Guys, need a hand here. I have no clue how to calculate the O.D of a tire but this is what occurred.

CC sold me those Mag wheels. The back ones are 18" diameter. So we did some best guessing and came up with a tire size of 275/35/18. The old size was 255/50/16 (Stock for an '84)

So what has happened is that this new size is a bit smaller. it has rendered first gear to be a little light out of the whole and I've lost my beloved 60' starts. When I get the clutch all the way out...it's almost time for a shift. My '84 loves the low end Grunt much more than the high end revs....

I want that Low end back, so I need to get a larger diameter tire.

can anyone tell me what size or how to calculate a larger size?

Tanx

I think you would need a smaller diameter tire?

Use the first to find the tire height of the original tires vs the new then plug them into the 2nd to find what tire height you need with 18inch rims to get the same gear ratio back
http://www.1010tires.com/tiresizecalculator.asp

strike that here is a do all site
http://www.kabamus.com/garage/gears.html

Im the opposite. the 3.08 gear is crap for my motor and autox. Ill be running shorter tires for stiffer sidewalls and a lil lower gearing

The first link works for tire size comparison

Overall diameter:
16" Stock Overall Diameter: 26.03 in 661.16 mm

18" Upgrade overall Diameter: 25.57 in 649.47 mm

If I go to 275/40/18 it is:
Overall Diameter: 26.66 in 677.16 mm

So I am off 1/2" either direction

Here is another link that I have used.

http://www.miata.net/garage/tirecalc.html

I'm running a 25.5" tire in back on mine.

I don't understand your description. A smaller wheel is like having more gear, so if anything it should have a little more torque.

The diameter difference is 1.8%. That'd be like going from a 3.07 gear to a 3.12 gear.

I think it's in your head.

How about a 285/35/18?

^^that is only .7% difference or

(Stock)Overall Diameter: 26.03 in 661.16 mm

(New) Overall Diameter: 25.85 in 656.59 mm

That may work out well. Only 4mm difference in overall OD. That's like running bald tires.....

IMHO you problem is the difference in rubber compound between the old & new tires & possibly a difference in footprint size.

Suggest comparing compounds with a durometer.

255 50 16 = 799 revolutions per mile

275 35 18 = 811 revolutions per mile.

I cracked some of my manuals and found the nominal size for the 255/50R16 was 26.1 inches and the nominal size for the 275/35R18 is 25.6, so you are right, it's 0.5 inches shorter. If you want to stay with 18" rears, the best size option I see is 295/35R18. Nominal size on that is 26.1, same as the 16" OE. The chart I'm using is only size specific, not brand or tread, so you will still have to find what might be available in your corner of the world in that size.

Good Luck

JD

Tanx!! I'll check out 295s

Will 295s even fit a 9.5" wheel? Check the tire specs on tirerack.

Go a little taller....try 275 40 18 GSD3 According to the tirerack the specs are 26.8" diameter and 780 revs per mile. Since they are on close out price is PDG too.

I like them on my car.....and they fit a 9.5" rim...


Mike

GSD3???

Quote: Goodyear F1 GSD3 Great street tire for dry and rain not so great in the snow....
Right now 198 bones each. Buy set of four 80 bones back in the mail.......
I went with this size because IMO it was best match to replace my 285 40 r17 GSD3s that were on the car.



Mike

seriously i don't know how you are realistically seeing a change in anything. unless you are spinning. i have never noticed a difference in output of my car with swapping tires.

^^if not then you aren't very perceptive.

Changing the O.D of a tire on the rear is a very quick way to change the gearing.....as evident by the post in this very thread.

If you can spin your rear wheel and count how many times the drive shaft turns in order to see what the gears are in the differential....wouldn't it make sense to you that a very large tire would change anything versus a very small tire??

Many of the guys that race at our frequent drags run smaller dia. rear wheels and tires at the track for just that reason. I'm not sure what they pick up, but I have monitored this to see what people are doing. I am getting ready to run mine for the first time in April and I have been observing for a couple years now.

Quote: today isn't my first day driving cars. i have never noticed a difference from going from street c5 wagon wheel rims/tires to my drag race setup which was 27x10.5-17 slicks or 245/50/16 dr's.

I think you're on crack John. We're talking less than 2% difference either way. The difference in tread wear from what you removed probably cancels that out.

If I gave your car 2% more horsepower you'd never notice it. Or if I made your wife's tits 2% larger or your tires 2% smaller.....

Stick it out, do some burnouts, and then put 285/35s on there next time. Those are the tires you should have went with, I just wasn't aware that was a common size at the time.

Oh yeah...that's what I'm doing...I'll wear these out pretty soon...I'll go back to the used guy again and try the other size.

Scroll down to the bottom of this site for a great Tire size calculator

http://www.joby.se/corvette/div/corvette_wheels/