Rpm's too high?
Junior User
Joined: Apr 2002
Posts: 90
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From: oldsmar 34677
Rpm's too high?
I have an 84 w 351/ C6. I have 33x12.5 tires on there. I don't know the gears that are in there, but right now I am at 2500 and only 55 mph. This seems really bad too me. Do I just have a high gear or what. And when I eventually get my 38's or 40's what gear should I put in.
Thanks
John
Thanks
John
Postmaster
Joined: Aug 2001
Posts: 2,882
Likes: 0
From: Nevada
Rpm's too high?
Your speedometer is, I believe, calibrated for 26" tires. That means it's way off with 33" tires. You're probably closer to 70mph than 55mph. An '83 should have a 3.50 rear end.
Here's a calculator that will, with an accurate speedometer, calculate your rear end ratio.
http://www.smokemup.com/auto_math/rear_end.php
With 38-40" tires, you're going to need some short (numerically high) gears to get any power to the ground. And that means changing the rear and front ratios.
Here's a calculator that will, with an accurate speedometer, calculate your rear end ratio.
http://www.smokemup.com/auto_math/rear_end.php
With 38-40" tires, you're going to need some short (numerically high) gears to get any power to the ground. And that means changing the rear and front ratios.
Senior User
Joined: Jun 2001
Posts: 236
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Rpm's too high?
I am betting you have 4.56 gears in that thing if you are really at 2500 rpm at 55 mph. I can't say for sure, but try playing around with the numbers in this gear calculator, and see what you end up with:
https://www.ford-trucks.com/articles/gearcalc.php
https://www.ford-trucks.com/articles/gearcalc.php
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Senior User
Joined: Jul 2001
Posts: 237
Likes: 0
From: Clearwater USA
Rpm's too high?
Lift the rear tires off the ground put tranny in nuetral. Crawl underneath the rear, mark the drive shaft mark the one tire that turns when you turn the shaft, turn the drive shaft count how many times the shaft turns compared to the tire. You will get a pretty close idea you will be able to tell whether or not it is say a 3.55, 4.11 or 4.56. Just go slow and count the turns. Hope this helps and Godd Luck,
Rick
86 302 EFI, bored 60 over, AOD,
Headman Headers, Flowmaster 40 series,
2 inch dual exhaust, MSD ignition
Rick
86 302 EFI, bored 60 over, AOD,
Headman Headers, Flowmaster 40 series,
2 inch dual exhaust, MSD ignition
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