Speedo off, how do I formulate MPG w/new tire size?

[yfzjen]

I saw the formula for figuring this somewhere and wrote it down, and have since lost it.

I have 265-65-17 tires now that are 30.6" tall and I would like to swap to 265-70-17 tires which are 31.6" tall. I checked my speedometer today with a handheld GPS unit and when the dash gauge read 75 mph I was actually going 72.7 mph. Are the tires going to correct this or over correct? When I change to the new tire size how would MPG be calculated accurately? Like I said I ran across how to figure this but can't find it now. The more I tried to figure it the more mad and confused I got! Can someone help me out?

[dross]

its actually pretty easy....if you go larger on the tire size the speedo will read slower than you are actually going, to find out the percentage check out www.dakota-truck.net/TIRECALC/tirecalc.html

[SuperBusa]

Originally Posted by yfzjen

I saw the formula for figuring this somewhere and wrote it down, and have since lost it.

I have 265-65-17 tires now that are 30.6" tall and I would like to swap to 265-70-17 tires which are 31.6" tall. I checked my speedometer today with a handheld GPS unit and when the dash gauge read 75 mph I was actually going 72.7 mph. Are the tires going to correct this or over correct? When I change to the new tire size how would MPG be calculated accurately? Like I said I ran across how to figure this but can't find it now. The more I tried to figure it the more mad and confused I got! Can someone help me out?

Pretty easy to calculate from scratch. The 31.6" dia tires are 31.6/30.6 = 1.0327 times taller, or 3.27% taller. This also means their circumference will be 3.27% larger since the relationship between diameter and circumference is linear (C = pi x D).

If your GPS said 72.7 mph when your speedo said 75 mph then the speedo is 2.3/72.7 = 3.16% high. If you put the larger tires on the truck, then you will actually be going 3.27% faster down the road at the same reading on your speedo. So for example, if your speedo said 75 mph when you have the larger tires on the truck, you will actually be going 3.27% faster than when you had the smaller tires on ... which was 72.7 mph per the GPS.

72.7 x 1.0327 = 75.08 mph (actual) ... so yes, it looks like it will make the speedo about perfect.

Of course this could be seen early when it was shown that the speedo was 3.16% high and the bigger tires would make you go 3.27% faster -- since the percentage change is just about equal, and in the right direction they cancel each other out.

[yfzjen]

Busa you are one smart fella! Someone else was trying to tell me that 1" taller would be about 5 mph difference, and I was like well poop it'll still be wrong just in the other direction. I finally sat down and figured out the ratios for the tire sizes but just couldnt get past the brain fart of how to figure in the already off speedometer. Thanks to both of you guys for the information I needed. Now if I can just decide on which tires?

[JuliaGoolia]

I am in the same situation. I have just changed my tires last week. And when I check the speed before I changed them compared to the GPS I was also off by the same amount. Someone told that GPS is not accurate that it can be off. Does anyone know if this is correct?

[yfzjen]

The ones i've used have been pretty accurate so far. We had one of those road side trailers that flash your speed on the hwy one day and I was 2-3 miles off from it also. I used a handheld GPS and a cop friend radared for me and both were accurately the same. I imagine the handhelds could be off a lil though. Have you checked yours with the GPS since you changed tires to see if there were any changes?