The Stovebolt Site Forums The Shop Area Driveline Speedometer adjustment needed....

My 49 half ton is wearing 16" wheels with a total dia of about 28". I just went from a 4:11 rear end to a 3:55 and I know my speedo is off but not sure how much. Anyone give me some hints on this...say what would 30 on the speedo be in actual speed ? What part do I need to adjust it ?

Mark,

If the original rolling diameter was 28", then the change from 4.11 to 3.55 would result in an approximate speedometer correction of about 16%: 30mph would really be approximately 35mph.

Jim Carter's sells an adapter. You provide them information and they custom build an adapter. Their price is very good.

Before JC listed an adapter, I bought an adapter from The Speedometer Shop that resulted in my speedometer/odometer being within 1% of correct at 60mph. I used Method #1, but I measured 10 miles and I did it 5 times and took an average. I gave them my results and they calibrated a ratio adapter.

A 28" rolling/loaded diameter is a little larger than an original 16" tire.

My 16" radials are about 31.5" rolling radius.

Thanks Tim. I took the tire measurement at night and was really only close.. I'll take a more accurate one tomorrow... Thanks for the referral to the adapters

Mark

Don't know how you're measuring you're diameter Mark, but if
you make a 'mark' on the garage floor and the tire and roll the
truck back one revolution that should give you a pretty close
reading to calculate the true loaded/rolling diameter that Tim
is referring to.
DG

So that give you the circumference then divide by 3.14 if I recall my HS geometry

If you have a GPS,I would suggest comparing readings between the GPS and your speedometer. That will allow you to calculate the actual percentage error you have.

I have been around and around trying to get a ratio adapter that works...ie, makes my speedo read the correct speed. I started with Jim Carter and the company he used did a crappy job and is now out of business. I sent what they made to The Speedo Shop in Riverside..still it's not correct. This has been going on a while and costs a few bucks each time. I guess I need better suggestions of shops that can do it right !!!

Does anyone make an OEM style speedo that is already calibrated for the 3.55 rear ??

Any ideas ?

Thanks,
Mark

I picked up a ratio adapter at the swap meet last week end.
It appears to be an older one made by Stewart Warner. It has a marking on it of 1.2020. I don't know if is anything that would help your situation or not. It would be good to know what the code is in relationship to MPH corrections.
Fred

I dunno the answer to that. I think they all are similar and it's just the gears inside. Not sure but I think my speedo is about 16% slow

Just trying to contribute.........This site may help someone.

http://www.onlineconversion.com/bigger_tires.htm

Thanks Amigo... I checked it out... I may be able to make it work for me

Call Jim at PATC Transmission (888)877-1008. He will want you to drive your truck, while use a GPS type device to determine actual speeds, make note of what your truck's speedo is reading when you are actually cruising at I think 30, 40 & 50mph. I also went with 3.55 in my truck and found that I was about 20% slow. I bought a in stock 19% speedup ratio adapter from him for $62+postage. Now it reads dead on.

Bingo...just the kind of answer I was hoping for !!!

Thank you

Hi
I got a conversion gear from www.transmissioncenter.net
Mine is set up for gearing from mph tó km/h but you can get them with aalmost any ratio from 0.5 to 2 or so.