3.08 to ?????
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3.08 to ?????
After much confusion, I decided to mark my driveshaft, and push the truck forward one tire revolution, and count driveshaft turns. Turns out I have a 3.08 which is what the tag says on the door. Funny thing is at 55 mph, I'm turning 2050 on the RPMs with 31 inch tires, and a M50D tranny. I would have thought it would have been much lower. Anybody know the final gear ratio in that tranny? 89 F150 4x4. So anyway, to get the best combination of fuel economy and acceleration, which gear should I put in it? Because it doesn't move too fast and it sucks gas bad. So which gear would be best for the economy? which would be best for power? which is best for both? Its got a fresh 302 EFI. Tell me what you guys think. Thanks.
Timmy
Timmy
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From: South Dakota
The overdrive(5th gear) is 0.80 or 0.79, so if you run in 4th gear(direct 1 to 1) that would IN EFFECT give you a differential ratio of somewhere between 3.73 and 4.10. (3.85 to be exact using 0.80 if my thinking is correct) That should help you in decideing on an axle ratio. I just noticed it's a 4X4, so it may be simpler to go with smaller tires rather than change both differentials.
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yup
I was thinking that driving around in 4th gear would be a good test of economy, except for acceleration, I do a fair amount of city driving, so I am not sure how that would play out. I don't want to put any smallter tires on it that I already have. I'll find gears. but what I wonder is how my speedo was off when I first got the thing. it was sayin 2350 at 55mph in 5th gear. When actually 55mph is at 2050. Checked against three other vehicles and my dads GPS. So it must be right. How was it so far off? Eventually the drive gear on the output shaft in the Tcase stopped spinning. Who knows. right now its tough to know what my mpg is, lacking a speedo and all, so how do I fix that? But I know that its bad mpg. Real bad. Thanks.
Timmy
Timmy
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From: Harrisonburg, VA
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no sense
What doesn't make sense is how I can be turning 2050 rpms at 55 mph in 5th gear with a 3.08. In 4th at 55 mph, its about 2700 or so. how is that so high? Shouldn't I be more around 1600 at 55? Something must be wrong, could my tach be inaccurate? Tim
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exactly
You are exactly right, but the math doesn't seem to be consistant. it seems that the tach may be innacurrate... anybody else have this problem? How can I measure the actual RPM? I should be at about 1600 in 5th, and I am way high. I can't think of any other explaination. by the way, my speedo is INOP so maybe that has something to do with it. Don't htink so though... what do you all think?
Timmy
Timmy