i have 3.55's and 35's on my truck and it fly's off the line killing the majority of other vehicle's but it runs out of rpm's at 75mph..but i get 15mpg highway and 13mpg city(this is when i had 33's on haven't had the 35's on long enough to know)...i'm wondering how much the tire's throw out the speedometer..and if swapping to lower gear's will better my mpg or make it worse..and how much it will take my top end speed up..im thinking like 3.08's any opinion's?

 

What size tires was your speedometer calibrated for? (stock size)

There is some long drawn out equation for finding true error......I think it is available online somewhere.......

And what do you mean by 'runs out of rpm's'? Are you hitting redline? What RPM do you run at 75mph?

Here ya go. Error calculator.

 

If anything you need taller gear such as 4.56 for 35's.

 

225/75r16 and im running 35/12.5r20 and i dont hit the rev limiter i just run into the higher rpm's like 4-4500 i'm not a believer of running high rpm's for extended period's of time because a normal drive on the highway for me is 5-10 hours and wouldnt 4.56's kill my highway speed and make it so i have to shift more frequently?i have no acceleration problem's i kill my buddies chipped cummin's all the time...

 

Are you sure you have 3.55 gears, and are you going into OD

 

Sounds like you aren't making it into fourth gear. (and probably no TCC lockup either.) With 265's, and 3.55 gears, I am turning about 1750 RPM at 60 miles per hour. With larger tires, and the same gears, you should be running even less.

Given the original tire size, your speedometer is probably off by close to 10 mph at 60...... (an indicated 60 mph would mean you were actually doing closer to 70.)

 

yes positive i have 3.55's and yes i'm in 5th gear in my 5 speed..

 

Ooops. Manual trans? Is your transfer case stuck in low range?

 

There's a whole math calculation that is used to calculate the proper gear ratio to tire size. The gears should be changed to IMPROVE fuel mileage and speed off the line as well as when the torque converter kicks in, it drops the RPMs down when cruising- this causes lugging uphill / under loading conditions.

I should have that formula around here someplace but, off my head you looking to change them up to a 4.11 or possibly higher even to say, 4.86. On mine, running 3.55, and 33s, it lugs under load and, I've had to compensate by disengaging the TC by switching off the OD or kicking it down a gear when going up super steep inclines when towing. Unloaded it's not nearly as noticeable. I'm going to 4.10 - 4.11 on mine this summer when I get some time to change them out.

CM

 

Ur speedo must be way off, High gear with those rpms u must be flying with the 35's. Thats fast for my little 265's, around 90mph in drive if i remeber correct, I thought my speedo was off because i went by the door sticker for tire size and used one of those error calculators. I double checked it by doing a mile-marker to mile-marker run at a steady 60mph, should take 60 seconds. Mine is so close i call it dead on. It might not be a perfect way to tell but it works to keep u from getting a ticket for the speedo being off