Speedometer/Odometer Questions

So I just installed bigger tires on my truck and using the tire calculator, I got what Speedo reading Vs. Actual speed and can live with it being off but wouldn't this also change the Odometer readings as well? By my guess, the Odometer wouldn't be accurate either; it would read fewer miles than actually traveled. How can u tell what your MPG is?

 

installing bigger tires actually made my speedo correct which was off from the factory.

 

YES - your ODOMETER won't be reading accurately. You'll be putting 'more' mileage on the odometer than you're actually traveling.

If you have a GPS, you can check the odometer to the actual distance on the GPS.

Your best bet - is to buy something like a Scangauge II or other trip computers that can actually calculate all this stuff for you (including MPG) after you set it up properly.

 

Yes, it affects the odo which is the worst part really. I wouldn't be without a Scangauge as mentioned, but the factory odometer is a very useful thing if it is accurate. I installed a Dakota Digital speedo compensator and calibrated mine to better than factory. On my truck I noticed that when the odo was set perfectly, the speedo still reads a couple of MPH high. I believe that is an offset that the factory uses intentionally to avoid the problem of possibly reading low. No matter to me because the Scangauge is much more accurate for speed, and it is all I use. I use the odo for navigating, maintenance, and other uses, so I want that accurate.

 

I thought since the tires are larger, the odometer would be reading less miles, since the overall diameter of the wheel has increased. Using the tire calculater, if the speedo says I'm traveling at 65 mph, I'm actually traveling at 69.82 mph, so that would mean at the end of 1 hour, the Odometer would read 65 miles not 69.82 miles traveled. Right???

 

That is incorrect, if you are going with a bigger tire. With my 35s my speed is off by 12%. So when im going 70 on my speedometer im actually going 77, and in turn my odo is lower than what it should be. if I go 200 miles according to my odo i actually went 224 actual miles

use a GPS to see what you actualy travel and compare with your odo to see how accurate it is.

 

youi are correct

 

Yip. Perhaps I'm a simpleton, but these discussions always leave my head hurting. I've relented and just decided the speedometer is close enough...

 

Found this to be true with mine also.

 

bottom line bigger tires = spedo reading lower mph then u are acvtually driving
and bigger tires = less miles registered on odo

 

Yeah, you and me both. I was thinking more along the lines of the driveshaft turning more per revolution of the tires - and that's where I'm assuming it's more mileage. But its actually the opposite.....

My bad...go crawl into a hole now.

 

I've read repeatedly that the bigger tires (265 in my case) will correct the factory's "fast" speedo. In my case, my speedometer still reads 3 or 4 MPH faster than my GPS when I hit above 60MPH or so.

Like I said, close-a-damned-'nough.

 

Is this after you calibrated the ultragauge? If so how did you cal it?

 

My speedometer reads 2mph low. I have to go 67mph on the needle to actually go 65mph. I have a scanguage II.

The data on the scangauge comes directly from the computer. The GPS and the scangauge MPH are both the same, which tells me the data in the computer is reading accurately.

It's the speedometer calibration (needle) on the dash that's not accurate.

 

Jandy (cool name, btw), are your tires stock?

 

from the online tire size calculator

To figure out how far your ODO is reading, use the tire size calculator. See above, the 3.3% in the lower right hand corner? Multiple your ODO miles traveled by 1.033. Then divide that by gallons of gas used to get accurate MPG.

 

My speedometer was off by 4 mph @75mph and was actually doing 71-72mph and my rpms was sitting at 2300rpms, now that I got bigger tires (265/70/17) im actually doing 75mph and rpms dropped to 2100 rpms. I first noticed caused my girlfriend was following me and told me why im going so slow so then i looked at the gps on my phone and yup sure enough. I also get better mpg on highway cause rpms dropped at my crusing speed. The odo was my concern to.

 

The same signal feeds the Scangauge that feeds the speedo. If one is wrong, the other is wrong. The difference is that the Scangauge can be calibrated to compensate that.

If you install a DD compensator, both the Scangauge and the Speedo/odo will be affected. You set the speedo/odo first, and then calibrate the Scangauge last.

People get cornfused about which way the speedo is off because they often regear with larger tires. On a manual transmission, the signal is generated at the transmission/tcase. Lower gears make the speedo read high. Bigger tires make the speedo read low. Usually with gears + tires the end result is a speedo reading high. On an auto, the signal is taken from the wheel sensors. In that case, the gears have zero effect, and the speedo reads low with the taller tires.

A Scangauge or Ultragauge is the best way to bypass the problem with the Speedometer. They can be calibrated and they are accurate. I never even use my speedo even though it is calibrated. If the odometer matters to you, the only way to fix it is with a digital compensator.

 

Thanks - and Yes....they're stock.