Engine RPM at Hiway speed

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340dustr

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Hey all, I have a 70 Duster 340 auto with 3.23 rear sure grip. At 60mph, the tach reads 3500 rpm. This seems high to me as I used to have a Dart w/340 and 904 auto and 3.23 sure grip and it would rev at 2800 rpm at 60mph. Could the 3rd gear ratios in the trans be that much different between the 727 & 904? The tach was rebuilt last year and the engine sound at 60 seems to match what 3500 rpm sounds like. Any thoughts?
 
Both trannys have a 1-1 final drive ratio. There must have been different sized tires or
the speedometer gear was incorrect in one car
 
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An inch in tires size is ruffly equal to a 0.14:1 in gear ratio.

2.94 with 24" tires and 3.55 with 29" tires is the same overall ratio.

24" tires is equal to 1:1
26" tires is equal to .92:1
29" tires is equal to .82:1

Just divide 24 by your tires size to get the final ratio.
 
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How tall is your tires and what gear tooth installed in the trans? Use the search and there are several gear tooth charts posted on this site. If the speedo gear is correct to the chart for your tire size and axle ratio, i'd look at the trans. Could be stuck in second as stated previously or a slipping converter
 
racerdan80 said:
How tall is your tires and what gear tooth installed in the trans? Use the search and there are several gear tooth charts posted on this site. If the speedo gear is correct to the chart for your tire size and axle ratio, i'd look at the trans. Could be stuck in second as stated previously or a slipping converter
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Tire size is 215/70/14. Height of tire is 25.25". Speedo gear is yellow-not sure of the teeth but I used the charts corresponding tire size to gear ratio to select the pinion. Trans shifts up to 3rd gear.
 
340dustr said:
Tire size is 215/70/14. Height of tire is 25.25". Speedo gear is yellow-not sure of the teeth but I used the charts corresponding tire size to gear ratio to select the pinion. Trans shifts up to 3rd gear.
Click to expand...

Same tire size on both cars ?


with zero slippage should turn 2600 rpms so your old car seems right this one is way off.
 
You can test you speedo using your smart phone. On Google Maps it shows the speed you're traveling. At least then you'll know its correct.
 
There is also a smart phone app for speed using gps and it's right on the nose as far as I can tell.
You can also cruise with another car and compare the differences.

Both free info.
 
Something is definitely fishy. No way you should be at 3500 RMP with that setup.
 
273 said:
Same tire size on both cars ?


with zero slippage should turn 2600 rpms so your old car seems right this one is way off.
Click to expand...
Not sure of the exact tire size on the old car, but I know the tires were comparable to what is on the Duster now. If there is slippage, would it be in the trans or converter? Any way to eliminate one or the other?
 
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