Speedo reads high, and ODO reads low ?

[Ja$on]

Not sure how this is possible ? I thought they both work off of the sensor/gear attached to the axle in the rear housing ?

I've been trying to calculate my MPG after doing some maintenance, and noticed.

Speedo reads 3-4mph high at 40mph = speedo reads 40mph and really only going 36mph.
And ODO reads 7.5% low. I drove 15.8 miles, and only registered 14.7 miles

I believe the truck has 3.73 gears. I have nitto 305/55/20, and they are 33.07" tall per Nitto, will still measure. I need to check the revs/mile # that is in there now. My friend has some high end diagnostic tools, so hopefully his scanner will let me in there.

How is it possible that the speedo reads high, and ODO reads low when their calcs come from the same info ?

I've been doing quite a bit of maintenance trying to the the truck running clean, and the best I have computed is about 13mpg, driving with a soft foot, but I now know my ODO is off by 8% or so, fairly significant. Mods in sig

Any help is appreciated. Thanks

 

[JLDickmon]

the answer to both your questions is the taller tires. You're 2" taller than OE

Less revolutions per mile.

 

[VanoFord]

agreed. if you have a smartphone, you can download "tire calculator" and you can enter in your original tire specs, w/ what you are running now, and it will give you both degrees of error.

 

[DEEZUZ]

Jason. I started watching my milege on my truck.

Last week I filled up on Sunday, 18.7 gallons, I wasnt empty, I was a hair above 1/4.

Trip read 189 miles

I filled up tonight, 18.3 gal. Just a hair over 1/4 again. I did themath and my odo tracked exactly 189 miles...

I have to figure out how much my speedo is off to get my exact mpg. Just a rough estimate of milege, shows my week of driving should have grossed about 240 miles, instead of 189...

Ill see what I get on this tank on my trip to see if my driving is average..