Need help with choosing a tire diameter

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December 04, 2008, 01:26:13 PM

I did a search here already but I need a little help guys. My specs are 67 Dart GT mini tubbed, 383 mild street motor, 2800-3000 stall, 3.91 gears right now but would like to switch to the 4.30 I have. I am getting ready to buy a set of Hoosier quick time pros 11.5 in wide on 15in rim. I am unsure whether to buy a 28 in diameter tire or a 29 in? What rpm do I want to be at the end of the quarter mile? I would like to run somewhere in the 12's with this car. Although everything is streetable about this car I don't drive it very far on the street so quarter mile performance is the first priority. I had intended to get 29x11.5x15 tire for it but started to second guess myself on the diameter. Thanks for all the help.

have u modified the front opening of the quarter to enable a 29 to fit?

Yea, the rear end has been moved back and the front of the rear wheelwell have been modified forward about an inch so there is enough room for a 30 in tire that I tried.

It really depends on where your motor makes its power. Search for drapstrip calculators, you'll find some interesting ways to find out alot about stuff you'll need to know.

My thinking is if you use the 4.30's you'll need the 29's to tame down the gear at the top end. If you stay 3.91, you may need the 28's to keep the rpm's up.

Not trying to hi-jack, but in the late '80s, I bought a 4.88, to use with 30" tires, the calculator said 6500 rpm in the traps.
That seems to have changed, now it seems I have too deep gear.

Come on guys don't be afraid to give your two cents here. I got to put my order in tomorrow. Thanks

I would go with a 28 inch tire. With the 3.91s you should go through the traps at aroune 5700- 5800. This is with a mild motor. The 4.30s should raise the RPMs around 400 through the traps. A two inch increase in tire size will raise the RPMs around 200 RPMs.

I run a 27" tall tire and 3.91's and go through the traps at 6100 6200 with a mild 383 if it helps any. just my 2 cents

Quality of the torque converter will effect your calculations. Some have lots of slippage.

My dart was at 6300 at the stripe - 1/4 mile .

28x11.5 hoosier qtp

and 4:10 gear

My RR uses 4.57 gears and 11x28 slicks. 6800 shifts and 7000 RPM through the traps.

Alot has to do with your cam and engine combo. You could have all the right stuff for a 9 sec' car, but if your engine and cam only work to the 12's, the 9 sec' set up is going to hurt rather than help. If your cam is done in it's effective range of RPM by 6000 RPM going through the traps at 8000RPM means the set up is a bit off. Which you know. You shouldn't be much higher in the RPM's than your shift point going through the traps. Running out of RPM's too soon ("...run out of motor...") and your not getting any help from the engine by spinning it past your shift point and holding it. This is also a sign of slippage or a lack of traction at the line so check the 60ft times on the time slip so you don't over correct. Having RPM left when you get to the trap shows room for improvement; get more gear or smaller diameter tires to get the RPM's back up. Sometimes you have to try several different things, one by one, until you find that magic combination, convertor, cam, RPM, gear, which is specific to YOUR car and combination. To give you an idea, a 500 CID should, by all means, out run and out perform a 400/451 given the same car and parts. Logic says yes, but reality was the 400/451 using the 500's parts was a better match and went faster than the 500.

so, a 4.88 with 30's should keep rpm below 7000?