I have an 83 W150 chassis under a 78 D100 cab with a 727 transmission. The speedo gear chart calls for a 30 tooth gear with my 31" tall tires and 3.55 gear ratio. There's a 29 tooth gear in it now and a new cable. The cable turns easily in my fingers with no indication of any resistance. The speedometer needle flutters a bit at highway speed and more so at lower speeds. Speedometer can read 65 on the highway but I don't feel I'm going that fast and I'm not gaining on anybody. Speed limit is 50 and most people drive 55-60. What's really odd is that the odometer counts 48% high. So both speedometer and odometer read high but at different amounts. I can correct the speedometer with a different gear but that won't bring the odometer in line. I'm thinking of sending the speedo to Redline this winter to have it refurbished. That should take care of the needle flutter.
What are your thoughts on all of this? Do I need a different speedometer that counts the miles differently? Do different speedos have different odometer gearing?
Thanks for your help.

 

i would have the odometer roll over to 0 and reset the tripometer to 0 and see if they count the same. as far as seeing how far off you are do you have a smart phone? if so use google maps. it should show you the speed your traveling so you can see how fast you are actually going vs what the speedo says.

 

I can't help with speedo vs odo calibration....

But overall calibration can be done with a ratio adapter. Many vehicles had them stock, and your local old-school wrecking yard may have a whole bucket or shelf full of them.

https://recordtechnologies.com/products/ratio-adapter

 

The ratio adapter is about four times the cost of a Speedo gear. A little pricey.

 

LInk was for picture and data reference. They were $5 items (used of course) back when i worked at a wrecker. My parts trucks often have them too.

 

I have no trip meter (I don’t think, not home for a while to check). Which raises the question, could I have the wrong speedometer?
Thanks for the tip on using Google maps.

 

i think it would depend on how your truck is optioned but if the 70's dash is like the 80's dash the speedo are interchangable and you can swap in a one with a trip. my 81 was a base model without trip, oil pressure or temp gauge. it just had the dummy lights. i got another instrument cluster and just swapped over all the gauges. everything worked as it should.

are you sure it has 3.55 gears. 3.55 gears were not very common till early 90's when overdrive started to become standard. 3.23 gears were more commonly found in the 80's and older trucks.

 

I’ll check it again but when I jacked up the rear and turned the tire it was 3 1/2 turns as I recall. That was a while back and my memory is…well, you know. Lol.