GPS versus Speedo

[hammer]

Any body out there ever take a check on the speedo versus the GPS? I did the other day & the speedo was right on with the GPS.When the GPS read 60 the speedo was right there too on 60.At least mine is right on far as speedo goes.This is one way to check to see if the speedo is right on or if its off.Just thought if anybody was curious about there speedo being right or wrong this is the way I check mine.Hammer,P.S. If the speedo ever quits too you always got anouther,the GPS.Hammer smile.gif smile.gif

[ryeguy]

My speedo exactly matched my Garmin GPS 3+ 'til I put on the 265's. Now the error is consistent and what I'd expect.

--Rob

[hammer]

Ryeguy,I agree with you 100%.When ya change tire size beyond the stock tires it will throw the speedo off.Stock tires or the same size replacement is fine unless you can get it recalculated to the size you replace if they arnt stock size. smile.gif smile.gif Hammer

[Joe.G]

Ryeguy
Exactly how much was it off, I have 265's on mine and calculated about 3 mph slow on the speedo. Am I correct?

[hammer]

Joe G,If ya had a GPS you could tell exactly how far its off.The gps would show your true speed,And the speedo would show what your doing in speed with the changes to the truck.Hammer,P.S. If ya know someone that has a gps see if ya can borrow it to check your truck. smile.gif smile.gif

[ryeguy]

3mph sounds 'bout right. I think it's like 2.8 or something. It's around 4-5% if memory serves.

--Rob

[Fatdaddy]

Does that mean if you put on 265's that when your speedo says 60 then you are really doing 63? Or, does it mean that you are really doing 57 when your speedo says 60? Just wondering.

[GBurton]

Fatdaddy asked
"Does that mean if you put on 265's that when your Speedo says 60 then you are really doing 63? Or, does it mean that you are really doing 57 when your Speedo says 60? Just wondering."

Short answer - Speedo = 60 but your actual speed is faster.

Long answer - The speedometer calculates your speed based on the RPM of the axle (tire) and the circumference of your tires. For every turn of your tire your will travel a set number of feet. If the size (diameter) of your tire increases, the circumference of your tires will increase and you will travel further for every turn of the tire. If the speedometer does not know about the tire diameter change it will compute your speed based on wrong information and the indicated speed will be off. Mechanical speedometers required a speedometer drive gear size change to correct the problem

[DaveRuane]

With 265's, your maximum speed is 104mph on the GPS. On the speedometer it says 100 (or slightly less). Your engine is governed here and is quite annoying...

In theory, when the speedo says 50mph, your GPS will read 52.

Can anybody explain what the hypertech chip, does and how, in regard to top speed for a 8.1L/Allison?!

[Joe.G]

Yes, checked the 265's with a radar and he showed me to be running 62 mph with my speedo showing 60 mph. I think the 2.8 % error would be very close.
Thanks for the info!

[Silver Bullet]

Anyone know what the percentage off is for 285's? I didn't compare my GPS and speedo prior to the tire swap! Thanks.