My car has a 3.73 rear gear, and M21 trans. Last time to track with a 26" tall tire the car ran 14.2x all day. Yesterday with a 28" tire the car ran 14.9x all day. How much of my change in ET can be attributed to tire height? Does anyone have a link to a conversion chart? Thanks in advance.

Judging from the E.T. change I would say you must have a small cubic inch engine, correct? (or an engine that needs to RPM) There is not chart that I know of that will calculate E.T. shange from tire size. There are too many variables. If you had a high HP car you would most likely run faster (more MPH) and quicker (due to better contact patch) with the taller tire. I know you said your ET was slower, but how was the MPH? How about the 60ft time?

I am assuming you are running the same type of tire as the 26" and that both hooked, Is that right?

Not a direct answer but it might help... Your effective new rear ratio compared to the old tires is 3.46:1. To figure this divide the old tire size by the new tires size and multiply by the rear ratio. (26/28=.928, .928x3.73=3.46) What were your 60' times on each run?

EDIT: Please note, the original tire diameter was 25"

so compared to 25" and 3.73's you have 3.33:1 (25/28=.892, .892x3.73=3.33) Again what were your 60' times on each run?

The taller tire makes your gear ratio lower (numerically) like DjD said. That in affect makes your motor rpm slower,which slows you down.
I used to run 2.73's and a 26" tire, so at the track I could only use 1st and 2nd gears (th350). I went to 3.73's ,and all else being the same, went faster due to the better gears and also got to use 1,2,and 3 due to rpm's. So you see how the effective gearing works and how taller tires affect it.

Here are some answers,

SB 302 cubic inch

101.5 mph with a 25" tire

97.9 mph with a 28" tire

I'm very embarrassed about the 60' times but here goes, with both sets of tires they were in the 2.4-2.5 range. Neither hooked, obviously.

My best pass last year was a 14.25 at ~101.7 mph with a 2.4x short time.

I had the car on a dynojet last saturday...245 rwhp. :-(

The 28" tires killed you, I think they can account for almost all your et and mph change. With 3.73's and 25" tires you are driving the car more agressive than most to turn 14.2's. Typically I've seen guys running 14.9 - 15.3 with a light launch and a 2.2 or so 60' in stock 302's with 3.73's and 26" tires. Your 302 doesn't make low end power so you need lower gears to rpm where the power is.

Yeah, with that motor(small cubes....high revver), and a 28 tire, youd be better with about a 4.11 gear. With the 3.73 gears, unless you were making a lot more power you were better off where you were with the shorter tires. Now its harder to get it rolling and it probably doesnt keep in the powerband as well when shifting through the gears.......that slowed ya down. Plus, the weather from last race to this time may have made the results even harder to guage cause you never know how it would have done if the weather were the same both times.

With your traction problems, you would actually be better with a 4.11 gear and keep those 28 tall tires rather than going back to the 25 inchers with the 3.73 gear. The reason is that the taller tire will hook better once the suspension is sorted out cause of a larger contact patch. And youve already found out that the motor dont like those tall tires with the 3.73's, so a 4.11 gear would basically bring it back to where it was before. Still though, Im not suggesting you swap gears right now. Id go back to the shorter tire and keep your current gears, but do what you have to do in order to get it to hook. That would be the cheaper route and if it works.....than it works.

I wanted to swap out my M21 for a TKO 600 trans anyway, so maybe the time is right now.

A 3.27 1st gear versus a 2.2, :beers:

I'll keep the 3.73 gears in there.

That is pretty much what I thought. The problem is not really the tires, it's more of a gearing problem. While it is true the shorter tire will "act" like more gear. The real solution if you want to optimize 1/4 mile performance is more gear for an engine of that size, that likes to spin. The 28" tall tire will help you hook better and MPH better than a shorter tire. The tranny swap you are thinking about would be great. Most guys I know that ran small cube "old" school engines rean 4.56 or 4.88 gears for drag racing. 4.11's would be ok, but you would still be leaving a lot of E.T. on the table.

It being a street car (I assume) I would do as you plan, keep the 3.73's get steeper tranny gears and hopefully get some tries that will hook.

What kind of work is done to the almighty 302

I think you might want to get it hooking and then see your ET get better, more gearing through the rear or tranny is only going to make it worse in the 60'. IMHO

What kind of work is done to the almighty 302

Unported heads/intake
1 3/4" headers
MSD Billet distributor & 6AL box
2 1/2" exhaust
30/30 cam

It only made 245 rwhp last week but the lash may be way out of tune.

I think you might want to get it hooking and then see your ET get better, more gearing through the rear or tranny is only going to make it worse in the 60'. IMHO

With the 2.2 trans 1st gear and 3.73's when I dump the clutch hard the car bogs real bad I'm totally out of the power band and it accelerates like a Honda civic. What I need is a heavy duty clutch so that I feel confident dumping the clutch at a much higher RPM, like 6k.

Launch at 6k :D

I leave off idle to about 2k depending on track comditions/prep.