Gearing with rpm rang

[Doug80]

I was on the freeway doing about 70 and my rpm's were a hair under to right in 3k. Truck is a 80 Chevy 350 with 31 by 10.50 tires on stock rally rims. Any idea what gearing would be in the rear end? Would 35 by 12.50 help lower that rpm down and make it more happy at highway speeds or would it make it worse? If it would make it worse what gearing should I look into putting in to regear the truck?

 

[SlickGTP]

Which transmission do you have? If you have either the TH-350 or TH-400, then 3.73s would put your RPMs around 2800RPM @70mph.

35's would bring the RPMs down to about 2500... The problem with taller tires is that it would hurt your acceleration. But, the reverse is also true in that if you re-gear to bring the RPMs down it would also hurt acceleration.

 

[skysurfer]

^X2. That's a good guess based on the info provided but there's a big possibility for error when the op says "about 70" and without exact rpm's.

Tell us what transmission you have, use a gps so you know exactly how fast you're going, and have the tach needle sitting dead-on one of it's indicator marks.

 

[Doug80]

^X2. That's a good guess based on the info provided but there's a big possibility for error when the op says "about 70" and without exact rpm's.

Tell us what transmission you have, use a gps so you know exactly how fast you're going, and have the tach needle sitting dead-on one of it's indicator marks.

It's an auto and at 70 between 65-70 for sure (was dealing with stupid people not know how to drive) and was the rpm was at 3k or 2800

 

[Doug80]

Think it was more on the side of 70

 

[77 K20]

At 70 mph with a 30.5" tall tire (31" tires are usually a bit shorter) and an automatic trans with 3.73 gears you will be turning 2,953 RPM.
If you would put on 35" tires (34.5" actual height) would change the RPM to 2611.

But bigger tires have more rolling resistance, cost more, weigh a lot more... so could quite possibly hurt your gas mileage.

When I drive my truck on the highway to visit relatives it is almost a 3 hour drive one way for me at 2850 RPM. Other than it being a bit loud it doesn't harm the truck.

 

[77 K20]

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This chart assumes you have a manual transmission, or an automatic with a lock up torque converter (which you don't have). So you need to add more RPM to what the chart shows for torque converter slip.

I used a different online calculator that determined average automatic trans torque converter slip. Your RPM is in the green zone which is a good balance of power and gas mileage.

35" tires would bring it down into the yellow area- less power.

How much highway driving do you do vs city driving? Do much towing or hauling? Drive in hilly areas much?

 

[Doug80]

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This chart assumes you have a manual transmission, or an automatic with a lock up torque converter (which you don't have). So you need to add more RPM to what the chart shows for torque converter slip.

I used a different online calculator that determined average automatic trans torque converter slip. Your RPM is in the green zone which is a good balance of power and gas mileage.

35" tires would bring it down into the yellow area- less power.

How much highway driving do you do vs city driving? Do much towing or hauling? Drive in hilly areas much?

My gf is about 3-4 hours away and I vist her in the summer when she is at collage and just wanted to make sure that it running that high would not hurt the motor so even with the 35s it's still in the safe range?

 

[Doug80]

Worst gas mpg don't rlly bother me. did not get an old Chevy for the mpg lol love the look sound style toughness more than having to full her up every week lol

 

[skysurfer]

It's an auto and at 70 between 65-70 for sure (was dealing with stupid people not know how to drive) and was the rpm was at 3k or 2800

"65-70" and "3000 or 2800" is too vague, and you still haven't told us exactly what transmission you have. You might as well just tell us you stuck your hand out the window and it was flying pretty good.