Speed calculation

[dylanray]

So my 1970 V8 40 has original gears which I believe are 4.11. I'm running 33x12.5x15 and was curious when my speedo says 50 or 60 what's my real speed? Tranny is a 4 speed from a late model or a 60 series (H42?)

Feels pretty fast to me and pretty good hum from tires, gear box and engine rpm. Mostly been on country roads so no highway speedo check markers to help yet.

Thanks

 

[huminajumina]

Do you have a smart phone? Download a free gps speedometer app. It will be, pretty accurate, that or watch your odometer on the interstate & compare it to the mile markers every .2mi

I changed my speedo gear to correct for 33's so my odometer is exactly on, but my speedo needle is off & breaks occasionally....

 

[RWBeringer4x4]

It's roughly the difference in size of your tires from stock, I think.

For example - I believe the stock size tires on a 70 Land Cruiser were 29's or 30's. So your 33's are about 10 or 13% larger than stock. This, you're actually going about 10 or 13% faster than your speedometer says. So, if your spedometer says 50, you're really doing 55 or 56.5mph.

Not sure if that is exactly the way to do it, but it has always gotten me close!

 

[bikersmurf]

50=58 ish
60=70 ish

Usually use RPM calculator. By design, the guage is +/- 5.

 

[dylanray]

Thanks all who responded with useful info. I downloaded a gps speed app and will also try the free speed markers once it has seatbelts!

 

[1911]

http://www.grimmjeeper.com/gears.html

 

[Fast Eddy]

It's roughly the difference in size of your tires from stock, I think.

For example - I believe the stock size tires on a 70 Land Cruiser were 29's or 30's.

Not roughly, exactly. The original tires were 28s, so with 33" tires it is 18% faster than it shows on the speedo. 51mph is about 60.

 

[JohnnyC]

For 20yrs ive just added 10 mph to my speed ... That works for me

 

[RWBeringer4x4]

Not roughly, exactly. The original tires were 28s, so with 33" tires it is 18% faster than it shows on the speedo. 51mph is about 60.

Well - glad I've been doing it right all this time!

Also - I'm shocked that FJ40's came with 28in tires. Seems so tiny by modern standards!

Anyway, dylan, I think this is your calculation.

Original tires: 28"
Current Tires: 33"

33/28 = 1.178 or 17.8% increase over stock

50mph + [50(.178)] = 58.9mph.

When your speedo says 50, you're really doing 58.9 mph.

 

[JohnnyC]

Rick that is IF his tire is actually 33" tall... most 33's are less like 32.5" or so .... but still close 58 abouts probably... much easyer to just add 10mph for us lazy guys

 

[RWBeringer4x4]

Rick that is IF his tire is actually 33" tall... most 33's are less like 32.5" or so ....

Now you're just nit-picking

 

[Fast Eddy]

Rick that is IF his tire is actually 33" tall... most 33's are less like 32.5" or so .... but still close 58 abouts probably... much easyer to just add 10mph for us lazy guys

Now you're just nit-picking

And really, the "height" of the tire is 2x the lower radius, measured from the center of the axle to the ground. There's a flat spot on the bottom. Nit-picky, but true. Tires are "smaller" if they have less pressure or more load on them.

 

[surfpig]

Not roughly, exactly. The original tires were 28s, so with 33" tires it is 18% faster than it shows on the speedo. 51mph is about 60.

My piggie with 33s was off by about 15%.

Actually, 16.666% when they were new.

 

[BDSeff1fitty]

Shoot me a PM with your email. Don't know how to upload an excel to Mud.

I got bored last night and made a LandCruiser Speed Calculator. Need to know RPM to get real speed.

Allows you to choose transmission from J30, H41, H42, H55, SM420, SM465
Choose 3 speed case, 4 speed case, Orion 3:1, Orion 4:1 and Split case
multiple axle gear options, tire sizes, and RPM's.

Edit: I could probably modify it to show a "stock" drivetrain then current drivetrain to compare speeds at what ever RPM you choose

 

[dylanray]

Shoot me a PM with your email. Don't know how to upload an excel to Mud. I got bored last night and made a LandCruiser Speed Calculator. Need to know RPM to get real speed. Allows you to choose transmission from J30, H41, H42, H55, SM420, SM465 Choose 3 speed case, 4 speed case, Orion 3:1, Orion 4:1 and Split case multiple axle gear options, tire sizes, and RPM's. Edit: I could probably modify it to show a "stock" drivetrain then current drivetrain to compare speeds at what ever RPM you choose

I'm good I figured it out yesterday on the highway with speedo marker check posts

 

[dylanray]

Shoot me a PM with your email. Don't know how to upload an excel to Mud. I got bored last night and made a LandCruiser Speed Calculator. Need to know RPM to get real speed. Allows you to choose transmission from J30, H41, H42, H55, SM420, SM465 Choose 3 speed case, 4 speed case, Orion 3:1, Orion 4:1 and Split case multiple axle gear options, tire sizes, and RPM's. Edit: I could probably modify it to show a "stock" drivetrain then current drivetrain to compare speeds at what ever RPM you choose

But thanks excel is awesome!!

 

[Vae Victus]

What if I'm running 33' tires but change the gearing from 4.11 to 3.70? If the tire difference is 17.8%, what does the new gearing do?

 

[JasonH]

http://www.4lo.com/

 

[JohnnyC]

http://www.csgnetwork.com/automotiveconverters.html
^^^ bunch of Automotive Converters and Calculators here ^^^^

 

[JohnnyC]

TTT