New tire size and some math questions

[hat_man]

Hello everyone.

I recently got new tires for my truck and ended up getting a different size from the original. The tires I wanted didn't come in the same same size or a comparable size that had the same diameter. That and I couldn't find a good 14" tire that was good in the snow and ice. I drive 120+ miles a day and going off the road in the winter is a big concern. Having to pay a tow and repair bill eats up any fuel savings a LRR tire or lighter rims might gain me.

I know the speedometer will be off by about 3mph and I'm sure the odometer will also be off. I am having trouble with the math in trying to figure out if the new tires are going to hurt my FE or not. I know I can put 10 gallons in the tank and drive 300 miles for a 30mpg average. This is fairly consistent as I drive the same route everyday. The new tires are heavier so I would expect some FE loss, but with the larger diameter it may be offset from the final drive being slightly taller?

Here are the particulars on the old (22/70/14) and new (215/75/15) tires.



The old tires are about 764 rev/mile and the new are about 728 rev/mile and the old diameter is 26.4" and the new is 27.7" (about 5% larger)

The question is.....with the odometer being slightly off because of the larger tires......if I put 10 gallons in the tank approximately how many miles (on the odometer) should be equal to the 300 I used to see? The simple answer seems to be 5% less or 285 miles, but is it really that simple? I just want to see if I'm still getting about the same FE with the new tires or just how much I have lost. I know this isn't the best time to try and figure out exact FE numbers as it's winter and I'm burning winter blend fuel, but I have to start somewhere.

Thanks for any help.

[user removed]

300X728/764= the new odometer reading

300X.9528=285.86

Correct the odometer reading by that factor should get you there. 5% is very close which would be 285 without the fraction. Basically you now record 285 miles while actually travelling 300. For your fuel log just adjust the distance up by 5% or a tad more.

300/285=1.052631579 or 5.2631579%

regards
mech

[BabyDiesel]

Old Mech, would not the 764 & 728 need to be switched? Larger tires cover more distance per revolution than smaller tires, adding distance.

(300x764)/728 = 314.84

I believe you would add the 4.95% to the odometer when you fillup, hat_man. That would mean in 300 miles, you have really traveled 314.84 miles. I have had to do this to my car as I went up 3.4% from the original tire size.

Let me know if I have let my sweet tea sugar high write nonsense lol!

[aardvarcus]

Hat man,

What brand/model of tires did you buy? What was the original equipment tire size which should be on your door pillar? I have found that actual tire diameters can vary significantly than the theoretical measurements that the online calculators use. I would suggest looking up the actual diameter of the OE recommended tires and the actual diameter of your new tires.

For an example, if the OE tire size was P215/70R14 which in a BFG tire would be 25.9" and 803 Rev/Mile and you went to 821 BFG Long Trail T/A Tour 27.7" and 751 Rev/Mile, then your odometer would be off by 6.9% (803/751=1.06924). This means for every one mile on the odometer, you actually traveled 1.069 miles.

As for your speedometer you could divide by 1.069 to get your actual speed, aka 51.5 MPH on the dash means 55MPH in real life. However the speedometer may have been off from the factory so I would suggest though that you borrow a gps and figure out what speed on the dash equates your desired cruising speed.

[user removed]

Originally Posted by BabyDiesel

Old Mech, would not the 764 & 728 need to be switched? Larger tires cover more distance per revolution than smaller tires, adding distance.

(300x764)/728 = 314.84

I believe you would add the 4.95% to the odometer when you fillup, hat_man. That would mean in 300 miles, you have really traveled 314.84 miles. I have had to do this to my car as I went up 3.4% from the original tire size.

Let me know if I have let my sweet tea sugar high write nonsense lol!

Old tires 764
New tires 728

Lower tire revs per mile with the new tires, means fewer signals to the speedometer for every mile travelled. The new tires roll further every revolution, thus the lower speedometer and odometer reading. I think I got it right, but I did go back and check the OPs figures.

To check the odometer with the consistency of your commute, assuming you know the distance of the previous reading, confirm the calculations with the distance of the new reading with the larger tires.

I just got my sweet tea sugar high.

Sometimes it's easier to fudge the factors. If the tires were twice the diameter,the speedo would read half the speed. Increasiing diameter will always reduce the instrument reading.

regards
mech

[BabyDiesel]

Originally Posted by hat_man

The question is.....with the odometer being slightly off because of the larger tires......if I put 10 gallons in the tank approximately how many miles (on the odometer) should be equal to the 300 I used to see?

I see now! Old Mech and aardvarcus are correct, I misread over that last part above that I bolded. That sweet tea got me.

To be awesomely exactly correct, the correct odometer reading to equal 300 miles would be 285.85 miles

[hat_man]

Thanks everyone.

aardvarcus....the original tire size from the door is a 225/70/14. I have had several different brands over the years so an accurate number may not be possible. The 764 revs/mile shouldn't be off more than 1-2% I would think. The new tires are Firestone Destination A/T's in a 215/75/15.

The only ones in my old size that I could find on Tire Rack are the BFG Radial T/A's with a overall diameter of 26.5" and 786 Rev/mile. The new ones have an overall diameter of 27.7" and 753 Rev/mile.

It looks like if I can hit 285 miles on 10 gallons then I'm at least in the ballpark. Any thoughts on the trade off in weight and final drive? The new tires are 28 lbs apiece and the BFG's are 25 lbs. I have a feeling the new rims are a bit heavier also but I forgot to weigh them before the new tires were mounted.

[arcosine]

The new tires may be lighter, since 215 vs 225. i would expect FE to increase by 2%

[aardvarcus]

Hat man,

Your new ones are 753 and your old ones were 786 so that is 4.4% difference. The tires look slightly aggressive but not too bad, but it is possible that tread pattern may hurt you a bit on rolling resistance. It is a hassle, but I actually keep two sets of wheels and tires for my Tacoma, a set of pavement pounders and a set of off-roaders. If you do a significant amount of street driving and only occasionally need the aggressive tread pattern, it is a good way to save some fuel on the road and save the wear on your aggressive tires.

Your wheel weight will mainly hurt you on acceleration and deceleration, more of an issue in stop and go. The better gearing/final drive ratio will mainly help you at steady state, so that is more of a benefit for highway driving. The taller tire will also lift the truck slightly, giving you increased ground clearance but hurting your aerodynamics slightly. Your exact commute and driving style will determine the rest, but I would expect to see at least a small gain.

[CapriRacer]

And keep in mind that a tire's rolling resistance can vary greatly - much more than the affect the difference in diameter has. So depending on what is being replaced, the A/T tires might be worse.