Wheel and Tire Size after 4.10 Gears

[derieu]

Hi all,

I have a 2014 SS 1LE, I will be doing a full car build soon to really make this car shine on the road course.

I have decided on going with an eaton truetrac + 4.10 gears for the differential. I was going to stick with my factory 20" wheels and similar tire sizes, but I am considering dropping down to a 19".

Will the more aggressive gearing (4.10 from 3.91) plus the tire size drop (going from a 295/35/20 to a 305/30/19) negatively effect me?

Am I better off staying with a 20" wheel while I have the 4.10's for now? This is a street and track car.

[Norm Peterson]

Originally Posted by derieu

Will the more aggressive gearing (4.10 from 3.91) plus the tire size drop (going from a 295/35/20 to a 305/30/19) negatively effect me?

Overall, that's 11% fewer mph at your shift point . . . in every gear.

Any straight where you're getting closer than that to your shift point now, you'll be upshifting before that corner. If you're not using 2nd very much now, you may use it even less frequently.

But there's a bit of track-specificness involved - you could, for example, find the new overall setup puts you a lot closer to redline down one or more of the longer/faster straights than you're currently able to reach, without having to grab that "one-higher" gear. And any corner where the same gear is still usable will, of course, give you stronger acceleration on exit.


FWIW, it cost me at least one extra upshift and one extra downshift on both of the courses at my home track, with the only change being a tire diameter change from about 27.1" diameter down to about 25.9" (4.5% fewer mph, shifting at the same rpm).

I completely understand having a few uncertainties about this, since I've been mulling over the plusses and the minuses of a couple of powertrain mods myself. In a whole lot more detail than I've provided above.


Norm

[derieu]

Originally Posted by Norm Peterson

Overall, that's 11% fewer mph at your shift point . . . in every gear.

Any straight where you're getting closer than that to your shift point now, you'll be upshifting before that corner. If you're not using 2nd very much now, you may use it even less frequently.

But there's a bit of track-specificness involved - you could, for example, find the new overall setup puts you a lot closer to redline down one or more of the longer/faster straights than you're currently able to reach, without having to grab that "one-higher" gear. And any corner where the same gear is still usable will, of course, give you stronger acceleration on exit.


FWIW, it cost me at least one extra upshift and one extra downshift on both of the courses at my home track, with the only change being a tire diameter change from about 27.1" diameter down to about 25.9" (4.5% fewer mph, shifting at the same rpm).

I completely understand having a few uncertainties about this, since I've been mulling over the plusses and the minuses of a couple of powertrain mods myself. In a whole lot more detail than I've provided above.


Norm

Yes, I understand this much. I made a quick excel spreadsheet with my transmission gearing to give me a few different tables of what speed i'd be going in certain gears (at specific RPMs) to compare.

It does seem like my 2nd, 3rd, and 4th gearing would be better than the extremely tall tires, right now in most cases im in the middle of my gears so this should in theory stick me higher in my powerband. Definitely might be one of those cases where you try it and see what happens.

To my surprise it seems like i'd actually get better street mpg (in theory) because i'd be turning just over 1300 RPM in 6th going 46mph. Might lose a few on the freeway though.

[Norm Peterson]

Originally Posted by derieu

Yes, I understand this much. I made a quick excel spreadsheet with my transmission gearing to give me a few different tables of what speed i'd be going in certain gears (at specific RPMs) to compare.

If you didn't use the revs per mile data from TireRack or other sites with that data, I hope you allowed yourself about a 3% downward correction in mph from speeds computed from tire diameter dimensions. For most situations it's not necessary to be that picky, but working with speeds on track is likely to be an exception.

Something else to consider . . . you generally lose a mph or two when you have to upshift at 120 or so, and it takes a finite amount of time and remaining straight length to recover the scrap of lap time it cost you.

It does seem like my 2nd, 3rd, and 4th gearing would be better than the extremely tall tires, right now in most cases im in the middle of my gears so this should in theory stick me higher in my powerband. Definitely might be one of those cases where you try it and see what happens.

No matter how much calculating you do, you still have to drive the damn thing to find out for sure that it really did work for you.

To my surprise it seems like i'd actually get better street mpg (in theory) because i'd be turning just over 1300 RPM in 6th going 46mph. Might lose a few on the freeway though.

Entirely possible, even likely, as it's easier to find yourself using higher gears for cruising at lower speeds but you won't have the option to grab that "one higher gear" out on the highway. Been there.


Norm