RPM at 60 mph

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grassy

grassy

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Folks,

We are turning about 3500 rpm at 60 mph. We have an a727 trans with 3.23 gear set and 17 inch wheels..I thought it should be slower than than..

This normal ?

Thanks
 
That doesn't sound right at all. What sort of converter do you have? I run 3100 rpm at 60 mph with 4.10 gears, 28" tire, with an 8" 5500 stall converter on the 727.
 
I run a Chrysler TQ 2200 stall I believe.

I was actually expecting 3000 rpm or less...speeds over 60 gets kinda buzzy...
 
Ian,

Rim size is not relevant, it's the overall tire diameter that is important.

Taller tire = lower RPM.
 
I just caught this on another thread..

"My a100 has a 273/727 with a 3.23 ratio and 28.00" tall tires. It is just a driver/cruiser. And at 60mph I'm running about 2350 rpm"

Crackedback,

Makes sense..I go out and measure.

Thanks
Ian.
 
Either the tach is way off, or the tranny is slipping badly. With a 3.23 gear I would expect 2500 RPM or so. There are websites with formulas that you can use to see exactly what the RPMs should be. You need to start with the diameter of the tires, of course. Here is one of them: http://www.4lo.com/calc/geartable.php
 
26 inches.

I have been in cars with slipping autos... with this car you put your food down and it scoots..no slipping at all.

Could the tach be off ?
 
According to the charts, I should be @ 2500 rpm..drats.. I am off by 1K..
 
mopowers said:
Is your speedometer accurate?
Click to expand...

As far as I know. It is electronic so I don't have to worry about old speedos nor the associated cables.

Sounds like I need to do find a flat, straight chunk of highway and have him chase me.

This is good stuff.

Thanks
 
That's a long ways off from correct. 3500 should be in the 80mph range.

Gear in trans correct? Try a GPS app in a smart phone to try and verify actual speed.
 
Could the tach be set for a 6 cylinder? Most of them have settings for 4,6 and 8's.
C
 
clhyer said:
Could the tach be set for a 6 cylinder? Most of them have settings for 4,6 and 8's. C
Click to expand...

Thought about that ...pretty sure it is hard over..but I shall check.

GPS is even a better idea that one car chasing another...thanks..
 
Also check the ground wire on your tach.
Mine was corroded and for some reason read high on the Auto meter I have.
 
Good ground...was just working on that last month.

1968...ouch...

We should sort this out this weekend.

Thanks all..I shall be back to report results..
 
There are many free speedometer apps for smart phones. They use GPS to calculate your speed and they work pretty well, especially at a constant speed.

Rather than try to measure your tires with a tape measure, do a Google search for "tire size calculator" and type the size into the calculator to get all the dimensions of your tire including diameter.

Once you know the diameter, use one of these calculators to computer your RPM at any given speed. http://www.ringpinion.com/Calculators/Calc_RPM.aspx
 
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