Assume a 2400 lb total weight car, about 125 whp, nominal 23 inch diameter tires, actual final drive ratio is 3.736. Going to a 195/50-15 drops the effective ration down to 3.86. Is this enough of a difference to matter on corner exit and acceleration on the straits?
It depends. Without knowing what it does to your shift points, it's impossible to say. If it leaves you in the same gear but a little better in the powerband, it'll be faster. If it puts you up a gear, your actual ratio at that speed might be taller and you'll be slower.
That depends, are you measuring the results by feel or stop-watch. At just over a 3% difference in overall ratio, I think you'll be hard pressed to feel anything much more than a placebo effect. But if chasing thousandths of a second, it has the potential to make the difference. Of course if going from 205/50 to 195/50 in the same tire, there might (or might not) also be some amount of negative grip effect there, reducing potential exit speed by that amount as well. Although the shorter sidewall and more 'stretched' (or less 'squeezed') tire might also speed up the transient response and/or decrease tire deflection by some amount as well. And so on, and so forth.
In other words, the only way to know for sure what exactly the net effect would be for that combo on that car on that course on that day with that driver, is to try it.
^^^What he said. You won't notice much of a difference. I once went 4.083 to 3.90. Literally it was a 200rpm change in fifth on the freeway. Swapped transmissions, now that was a change.
with light weight and HP, a higher ratio is usually best. The Torque and HP multiplication through the final drive helps acceleration. Two of my favourite cars had close ratio boxes and high ratio transmission. The Fiat 124 had a 4.40 to 1 differential and the BMW 318ti had something like a 4.10 to 1 diff. Both accelerated well to a point. The BMW would actually stay with the more powerful 6 cylinder cars till about 50mph, that's when you ran out of revs and had to shift to third.
FWIW, my IT7 car feels noticeably better out of corners with a 5.12 than a 4.88. Slightly less weight and power to what you described.
My Miata (similar power a bit less weight) on similar size tires came factory with a 4.1 rear end and I swapped in a 4.3 a couple years ago. It makes a solid difference.
10 turn road course, the 195/50 would give a powerband placement improvement in turns 1, 2, 6, and 9, still in the fat part of the band without running out of revs in the other 6 corners.
But you some of you feel that in terms of straight-line acceleration a .124 change in ratio won't have a measurable improvement, but some of you do think it will. I spose the real question should be, will the car run a faster lap with the slightly quicker acceleration from the improved torque multiplication of the 195/50, or faster using 205/50 to put a little more rubber on the pavement for cornering...??? But then again, good acceleration during trackout is also an aspect of good overall corning speed...???
going from 2.73 to 3.27 in my Challenge Firebird felt like adding 50hp.
Going from a 2.73:1 to a 3.55:1 resulted on a 0 second Improvement in quarter mile times when I did it to my 270 whp (at the time) Fairmont. The problem was that I now had to shift to fourth instead of crossing the line at the top of third.
Changing from a 3:08 to a 3:73 in my Mustang made 1st gear even more useless but every other gear way better.
akylekoz wrote: Changing from a 3:08 to a 3:73 in my Mustang made 1st gear even more useless but every other gear way better.
Considering that first gear is only used in the paddock and for getting out onto the track...
The effects of the change are almost entirely based on the power band of the engine and the environment of use. If the change is enough to keep it on the pipe when you need it to be then it's perfect. If it's a vtec type deal and it doesn't stay on the pipe when you shift then youll hate it and it'll be slow. If a gearing change resolves that issue then it's enough. If it's a big block with a big flat torque curve then you won't notice. Not my most eloquent post ever but I hope it helps.
I think that level change is chasing Nths.
You'd likely be feeling an equal effect of the lighter tires (all things being equal). All 4 tires being just a size smaller could be 1/2 lb each but since they are effectively "gyroscopes" there is less rotational inertia to spin up.
If it were my car I would go with the wider tire. This small of a change in the final ratio will help a bit in the straights but the speed you'll lose in braking and in the turns due to less grip may result in no change in lap times or even slower ones.
jimbbski wrote: If it were my car I would go with the wider tire. This small of a change in the final ratio will help a bit in the straights but the speed you'll lose in braking and in the turns due to less grip may result in no change in lap times or even slower ones.
See, this is what is making my brain hurt! I suspect my best course of action is to suck it up and just buy another pair of wheels for the 195/50s (FWD so use on the front) and try it. I also suspect that which size is fastest will depend on the track and even the weather.
I have a good bit experience with what you are trying to accomplish. In the mid to late 80s I ran a 125cc GP bike, there are crazy things like purposely going to the smallest size chain possible as well as tires so, pulling the clip on bars in at the steepest angle possible etc. The experience has always influenced my approach.
On my SCCA showroom stock NA Miata that I ran in 96 & 97 I ran 185/60-14s and 195/60-14s. So on the 185s I could turn lap times .3 faster than the 195s, the rub was it was much easier to overdrive the smaller tire and once you did the car would take longer to bring back from the brink. What I ended up doing was running 185s in qualifying and then switch to 195s for the race. If it was really cold out I'd stick with the smaller tires.
The smaller tire will have less rolling resistance as well as being lighter, this will contribute as much to lap time as the gearing.
As for the cornering; once you turn into a corner are you pretty much a passenger hanging on by your fingernails? If not, you are either not trying hard enough or have way more tire than you need.
On a car at this power level the brake pedal is not there to slow the car, it's there to rotate the car on corner entry. If you are using the brakes to slow the car any place other than the end of the longest straights you're doing it wrong. Note mine is a horribly skewed (sp) view brought on by reeeally gutless cars.
On cars that hit the aero wall around 95 mph you end up doing bizarre things to eek out the last bit of time. I'd say try the smaller tires as planned. One of the biggest clues is are you devouring people in the corners then get get raped on the striaghts? If yes the smaller tires will help.
As is obvious by the responses so far there are a lot of variables that will effect whether or not you'll see an improvement. One of the variables that hasn't been discussed so far is how consistent of a driver you are. The change you're talking about is going to be a bit subtle and if your lap times aren't consistent and you're theoretical best lap is significantly lower than any of your actual best laps then you're not likely to see an improvement in lap times. In fact, if the change makes it easier to spin the tires on corner exit (and it might) they may be worse.
Having said that (or typed it) tires are consumables so I'd give it a shot on the next set and see what happens.
Going from a 3.27 to 3.73 only got me .15 at the drag strip. Car was a 95 T-Bird V8.
Going from 3.07 to 3.73 in my '77 C10 made around town a bit more fun, but gave me an increase in fuel economy on the highway because the snotty cam was a lot happier at 3000rpm.
When I swapped the 4.10 axle from my parts truck into my last Ranger in place of a 3.73 it made a very noticeable difference in acceleration.
You'll need to log in to post.
