Speedometer Calibration

[Doc]

Forgot to ask about this in the previos thread.

My truck came with 235/85/R16 tires and a 4.10 rear differential. I changed the tire to 285/75/R16s and the differential to a 3.55. When I checked my speedo calibration after the changes it was right on the money. I tested it by driving 5 miles at 70mhp, 5 miles at 65mph, and 5 miles at 60mph. I timed the distance with a stop watch and came up with these results:

5 miles @ 70 was actually 69.98mph; 65 was actually 64.90mph, and 60 was actually 60.11mph.

I thought I would need to have the speedo machine calibrated. Does the computer do it automatically. It just seems weird to me that I didn't need to make any changes after adding over 1 inch to the tire height and changed the rotation in the rearend.

 

[lgettler]

I believe what took place is changing your tires to low Profile 75's lowered the speedo by 4 mph and replaceing the rear end to 3.55 raised the speedo back up to normal, I know I ran 75 low profile and my speedo dropped by 4mph fand now I am back to 85s and the speedo is right on. There is a website that gives you actual speed of any given tire size.

Generaly the ear end change will change the RPM by about 500 RPM.

look at this sete http://www.miata.net/garage/tirecalc.html

 

[Doc]

From what I could find the 285/75 tires are just a tad over 1" taller than the 235/85s. I did get the drop in RPM from changing the rear end, but can't figure out why the speedo didn't change. Oh well, it's not really that important. I just wanted to be sure I'm not messing the computer up.

 

[Mr_Roboto]

Does your truck have the VSS in the trans tailshaft or in the rear axle?

Trans mounted reads driveshaft RPM and would be affected by a gear change. Rear axle mounted reads AXLESHAFT RPM and is not affected by gear change.

Either one will be changed by tire diameter.

 

[Rawpower]

You can change the gear within the axle sensor correct to accomodate for larger tires?

 

[Mr_Roboto]

You can change the gear within the axle sensor correct to accomodate for larger tires?

If you are talking about the VSS in the rear axle, there is no "gear". There is a toothed tone ring which is the same for all axle ratios, and speedo adjustments are done electronically.

The VSS in the trans works just like a speedo cable (and goes in the same spot as a speedo cable) and can be adjusted using different driven gears.

Now there are also aftermarket devices that intercept the VSS signal and change them to compensate for big tires etc. However that is overkill for minor gearing changes.

 

[Black dawg]

I would go by the odo over a known accurate distance, speedos generally read about 3mph fast in stock form. so if you make the speedo dead on your odo will read slow.