New Gears & Tire Size

[1969K10]

So, I"m finding the 4.10 gears in a 14-bolt are easier to come by than replacing the 3.73's in it. I might have found a D60 for the front. So here's my ? I'm running 33" tires with 3.73 gears. When (if) I go to 4.10's, I'm going to need a 36-37" tall tire to maintain the speedo. Does that sound correct to anyone who know's what they're talking about?

THX

[69CvyPickup4x]

ttt for ya. i have 4.10s and im running 35's. problem is... i dont have a spedo. but i will get one sometime. i had a spedo before i switched to a 205 tcase and it was acurate up to about 45mph.. but i think the cable was just messed up.

[Zoomad75]

Here is what I figured out. First off, are you sure your speedo was accurate with the 33's and 3.73's? My bet is no as most don't get the speedo corrected after moving up sizes in tires. So what that means is, you have to compare to what would have been stock or around 31" tires with the 3.73's. Look at it this way:

Right now: 3.73's, 33" tires works out to 2279 rpm @ 60 MPH
Projected: 4.10's, 33" tires works out to 2505 rpm @ 60 MPH
Stock: 3.73's, 31" tires works out to 2406 rpm @ 60 MPH
(this is based on a 1:1 gear ratio in high gear, no OD)

To put it another way, when you moved up to 33's with the same gear ratio, your speedo started spinning faster (each tire revolution travels further with a bigger tire), by the amount of 3 mph @ a true 60 mph.

Now from what I've figured you aren't going to be too far off with the 4.10's. But if your speedo was accurate with the 33"/3.73 combo and then changing to 4.10's then to correct the speedo by tire size alone, you would have to run a 36" tire.

I'd check to see how accurate your speedo is now before you change the gearing. Unless somebody has added a different gear on the speedo drive at the trans or a ratio corrector box at the speedo, chances are your setup is off to begin with.

[msgross]

you can buy a gear reducer/multiplier (don't know the real name) to change the speedo cable ratio. It connects in line with you t-case and cable. Don't ask me the ratio that you need though....

I wouldn't change gears to make the spedo look right but I would change gears to make your tach run what you want (power vs. mpg)

[Towstrap]

this is some good info

http://www.4lo.com/calc/gearratio.htm

[1969K10]

Thanks all. An accurate speedo is not my main concern. I'm worried about added stress on an already tight gasoline budget (kinda). As I am, whatever the speedo reads, you got to add 1/2 again for actual speed - reads 10, traveling 15 - reads 40, traveling 60, etc...