owensjt

I have a 2002 ES 300 and just recently replaced the factory wheels with a set of 18" Kaotik Z5s. The tire size I chose was 235/40/18 compared to the factor set up of 215/60/16. Just wondering by plus sizeing how does that effect the speedometer and odometer readings? According the to tire size calculator below there should be 1.7 mph difference.


http://www.1010tires.com/TireSizeCalculator.asp

bitkahuna

iTrader: (20)

From my online tire size calculator / comparison page:

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215/60-16:
--sidewall (mm) = 129.0
--diameter (mm) = 664.4
--circumf. (mm) = 2087.3

235/40-18:
--sidewall (mm) = 94.0
--diameter (mm) = 645.2
--circumf. (mm) = 2027.0

(2.89)% difference in circumference.
-----------------------------------------------

My calculation shows the 18s you're running are smaller circumference than stock, which means your speedo is now going to read higher than stock. So 60mph stock will read 61.7 mph.

It seems a 235/45-18 would have been much closer to the 16 size. Don't sweat it though, the *stock 17s* have 235/45-17s which are ~3% smaller than the 16's size!

SeattleGS400

iTrader: (1)

Bitkahuna:

I think that online size calculator is for ideal tire sizing--i.e. what dimensions a tire of a certain size should be, and the caculations are theoretical. However, all manufacturers have some discretion as to how close to those ideal dimensions their tires should have, so it's not always best to get use an online calculator for the exact differences (they are good for approximations, however). The best thing to do would be to compare the exact tire specs from the manufacturer data sheets (available from each tire manufacturer's websites) and compare them across the board.

For instance:

16" Stock GS4 size: 225/55R16
17" Stock GS4 size: 235/45R17

From the website's calculation, you're getting the 17" size about 3% smaller, however, let's just say we put on Michelin Pilot Sport A/S on the each of the above rims, and here's what we get:

http://www.michelinman.com/assets/pd...lotsportas.pdf

Michelin Pilot Sport A/S

Tire revs per mile at 45 mph

16": 806 (overall diameter 25.7")
17": 811 (overall diameter 25.2")

From this, just looking at the tire revs per mile at 45 mph, we see that the 17" tires will spin about 0.6% faster than the 16" tires. Now let's say we decide that we don't care about the all-season capability and instead just want to go for the Pilot Sports instead:

http://www.michelinman.com/assets/pd...pilotsport.pdf

Michelin Pilot Sport (regular)

Tire revs per mile at 45 mph

16": 806 (overall diameter 25.8")
17": 822 (overall diameter 25.2")

We see that even though we're using the same theoretical tires sizes even from the same manufacturer, the regular Pilot Sports in 17" has a 1.9% faster rotation than the comparable 16" size (but still actually 1.1% less that the theoretical calculation).

In the 16" size, even though the Pilot Sport A/S is slight larger in diameter than the Pilot Sport (regular), it still rolls at the same revs per mile. This is probably due to the how much the tire might "give" due to how it's made--that is, the PS A/S might be a little flatter when rolling. Also, note how the 17" PS A/S and PS have the same exact overall diameter, but they still roll at different rates (the regular PS rolling about 1.3% faster than the same size PS A/S).

I think the theoretical calcualtion is a good place to start for comparing the changes in tire and rim sizes to the what you might get in the speedometer readings. Nonetheless, to get the actual difference in speedometer readings vs. different tire/rim size, you should use the manufacturer's tire specs chart.

3dog

iTrader: (1)

that just amazes me that they would be the same size but revs per mile are different. i never gave any thought to this until SeattleGS400 brought it to my attention. Som that is some nice work.

ivan
3dog