Tire size vs. odometer/mpg's

[mshu7]

Hey fellas, I'm getting ready to go get new tires on the Colorado in my signature. Stock tires are 265 / 75 R 15. I didn't care for any of the options in that size, so I'm doing what a lot of other Colorado/Canyon owners do with their tires - increase to a 31 x 10.5 x 15.

I know that the increase in size will throw off my speedo a bit, but will the increase throw off my odometer, which will in turn throw off my MPG calculations (trip miles divided by gallons to full)?

I know the increased weight will effect MPG's, but I'm curious about the size. I would try to use the "Miata tire size calculator site" but it doesn't offer standard size options so I haven't been able to figure this information out.

Also, if the increase in tire size does effect the odometer, is it possible to update the PCM with the new tire size so that it will reflect correctly on the speedo, odometer, etc.?

Thanks in advance.

By the way, I'm trashing some extremely cr@ppy Goodyear RTS's for a new set of Firestone Destination AT's. I'll report back with my impressions of the new rubber.

 

[SHOZ]

I like to get tires a bit larger than stock usually. This slows down the speedo and odo. It's been my experience with the newre electronic PCM driven speedos and odos that the speedo will read a couple miles an hour fast but the odo reads correct. To test yours get an OBDII reader and compare your indicated speed to what it says. My Garmin is dead on too for speed.

So what I have now is a 5% error reading slow on the odo. Pretty easy to figure the gas mileage adding that 5% back in. And the speedo reads more accurately.

Here is a good tire size modeler site.

http://www.rimsntires.com/specs.jsp

 

[Quattro Pete]

Yes, it will affect your odometer reading as well. However, the two sizes you're comparing are actually very close in terms of overall diameter... about 1% difference, so it's pretty much negligible for the purposes of MPG calculations.

I'm not aware of any way to recode the PCM to account for that. That may in fact be considered tampering and may be illegal.

By the way, these calculators support non-metric sizes like yours:
http://www.dakota-truck.net/TIRECALC/tirecalc.html
http://www.1010tires.com/tiresizecalculator.asp

 

[ARCOgraphite]

I would think if the odo is off the speedo is too. Years ago the module was programmable and used nominal tire size (circumference) input not wheel speed instead trans tailshaft speed. We would assume new systems utilise the ALB sensors and read actual wheel speed. I never coded this but if one knows engine speed the system should have a lookup for anticipated wheel angular velocity and then write a short term trim ( correction) which should be feature available in trucks given high truck tread and the anticpated wear and effect on speedo error. Answer I dont know definitively but would suspect that ODO and speedo errors go hand in hand.

 

[CapriRacer]

A couple of thoughts:

1) Use the tire calculator on www.1010tires.com

2) You need to buy a Load Range C in a 31X10.50R15LT and run it at 50 psi to get the same load carrying capacity.

 

[tommygunn]

This site will tell you how much a different size tire will effect your speedometer.
http://www.miata.net/garage/tirecalc.html

 

[Astro14]

I did the opposite swap once - my truck has 31x10.5R15 standard and I ran a set of 265/75R15s for about 45K miles. The P-metric tires rode smoother but the handling was softer (softer sidewalls, it seemed) and they were more easily damaged the way I used the truck.

Off came the P-metric tires to be replaced with the current BFG AT KO in the original size. Ride was a bit more harsh. Handling a bit more secure. Tire carcase more tolerant of actual off-road conditions.

They're so close in size, that I noticed no difference in speedo, MPG or other factors.

 

[Quattro Pete]

Originally Posted By: tommygunn
This site will tell you how much a different size tire will effect your speedometer.
http://www.miata.net/garage/tirecalc.html

Oh really? How do you enter size 31 x 10.5 x 15 in there?

 

[ARCOgraphite]

If you know final gear, just calc tire circumference easy math from there longhand.

 

[mshu7]

Thanks for all the great info & suggestions guys. After looking at the 1010tires site, the two sizes are much closer in size than I was thinking they were.

CapriRacer - I had read that somewhere about the PSI of the LT tire but forgot about it. I'm glad you reminded me.

So, I'm back from the tire shop with new tires. I have to go back in the morning to get an alignment because they were backed up with alignments today. Anyway, I am already LOVING the Firestone Destination AT's. It has really improved everything I hated about the GY RTS's. I'll be posting a new thread later in the week, after I get the alignment, get them aired up to 50 psi, and have some more time behind the wheel with them. Plus, I'll post pics.

Thanks again for all the great info guys!

 

[Errtt]

My current Toyota work vehicle is the first I have had the speedo agree with the GPS.
The wife's previous motorcycle was the most off I have seen - it read way faster than actual speed and I bought it new that way off the dealer floor. Then one of my Harley's is very close I noticed once while beside a new vehicle on the freeway and spotted the digital speedo reading, then afterward the GPS confirmed.

 

[Coprolite]

I do believe it is illegal to tamper with the odometer, even if you are making it correct.

 

[ahoier]

I would "think" the odometer and speedometer should be "linked" or "in sync" in some way or another.....?

Sooooo.....if you were to somehow "recalibrate" the speedometer (that is, how fast your car travels per mile), it should be spot on with how many miles you've traveled....right?

Just an idea....doesn't seem like a PCM/ECU mod at all to me..

And AFAIK, the only "odometer modifying" I could see being illegal, is changing the actual "mileage" to make it higher/lower....I can't see re-calibrating a speedo/odo being "illegal"...

 

[Sonic]

That's not really any increase, maybe a decrease. I checked the specs on the average tire at 265-75-15 (30.7") and 31x10.5x15 (30.5"), almost identical diameter. You would have to make a significant increase in height to throw off your speedo, you won't see any difference.