View Full Version : Tire size and speed results

03-20-2013, 06:23 PM
(Hope this is an OK place to post this question)
If you're not familiar I have a 59 T-bird Rat/street Rod kind of project. I have a 390 with headers, close ratio 4sp, and a 3.89 open rearend.
I have two sets of 15 inch rear wheels. One set are pie crust cheater slicks from "Towel City tires". The diameter is 28.5, the width is 8 inches. I also have a set of modern 225/65 15 inch that are about 25.5 to 26 inches in diameter. The car seems quite a bit faster with the smaller tires. It accelerates better, and it can spin them pretty easy. It won't spin the 28.5 inch tires, at least without burning a lot of clutch.
Does this sound normal? Or should i say, is this what you would expect? With the headers I should be around 350 HP and well over 427 torque.

I have no experience whatsoever with this kind of thing so thought I would ask you guys.
Maybe "Noncontrol" has an idea being that he has a 390 4 speed?

Here's an album of my car if you want to see the bigger tires.

Thx in advance, Dave J

03-20-2013, 10:10 PM
It's definitely easier to smoke smaller diameter tires than larger ones. People put larger and fatter tires on drag cars for traction. Smaller tires also have less rolling resistance.

Nice album!

Dakota Boy
03-20-2013, 10:21 PM
X2 on the comment above about the larger diameter tires.

Richmond's (Richmond Gear) website has a great drag calculator that you can play around with when considering tire sizes and rear axle gear ratios.

Dakota Boy
03-20-2013, 10:37 PM

I just looked at all your pics, and it seems to me that you are having WAY too much fun with this project.

All the really DIRTY work on my car is complete, and I find myself quite bored during these long Wisconsin winters now!

I think what you are doing with your Square' is awesome, and I'm glad to see that old clutch pedal/brake pedal setup of mine is being put to good use!

03-21-2013, 09:19 AM
Thx Gents, yeah I'm not too surprised that the old school slicks hook up more than the standard tire, but they are more of a show tire than traction tire so I thought I would be able to spin them with the open rearend. I am (mostly) surprised that they don't accelerate as well as the smaller tires.

I'll check out the calculator on Richmond's site when I'm ready to buy some good road tires.

Greg, your pedal set up was a life saver. I didn't want to go with a modern conversion kit. Can't tell you how much I appreciate that. If you ever think of a piece or part that you need let me know. I have most of the interior trim and some exterior that I will never use and I'd be glad to see someone put them to good use.

regards, Dave J