Why are rpm's so high
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Why are rpm's so high
I just rebuilt the 302 in my 79 f-150. i swapped out the pos 4 speed with od SROD for an NP435. The NP435 came out of an 84 f-150. I know 4th is 1:1. I have 235/75/15 tires. From what i have read they should make 722 revs/mile. I have a 3.00 rear end. Doesn't it make sense if i travel at 60mph I am traveling a mile/minute so my tires should make 722 rpm. Take 722 times my rear end ratio (3.00) to make my engine rpms 2166 at 60mph. I put a tach on and my rpms are between 2800 and 2900. That seems high to me. Can anyone figure out why this is?
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I just thought of something but don't think this should effect it. The speedo gears are specific to what rear end ration you have, correct? When i swapped trannies, i just unhooked the speedo cable from srod and hooked it back up to np435 assuming i didn't need to change the speedo gear since my rear end ratio wasn't changing. Was i wrong? Is my speedo gear why my rpms are so high? Maybe i was was actually going about 80 and speedo said 60?
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Can someone explain what determines what speedo gear you should have, I just thought it was the rear end. It has stock rear end and tires. What tranny I have shouldn't matter, at 1:1 ratio is a 1:1 ratio.
