3.42 to 3.73 with 31" Tires
 

I have 31" Mickey Thomsons on my '89 S10 Blazer.
I have a G80 with 3.42 ratio. My speedo is off ~ 2-3mph.
Will a gear change to 3.73 get me closer to the correct mph reading?
What would 4.10's deliver as far as the mph reading?
I know the cost & work need to swap them out.
I'd like the taller gears to help the get-up-n-go along with the added help to the transmission.

gearing won't correct you're speedometer it will help with the loss of power but you will have to get your speedometer recalibrated via the computer to get it to be exact

Correct me if I am wrong on this...
 

I know the gear ratio change wouldn't correct my speedometer reading but I wanted to know how much closer I could get to the correct actual speed display.

Going from a 3.42 gear ratio to a 3.73 gear ratio is a reduction of ~9%

When I travel at "true 65mph" my speedo displays ~67mph due to the larger tires (3.5% increase)

So I calculate that if I change gears to 3.73 it will reduce my mph display 5.5% (9%gear - 3.5%tire)

(67mph displayed - 5.5% = 63mph displayed)
If I travel at "true 65mph" my speedo will display 63 mph - I can live with that without a pcm tune.

I did just what you're talking about, although I switched to 3.73 from 3.42 gears then changed to 31" tires. 31" tires with 3.73 gears it put my speedo pretty close to accurate, according to my GPS (+/- 2Kmph). I did use a programmer to correct the ECU afterwards. I thought it was the best thing.

89 has a DRAC and you don't tune the ecm to correct the speedo. All yoou have to do is make it adjustable so you change the jumpers.

Digital Ration Adapter Controller Calibration


I have 3.73s with 31s and it isn't bad. Mine came with them factory tho. If I was gonna change I would go to 4.10s. If ya lucky you can find a front and rear in a junckyard so you don't have to set up gears.


Gearing does indeed change speedo read out same way changing tire size will. Changing gears isn't the way I would go about fixing a speedo it can be done. I lucked out and my 87 is a cable speedo so I've collected all kinds of drive and driven gears to adjust mine.

Thanks neo71665, you are truly wise on the old school 1st gen's
Going to hunt around the local junk yards for the 3.73 assemblies. Thanks to that cash-4-clunkers program the govt ran awhile back, there's plenty of 1st gen's that were scrapped.
I'll then tweek the speedo via the drac.
Thanks again