 3.55 now thinking I should have perhaps turned both at the same time, but will need assistance for that.

Thanks

Rog

[Matt]

If your LSD is working OK then you only need to rotate one wheel, as the other wheel is clutched to it and will automatically rotate in the same direction at the same speed.

As a very basic check that the LSD is working, just jack one wheel off the ground (handbrake off, gearbox in neutral). You shouldn't be able to rotate this wheel by hand, as the other wheel on the ground will be 'holding 'it

[Roger S]

If your LSD is working OK then you only need to rotate one wheel, as the other wheel is clutched to it and will automatically rotate in the same direction at the same speed.

As a very basic check that the LSD is working, just jack one wheel off the ground (handbrake off, gearbox in neutral). You shouldn't be able to rotate this wheel by hand, as the other wheel on the ground will be 'holding 'it

Hi Matt

Yes working ok, It was just that someone said a long time ago that the other wheel may slip slightly on the clutches and not rotate at the same speed as the other wheel being rotated which could somehow give a false reading, excuse my ignorance on this !

So looks like I have 3.55, its is effeminately not a 3.91, I will put a strobe on to check my RPM against the Tach.

Thanks

Rog

[Matt]

Great

3.55 is a little more in favour of acceleration rather than having low rpm on the highway.

3.23 would bring the revs down around 10% and is a good all-round ratio IMHO

you might even like 2.76 if comfort's your thing, given also that your tyre diameter is probably a little smaller than many people use this might not be such bad gearing.

With a 440 your car will still pull like a train if you downshift.

[Roger S]

Great

3.55 is a little more in favour of acceleration rather than having low rpm on the highway.

3.23 would bring the revs down around 10% and is a good all-round ratio IMHO

you might even like 2.76 if comfort's your thing, given also that your tyre diameter is probably a little smaller than many people use this might not be such bad gearing.

With a 440 your car will still pull like a train if you downshift.

Hi Matt

I now have set of Magnum 500 wheels fronts 225/70 R15 rears are 275/60 R15 ( Couldn't live with the Hubcaps ) I understand that they equate to 28 inch diameter which is only a tad taller that what I had originally

I guess I would prefer 3.23, but I can live with what I have as I don't do too many long journeys, I do like the acceleration and yes she does pull well.

Thanks

Rog