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mph / rpm ratio?

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Zimmy that sounds right to my car speeds...
 
here is mr road test results
all in 4th gear
40 mph = 2000rpm
50 mph = 2500 rpm
60 = 3000
70 = 3000
75 = 3250


so what is my rear end
tires are 155/80/13
 
60mph and 70mph have the same rpm?
 
[ QUOTE ]
here is mr road test results
all in 4th gear
40 mph = 2000rpm
50 mph = 2500 rpm
60 = 3000
70 = 3000
75 = 3250


so what is my rear end
tires are 155/80/13

[/ QUOTE ]

Woodie,
Are those in miles or kilometers per hour? /ubbthreads/images/graemlins/lol.gif Either your tach or your speedometer is off. To be doing 75 mph at 3250 RPM you'd have a 2.85 rear. A 3.9 at 3250 if 55 mph. Get a GPS or follow a new car to verify speed to be sure.
 
sorry typo,, 4 yr old at my knees before

should read

40 = 2000
50 = 2500
60 = 3000
70 = 3500
75 = 3750

sorry again
 
Woodie, something is not correct about your rpm/mph ratio. Faulty speedo or tach, or you have a 3.5 rear end.
 
Woodie, my '72 should be identical to your car, and at 3250, I'm spot on 55 mph. That's with 175-70's on it.
That was validated by a sheriff buddy following me in the cruiser for a speedo check, as I wanted to verify things after the tire swap.
Jeff
 
Woodie, can you get access to a GPS? They do pretty accurate speed calculations. I've used mine to verify my bike speed when I got a re-calibrated speedo face. Tunebug has no working speedo or tach, so I use a GPS when it matters.
 
When I first got the car on the road I was driving it at 60mph, my wife was behind in the cherokee, she said she was doing 60 as well. SO I don't get it, is 3750 rpms to low at 75 mph in 4th gear. I thought it was about right.
 
one other thought, some time in the next day or two, I will do a rough 0 - 60 time and let you know.
 
nope bought it in may of this yr. I did some minor body work, painted it, tuned it and droveit. I have had several minor issues, electrical, timing, and the sort, but nothing major. She drives real well. see pics at https://www.cardomain.com/ride/2354952
 
Is it possible that the diff actually is a different ratio? Or even the final gear had been replaced?
Nice hardtop, all looks good, sparky good.
 
I know the PO put in a new clutch....
 
dont let the older guys see that motor....
they still paint theirs black /ubbthreads/images/graemlins/wink.gif ....zziimmmmy
 
[ QUOTE ]
Fair enough.
What's the formula for that Jerry?

[/ QUOTE ]

(rolling distance in inches x RPM) / (diff ratio *1056)

so for your 69" roll and 3.9 diff, (69*4000)/(3.9*1056) = 67mph
 
I'll see if I can do that in my head when I see a cruiser next time. Until I get a new speedo, I put some whiteout dots on the bezel, that formula is great.
Thanks John.
 
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