New meats on the back some pics and questions

Put my track/summer tires on the back 18X10 deep dish with nitto 555R extreme drag 305/40. I was wondering how the different height tire will affect my gearing at the track, previously i was running a 35 street tire and was traping at 6500 rpm in third gear. These tires are taller (40)with 6/32 tread depth. I measured the height and it came to 26.8 inches, i adjusted my speedo with the xcal2 to 754 rev per mile based on the different tire height. Will i still be able to trap winding out third i have the stock 3.55 gears btw. My shift light comes on at 6400 and my rev is set at 6800 right now as well. I really dont want to have to shift to 4th right at the end as im sure it will hurt my et a lil bit. I am seeing 104-106 mph trap speed so maybe i will have to do the 3-4 shift especialy with some stickier tires, what do u guys think?

What was the previous tire diameter or size?

generally a taller tire will decrease your gearing a shorter tire will increase it

Went from 285/35/18 street tire to the 305/40/18 nitto dr.

so i went from 25.9 inches tall to 26.8 inches so an inch taller basicly. Thats does make sense because with the smaller tires i had i was seeing 2500 rpm in 5th at 70mph which always seemed a bit high for the 3.55 gear to me. So i should def be good to go then right?

Lightblade said:

Went from 285/35/18 street tire to the 305/40/18 nitto dr.

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Lightblade said:

so i went from 25.9 inches tall to 26.8 inches so an inch taller basically. That does make sense because with the smaller tires i had i was seeing 2500 rpm in 5th at 70mph which always seemed a bit high for the 3.55 gear to me. So i should def be good to go then right?

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Nitto's site says the 305/40 is a 27.7 tire. so your gearing goes to 4.03 effective. It was effectively about 4.30

You'll be fine, but IMO you'll lose a bit in the 1/4

Well as they sit now with 30 psi for street use they are 26.75 inches tall i just measured again to make sure. Their site says 27.7 and 750 revs per mile. Im guessing that fully inflanted and not on a car supporting weight. Im sure they will be alittle shorter when i deflate them at the track as well. So i have my rev per mile set at 755. I def wont loose anything in the quarter though even though i lowered my effective gear ratio. As the crap street tires are garbage best 60 ft was a 2.3 and they break loose a little everytime i shift at high rpm. The drag radials will more than make up for the lowered gear ratio im sure, thanks for the info!

stkjock said:

Nitto's site says the 305/40 is a 27.7 tire. so your gearing goes to 4.03 effective. It was effectively about 4.30

You'll be fine, but IMO you'll lose a bit in the 1/4

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??
what calculations did you use to get that?
Normally you figure original tire height divided by new tire height multiplied by gear ratio.

I don't see how putting a 27.7" tall tire on with 3.55 gears is going to result in him having 4.30s. You have to use his stock tire height as the first number in the calculation to get his true gear ratio. But even if you use his previous tire height of 25.8 and assumed that was his stock tire height (which it isn't) it would come out to a 3.18 gear set.

Assuming his stock tire height was somewhere around a 27.2 (just basing that on what my stock tires were) and changing to a 27.7" tire, that is going to give him 3.48s

ahh yea if you were pulling 2.3s an these DRs get you to 1.8s that will be a nice pick up

Hawgman said:

??
what calculations did you use to get that?
Normally you figure original tire height divided by new tire height multiplied by gear ratio.

I don't see how putting a 27.7" tall tire on with 3.55 gears is going to result in him having 4.30s. You have to use his stock tire height as the first number in the calculation to get his true gear ratio. But even if you use his previous tire height of 25.8 and assumed that was his stock tire height (which it isn't) it would come out to a 3.18 gear set.

Assuming his stock tire height was somewhere around a 27.2 (just basing that on what my stock tires were) and changing to a 27.7" tire, that is going to give him 3.48s

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LOL my bad - I got him confused with another thread I was repling to as well. I used his gearing at a 4.10 for those calcs.

Once again Doc you caught my goof!

Yeah i'm hopin for 13.3-13.5 If only my car wasnt so dang heavy.

So are you still running these 305/40's? Any feedback on performance... both at the track as well as everyday driving?

I am about to pull the trigger on some DRs and think I have narrowed it down to these or the M&Hs (275/45/18s). I was also considering the new NT05Rs (285/40), but given my minimal track plans, these might be a little less streetable as I don't want to maintain a second set of tires/wheels.

Pretty sure the circumference of the tire doesn't change no matter how much weight is on the tire so your revs per mile won't change just because the tire is squatted. The circumference will increase a little going down the track as centrifugal force does it's job but at your speed that will be negligible.

doogie said:

So are you still running these 305/40's? Any feedback on performance... both at the track as well as everyday driving?

I am about to pull the trigger on some DRs and think I have narrowed it down to these or the M&Hs (275/45/18s). I was also considering the new NT05Rs (285/40), but given my minimal track plans, these might be a little less streetable as I don't want to maintain a second set of tires/wheels.

Click to expand...

No, havent had them at the track yet. Had a flat in one that i repaired and now the 18x10 rims with these tires are not fitting with the 14" rotors on the rear so i am grinding down the calipers to fit them. As for dd use they are awesome it sucks going back to street tires! the 555R is longer life more user friendly compund, the nt05 and m&h will wear alot faster but will have better traction.

The most accurate way is to measure your tire radius from center of hub to pavement at the pressure you will be using at the track (there is some error due to circumferential growth from centrifugal force and temperature change and I don't know the math to fiqure it out)...double this number and multiply by pi (3.14159) to get your true tire circumference. To figure your effective gear ratio take your stock tire circumference and divide it by your new tire circumference and multiply by your gear ratio. This gives you your effectvie gear ratio. You can also calculate your ideal tire size based on trap rpm (peak horsepower?) and desired speed. Redline rpm divided by rear gear ratio (assuming fourth gear 1:1 transmission ratio... if in another gear you have to divide by that number first to get driveshaft rpm) times your tire circumference (calculated like I suggested) in inches divided by 1056 will give you your speed at redline. Hope this makes sense...been drinking a bit. You can also calculate engine rpm for any tire size/gear ratio by modifying the formula too...this way you know what a tire/gear combo will give you on the highway.

calculators

go to Wallace racing web site great calculators http://www.wallaceracing.com/Calculators.htm

awesome site