Original tire size information

[GreyFox]

Ok I’m trying to figure out why my speedometer is 10mph slower than my actual speed. My wheel is a 16” with a 235/85/R16. So I’m trying to determine the correct tire size or the original tire size. I’m looking to figure out should I change the speedo gear or the tires. I need new tires so I’d like to go that direction.

[Greg63]

It is much cheaper and easier to change the speedometer gear (if it is still available).
I have used this link in the past to figure out my speedo - https://transmissioncenter.net/produ...dometer-gears/

[rpmerf]

235/85R16 is what I have on my truck. I believe it was stock, but they may not be what came on your truck. There are calculators you can use to determine what speedo gear you need based on your tire size, rear ratio, and the speedo gear in the transmission.

EDIT: What is the year / make / model on your truck to help determine what would had been the factory tire size. Any options like heavy duty suspension or heavy duty tires?

[The Rocknrod]

Also there is tis helpful link to figure it out - http://www.bgsoflex.com/speedo1.html
And https://www.tciauto.com/speedometer-gear-calculator

[GreyFox]

Changes made on my is I kept the Eaton axle and changed out the diff to a '72 4:10 it was a 4:56 rear.

I still have the sm420 in my truck and have considered moving to the sm465 but I don't know if I can get the part that would allow me to fix the speedo setup in the transmission. I may have to figure out the fix by using an external increase method.

See the ebay search here: https://www.ebay.com/itm/GM-and-Dodg...0AAOSw95lbKRPH

[vince1]

ww.4x4review.com/sm465-4-speed-how-dissapointed-i-was/

Sm 465 is still only 1 to 1 is it not? Hopefully it is quieter than the SM 420. Maybe you should also consider going for a 5 speed? I guess it depends on if or not you are happy on how it drives down the highway, providing that's what you use it for.

I've changed mine from 3.73 to 3.07 which makes it easier up here now that we have gone to kilometers because now I just double the MPH to get KPH. Tires 235 75 15 are smaller than yours though. My plans are to replace the SM420 with a TH400.

[GreyFox]

Yes the sm465 is just a next generation sm420, the changes are that the granny is a 6:55 and the sm420 is a 7:06. The rest of the gears are the same. You gain a reverse light switch and a more mild granny gear. Going to the nv4500 would cost me around $3500 out the door and I only paid less than half of that for the truck. I'm convinced that the conversion to a nv4500 would be a bit too expensive. I think I should pull off the rear axle and get a Dana60 or 70 with a 3:73 in it.
The current performance is ok but once I reach 50 - 55 the rpms are very high so I hope to one day fix that situation.