highway rpms too high
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Joined: Jun 2002
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From: simpsonville USA
highway rpms too high
I've got a 68 F100 with a transplanted C6 and a stroked 390. I mixed and matched parts and rebuilt this puppy so it had what I wanted from the factory. PB, PS, AC, Auto C6, and disc brakes. None of this was on the truck when I got it. The truck is now a hoss but seems to run a little high RPMS on the highway. It is running 3500 at 65 mph? The motor is a 390, C6 tranny, and 3.25 gears. I checked the gears by raising the truck and turning the tire 2 times and counted 3.25 revolutions of the drive shaft. So 3.25. Seems awful high! Any ideas?
Ken
68 F100 390
74 Bronco 351W
Ken
68 F100 390
74 Bronco 351W
Postmaster
Joined: Dec 2002
Posts: 3,959
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From: USA
3.25 seems like a pretty high gear to be able to give you 3500 rpms @ 65 mph. What size are your tires?
I'm suspecting that the gear ratio is not 3.25, but lower (numerically higher). With 30" tires, no overdrive, an auto trans, and 3500 rpms @ 65mph, your gear ratio would have to be around 4.50:1 That's not a likely gear ratio for that truck iether, so maybe your tires are really small? If your tires were 27" the gear ratio would be around 4.10 with 3500 rpms @ 65mph. That seems to be a likely scenario. At any rate, a cheap way to reduce your rpms is to switch to taller tires.
I'm suspecting that the gear ratio is not 3.25, but lower (numerically higher). With 30" tires, no overdrive, an auto trans, and 3500 rpms @ 65mph, your gear ratio would have to be around 4.50:1 That's not a likely gear ratio for that truck iether, so maybe your tires are really small? If your tires were 27" the gear ratio would be around 4.10 with 3500 rpms @ 65mph. That seems to be a likely scenario. At any rate, a cheap way to reduce your rpms is to switch to taller tires.
Thread Starter
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Senior User
Joined: Jun 2002
Posts: 119
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From: simpsonville USA
ya, my tires are 27 inches so that could be part of it but my gears are 3.25 unless I am really missing something. I have been having some trouble with the 73 power brakes I swapped in with the discs. They have been dragging some so that could be part of it too. I did adjust my intermidiate band and it has been okay. Should I be dropping into a cruise mode on the freeway?
Thanks,
Ken
Thanks,
Ken
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