speed doesn't match speedometer
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Junior User
Joined: Mar 2009
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speed doesn't match speedometer
I have an 88 F150 custom with a 4.9L I6 300 2 wheel drive M50D-R2 transmission and 3.08 rear end. The truck is straining pulling loads that really shouldn't be a problem. Another problem is the speed being off from the speedometer. It all started when I put my new tires on which are 30" tall. The speeds are at the bottom.
Since I have a 3.08 rear end gear should I go 3.55, 3.73 or 4.10. I don't want to make it the opposite of what it is with a top speed of 65. So just to get back as close to factory as possible.
under 30 not so bad but
when it says 45 I'm doing 52
when it says 70 I'm doing 85ish
and the last time I hit 85 I was racing a 2001 f 150 that had the speed shutoff at 106 and I kept pulling past that.
Since I have a 3.08 rear end gear should I go 3.55, 3.73 or 4.10. I don't want to make it the opposite of what it is with a top speed of 65. So just to get back as close to factory as possible.
under 30 not so bad but
when it says 45 I'm doing 52
when it says 70 I'm doing 85ish
and the last time I hit 85 I was racing a 2001 f 150 that had the speed shutoff at 106 and I kept pulling past that.
Hotshot
Joined: Sep 2001
Posts: 11,958
Likes: 228
The I6 is a low revving engine. I would go with 3.55 or maybe 3.73 if you had 31 10.50 tires. Don't go 4.10. That gear is more suited for the 5.0 and 5.8. The 300 makes its power down low and doesn't need the lower gears, and it doesn't like high RPM.
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