Trans gearing for larger tires

tainlay · Post by **tainlay** » Mon Nov 21, 2005 2:15 pm

I've got some LT215-75R15 BF Goodrich Radial All-Terrain tires on my '74 Thing and with the stock 1600 it really moves like a slug through the gears. I'm looking for suggestions on re-gearing the trans to counter the increased tire size.

Regards,

Thomas

Post by **Marc** » Mon Nov 21, 2005 2:39 pm

The standard AV transmission for US-market `73/`74 Things has a 4.125: R&P. Your tires are about 14% taller than stock; the most you can compensate with a Type I trans would be with the "AM" trans (not found in the US, it was used on 1200/1300cc IRS bugs in Europe & Canada) which has a 4.375 - that'd get you halfway there. There's also a stock 2nd found in some Things that's 2.25 rather than the common 2.06 - it'd be hard to find one but they're around. The 1.32 3rd used in earlier bug boxes could also be used instead of the normal 1.26 (not a big change, but a step in the right direction). In aftermarket 3rd and 4th there are many ratios available, but those won't do you any good off the line and to change 1st & 2nd any more requires an aftermarket mainshaft which is pretty pricey, so for any lower gearing than that you'll pretty much need to either go with an aftermarket R&P or convert to an IRS II trans (worthwhile but a bit ambitious).

Bruce2 · Post by **Bruce2** » Mon Nov 21, 2005 11:55 pm

I'd suggest the same. 4.37 R&P with a stock 3.78/2.06 mainshaft. Then modify an early 1.32 3rd to work on the splined mainshaft. For 4th gear, use the .93 Beetle SSC gear.
I call this combo an economy close geared trans that uses all stock parts.

tooter · Post by **tooter** » Tue Nov 22, 2005 12:55 pm

Do you need a step ladder to get into your Thing or just a running leap? Also remember the added tire size is added weight and centrifical as well as gravitational forces when you turn the wheel. Have you considered what the added tire sizes do to the stress on your steering box and other front end components? Does it feel like you're turning a tank?

tooter

Bruce2 · Post by **Bruce2** » Tue Nov 22, 2005 11:56 pm

Maybe he goes offroad and needs the ground clearance.

tooter · Post by **tooter** » Wed Nov 23, 2005 8:24 am

Nahh.... probably just the pot holes in some cities!

Kubelmann · Post by **Kubelmann** » Thu Nov 24, 2005 10:46 pm

Fellers.. Y'all fergot to mention that rule one of bigger tires is bigger motor. YOu can play with gears all day long but if the motor won't turn the tires. I built a gear ratio to tire size calulator that i include in the RoMTOC CD but would happily pmail to nayone interested. It is a simple excel spreadsheet program I built to fogure tier size/gear ratio opitmal proportional ratios. It is a great way to figure out what you will get wiht differnt tires. Big tires create off the line issue and tned to beat little motors to death regardless of what you do to the gears.... All three work together as an enclosed system.. Engine power, gear ratios and tires size. It is like a three part ballet of performance.

RICKERACER · Post by **RICKERACER** » Tue Nov 29, 2005 9:12 pm

Okay gents, same question, different tire size. I run 31" tires both front and rear. Yes, it is like turning a tank. I go off road so I do need the ground clearance. I will have the power tu turn the tires, 2.0 porshe conversion, but did okay with the 1600. I am going to have my ZF limited slip diff installed (Thanks Bruce) and wanted to change the ring and pinion to a 4:86 ish or so. I know changing the individual gear sets may be a better option, but will my plan work? I am more interested in "crawling" with a 65ish mph freeway run on occasion. Thanks, Rick