Tyre Height Question

XTJG33 · 04-05-2014, 10:26 PM

Hi Guys,
I have a 1968 XTGT Replica with 4.11's in the rear running a 255/60/15 tyre (27 inches high). Last weekend i took it down the quarter and crossed the traps at 13.63 @ 99.84 mph @ 5100rpm. i was changing at 6100 and had a 1.912 60ft.
If i fitted a 26 inch high tyre, say 235/60/15, would i get a better 60ft time and ultimately and better 1/4 time. what rpm would i be crossing the line at?

GTB 351 · 05-05-2014, 01:37 AM

I tried this many years ago and it was a backward step. Having said that, it may not be in your case; there are so many variables.
Buy yourself a Moroso Power Speed Calculator. Available at any speed shop for less than $20.

Giant · 06-05-2014, 03:52 PM

I have played with diff tyre sizes as it effectively reduces or increases your diff ratio it can help obtain the correct gearing. I think a shorter tyre would help you. Try the calculators to work out what's best

mex351 · 06-05-2014, 06:53 PM

Ant's LS powered XD wagon used 255x60x15 mt's to go 13.2 by swapping to my 255x50x16 tyres it went 12.8.No other changes just rims and tyres.ONBOC went faster with the taller 255x60x15 combo but slower with the 255x50x16.

Each combo is different and finding the right 1 for you is the hard part.

XTJG33 · 06-05-2014, 11:02 PM

Thanks guys
I will try and get a Moroso Power Speed Calculator and see what it tells me. I have tried some online calculators but people say they are not accurate.

End of the day i will know the true answer if i make the change and give it a run down the quarter. but my gut feel tells me i have to get a better 60ft and ultimately a better ET

Woodykustoms · 09-05-2014, 09:21 PM

Be interested what you find what works for you, I think you are on the right track with the shorter tyre....

XTJG33 · 11-05-2014, 03:15 PM

I will let you know Woody when I do it

sweet na · 30-06-2014, 10:44 PM

I'm assuming it's a Toploader and that you're crossing the line in 3rd gear. With the 235/60-15 you'll pick up about another 300 rpm across the line and will definitely accelerate better but may not launch any better unless it's a drag radial. I'm sure you'll have fun trying and that's what it's all about at the end of the day!

XTJG33 · 01-07-2014, 12:08 AM

Quote:

Originally Posted by sweet na

I'm assuming it's a Toploader and that you're crossing the line in 3rd gear. With the 235/60-15 you'll pick up about another 300 rpm across the line and will definitely accelerate better but may not launch any better unless it's a drag radial. I'm sure you'll have fun trying and that's what it's all about at the end of the day!

Cheers, I have a C4, and I want to try a drag radial. I would hate to go out there and get wheel spin. I have tried to find some used 235/60/15 ET streets with some tread so I can get 2-3 passes out of them but no luck yet.
And yes it is fun trying

sweet na · 01-07-2014, 10:10 AM

That's good that you're running an auto, makes launching a lot easier. Just keep an eye out at the strip for guys running the smaller tyres and talk to them about selling them to you before they're worn out.

XTJG33 · 01-07-2014, 11:41 AM

Good thinking 99

99er · 03-07-2014, 07:15 PM

you should try to set it up to cross the finish line in top gear close as possible to redline.the bigger the tyre the more mile an hour considering you have the power to turn them quickly.generally speaking one tenth in the first half track converts to two tenths at the line.lots of variables.set it up for your max revs to when your camshaft stops pulling,in 1st gear take it to redline,the other gears to when your cam stops pulling.where is your max torque at rpm.all that said i think you need a smaller tyre to reach higher rpm at the line.or launch harder.how are you launching

XTJG33 · 03-07-2014, 08:16 PM

Thanks, i change shift points from 5500 by 200 rpm leaving the line at 2500 with not wheel spin and good hookup. Best time was when I changed at 6100. Crossed the line at 5100rpm at 100 mph. I want to cross the line at say 5700-5900.

99er · 06-07-2014, 01:37 PM

thats sounds like a good launch.looks like you need smaller tyres.but how much smaller.there will be a ratio as per inch in tyres as to affecting rpm at the line.i dont know what it is.for example if 1 inch down gives you 200 rpm, at the line you need to go down 3 inches to pick up 600 rpm.have you got any 14inch rims and tyres you could try

XTJG33 · 06-07-2014, 02:52 PM

Cheers, yeah would be good to know by how much the smaller tyre will affect the rpm. 14 inch rim with a 235/60/14 might be a good idea. Never thought of that one. Will look into it. Or even better, just build a faster engine....hahaha

sweet na · 06-07-2014, 03:16 PM

It's a shame you're in Adelaide, I've got some Toyo Proxes Ra1 225/50-14 which are 22.5" high and 8" tread width. Used them on my XY when I wanted to lower the gearing without changing diff ratios.

crappy408 · 09-12-2014, 03:02 PM

Quote:

Originally Posted by sweet na

It's a shame you're in Adelaide, I've got some Toyo Proxes Ra1 225/50-14 which are 22.5" high and 8" tread width. Used them on my XY when I wanted to lower the gearing without changing diff ratios.

If you think you could launch at a drag strip on those tyres you are tripping.