Do I need to regear if I go from 37s to 35s?

[DaRocket95]

Basically I bought my truck as is and have no clue what gears it has but I've had a mechanic and friends drive it and say the tranny is good. But I keep reading about regearing when you change tire size...Right now I'm running 37x13.5x20 and plan to buy new 18s and run a 35x12.5x18. Will I need to regear?

 

Edit* By the way at 65mph my rpm is about 1600rpms, not bad..i think?

 

I dirve a 2000 Silverado z71 5.3L with a 6 inch suspension and 4 inch body lift. Avg a little under 13 mpg. I'm taking the body lift off and have to go down to 35s (37s wont clear and dont want to trim)

[blackout07]

Not due to a drop in tire size, but it's probable that you have stock gearing and need to regear anyway.

[DaRocket95]

How would I check to see what I have? When I bought the truck it came with 37s that already have less than 10% tread. And the tranny and stuff runs perfect, wouldn't a problem have already occured by then?

[Jsdirt]

RPO code will tell you - usually starts with a "G".

 

Another way is to jack up the rear end, put in neutral, spin tire one full revolution, & count the number of times the driveshaft spins. Marking the shaft with white paint or a grease pencil helps. 3.73 times is 3.73 gears .... of course that 3 hundredths of a rotation is tough to measure by eye, but you can get a ballpark idea without pulling the diff cover & counting teeth.

[DaRocket95]

Okay thanks dont have time to check soon, so based on the stats i listed: 1600 rpms at 65mph. Does that sound like i have good gears?

[Duelin]

Has The speed-o ever been re-calibrated With those tires that you have on it now? If not then comparing RPM's to MPH doesn't mean much. It gives you false info. Also if you re-gear you have to do front and back as a matching set.

[DaRocket95]

I actually checked my speedo with an app on my phone and it seems to be almost spot on

[Duelin]

According to the calculator that I used (link below). At 1600 RPM's and 65 MPH and 20" tires you have a 1.47 gears. Not good! And as far as I know I don't think you can buy 1.47 gears.

 

 

http://www.richmondgear.com/101032.html

[DaRocket95]

Haha i dont have 20" tires i have 37" tires!!!

[doodeediddledodindee]

According to the calculator that I used (link below). At 1600 RPM's and 65 MPH and 20" tires you have a 1.47 gears. Not good! And as far as I know I don't think you can buy 1.47 gears.

 

 

http://www.richmondg...com/101032.html

 

 

1.47 are the ones for highway mileage

[DaRocket95]

Got it for free so no real biggie...anybody know if its okay or not?

[blackout07]

It's way under geared. If your speedo is right that is...

[Mckenzie]

can we get some pics of the truck?

[tyhuck]

You have either 3.73 or 4.10 gears depending on what "about 1600 rpm" actually means. By my calculation, 3.73 would put you at 1541 rpm at 65, and 4.10s would put you at 1694 rpm.

 

Here is the calculator I use.

http://www.novak-ada...dge/gearing.htm

 

I dont think the Richmond calculator takes into account OD. Seems like its based on a 1:1 final gear, not the .70 that our trucks have.