Question on tire size for big bore 40 hp and 4.37 gearing

[biblethumpncop]

I am having a Frankenstien of a bug put together locally. I am having a 1200 cc engine (40 hp) rebuilt with a big bore kit (1385 cc) that will run a 30pict1 in a full body bug (Class 11 style). The bug will have a 3" body lift that is made by Bugzylla Fabrications. I am having a Type 1 transaxle built with 4.37 gearing by Don's Bus Box. I was going to put a Flatback kit on the body, but have changed my mind and decided to keep it as a sedan (since it has a factory sunroof).

Here is the question......
What size offroad tire can I run? I want the same size on all 4 15" wheels. I was going to use 215/75r15 but wonder if the engine and transaxle combo would allow me to run 235/75r15 without any issue? I will be using it as a daily driver, no more than 65 mph on highways when I drive them. Otherwise, mostly surface streets. I want to play in the desert once in a while.

Please give me some advice! Thanks!
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[Russ Wolfe]

I think this would be best asked in the "OffRoad" forum, so I am moving it.
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Gary: OK. Ima poop.

[biblethumpncop]

I posted it already in the offroad section, but had placed it here due to the 40hp engine. Not as many over there use that size engine.
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[jsturtlebuggy]

If your transmission has stock gearing and you only have a big bore 40hp your going to have a hard time pulling any tire that larger diameter than stock size at highway speeds.
It can be done but I would not in my opinion use anything more than 27inches in diameter at that you are going to be using alot of 3rd going up any kind of incline.
Going to close ratio 3rd and 4th gears will help alot.
Way back in the late 60s and early 70s before a 1600cc engine became more available it was very common to change out 3rd to 1.38 to 1 and 4th to 1.04 to 1.
Another option for small engines was going to bus reduction gears.
Now day with bus IRS transmissions readily available with 5.38, 4.86,and 4.57 ring and pinion it is more common to switch to one.
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Joseph
Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
Having fun with Dune Buggies since 1970
Into Volkswagens since 1960

[biblethumpncop]

Thank you for the reply. I don't know what the stock gears where, because it would have originally been in a swing axle. I have the option of having 4.86 or 4.37 since he is building it for me.
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[biblethumpncop]

I just heard back from Don's Bus Box. The swing axle cars the 40 hp engines originally came in were 4.37 gears. Although the original tires were about 2" shorter, I imagine the hp gain by putting the big bore kit on should even everything out. I'm going with 215/75r15.

Thanks!
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[glutamodo]

[biblethumpncop]

Thanks for the info Andy. I see that the 225/70r15 are a 1/2" shorter than the 215/75r15. I may opt for them.
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[glutamodo]

Well, on a calculator it's only about .3" lower.

Factory original height on bias ply tires was around 25.8"- 25.9"
With radials it's more like 25.4" Here's a little chart comparing calculated numbers - (actual tire size can vary from the calculations though)

[slalombuggy]

Here's a gear ratio calculator where you can play with gears and tire sizes to give you an idea of where your engine will be at, at different speeds

http://www.et-studios.com/motorsports/gears/gears.html

brad

[biblethumpncop]

I bought some sport wheels that are getting powdercoated gloss black. I ordered some General Grabber AT2 225/70r15 tires for it. I should have the tires mounted next week. The transaxle is having 4.37 gearing put in it right now.

Thanks for the information.
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[dustymojave]

I ran a 1385 (40hp with 83mm big bore cylinders) for decades in my 58 Baja.

I found it best to keep the tire diameter down to about 28" and wound up with 215-75 tires with 4.37 gears in a stock swingaxle trans. This allowed me to climb substantial hills and pass modified 4x4s in the process, pre-run desert race courses, while keeping it real for the freeway in daily driver use.

I used 7" wide rear rims with stock back spacing and 5" fronts for clearance. With long axles, the rears just barely touched the inner wheel wells at full bump.
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Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[biblethumpncop]

Great information! Thank you!
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If you blame others when you fail, give them credit when you succeed!

[dustymojave]

Oh...BTW
That Baja looks like this now:

Not a 40hp anymore. It costs way more to rebuild a 40 than a 1600. So after nearly 3/4 of a million miles with small motors (the original 36 lasted 376,000 and the 40 nearly 300,000 more) it now has a 1776 with dual Kadrons, full cage from bumper to bumper, 10-1/4" of front travel with the original beam, arms and spindles, and IRS with dual shocks in the rear.

Not knockin' the 40hp, it just wasn't practical to keep it going.
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Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[Lotrat]

Hey Dusty, think you can snap another picture in the same spot? It would make for a pretty cool "then and now" thread.

[dustymojave]

LOL

That B&W photo was taken 30 years ago. Last time I went by there, that trail wasn't even there anymore. The site is in Kelso Canyon about 1 mile south of the west end of Dove Springs Rd. It's at the site of what WAS in the early 70s a ghost town by the name of "Sageland". By the time the photo was taken there were just a couple of building foundations and trails left. Other than the location being noted on some maps, there's no evidence of it anymore.

Would be cool though!
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Richard
Offroading VW based cars since 1965
Tech Inspection 1963 - 2012 SCCA/SCORE/HDRA/MORE/MDR +
Retired from building Bajas, Fiberglass Buggies and Rails in the Mojave Desert. Also Sprints & Midgets, Dry Lakes, Road Race cars. All types New and Vintage
SoCalBajas Member
Kicked Cancer's A$$...1st and 2nd round...Fight ain't over yet.

[biblethumpncop]

I had vw sport wheels powdercoated gloss black and mounted General Grabber AT2 225/70r15 today.

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If you blame others when you fail, give them credit when you succeed!