46inch Tires: Speedo-ReCalibrate??

[USAFSS-ColdWarrior]

Ok,I'm a new guy here and haven't been able to locate this info in searching the threads....

On a Duece.... When changing to 46 inch tires (from stock NDT's) is there an easy way to recalibrate (adapter?) the Speedometer for true indication?

Secondary questions....

What is the actual circumference of the NDT's vs. the 46's?

Has anyone had experience with MPG increase or decrease?

And, what about the "cruising speed"?
I'd think that this would be a major improvement but haven't found commetary on the topic yet.

Any help on this would be appreciated.

Thanks in advance,
John

 

[gimpyrobb]

John, an online calc says at 2500 rpm with 46s you should be doing 72mph. My gps says I'm doing 63. I'm running the XML tires. I doubt there is a military gear, but look for a speedo shop. I was told they can make/get whatever gear is needed and do the math for you. A $100 gps is now my speedo.

 

[USAFSS-ColdWarrior]

"Robb" (I presume your name),
Thanks! As for GPS - I figured that to be the quick fix.

Where did you find the online calculator? As I mentioned, I'm a newbie and didn't even know the diameter/circumference of the stock NDT's.

Have you increased MPG with the bigger rubber also?

Finally, here in San Angelo, TX, there are no Speedo-Shops (unless you're looking for skin-tight athletic wear), and we are 90 miles from anywhere. We are a city of approximately 100,000 (including everyone in a 45 mi. radius) and have to go 200 mi to get to anything much larger. Thank God for the internet... I'll find the Speedo gear online once I have a truck to put it in and the tires that necessitate it. LOL.

Thanks for the help!
John

 

[tm america]

15,5s or 395s are 15% taller than the stock 9.00s the speedo adapter you need is 15% overdrivin they have them at boyce equip and they know which one you need .give them a call they can help you out .cuz the math doesnt lie its all multiplecation.my speedo is right i checked the miles travled against the time it took on the way home from haspin and it shows from the time i got on the eway i was going 72.my speedo only reeds to 60 but it will spin the needle around to the o in hour at the bottom of the gauge .i think there is more of and error with speed when using a gps sinc the satalites might not be relaying info back in real time.my truck would do 60 with stock tires after i turned up the fuel

 

[Avn-Tech]

Group,

There is a speedo shop in Baersfield, CA (Fleet Parts). They have been in business for about 30 Years.

While I have not had any speedos worken on there, I did purchase 4 aircraft hangers from the owner. The transaction has been great (receved Owner financing).

If you have any speedo questions call him, he may be able to help you. see:

fleet parts bakersfield ca - Google Search

Tell them John Fischer sent you.


Laterrrrrr
Avn-Tech

 

[frodobaggins]

15,5s or 395s are 15% taller than the stock 9.00s the speedo adapter you need is 15% overdrivin they have them at boyce equip and they know which one you need .give them a call they can help you out .cuz the math doesnt lie its all multiplecation.my speedo is right i checked the miles travled against the time it took on the way home from haspin and it shows from the time i got on the eway i was going 72.my speedo only reeds to 60 but it will spin the needle around to the o in hour at the bottom of the gauge .i think there is more of and error with speed when using a gps sinc the satalites might not be relaying info back in real time.my truck would do 60 with stock tires after i turned up the fuel

55 + 15% = 63.25
60 + 15% = 69

 

[USAFSS-ColdWarrior]

Thanks - the 15% factor for the diameter delta was most helpful and the mph conversion simple enough.

The expected performance delta is another thing though. Without other factors, a simple 15% mpg increase could be assumed (ass+u+me method). Though that would be ignoring many other factors.

However, I know that the change in wheel diameter would be like changing the differential ratio - more or less torque inversely compromises the engine RPM to Speed Ratio, right?

Hence, am I correct to think that the larger tires would reduce the low end torque & fuel efficience while increasing the top end speed & MPG???

Assigning a 1/3 inefficiency factor to the impact of tire diameter increases, I feel safe to think that the MPG improvement at 2500 rpm might be 10-11%.

Thanks again for any and all input on this.
John

 

[tm america]

you also have to take into consideration that wider tires grow more with centripital force from them being rotated and 15.5s and 395s are 46.7 inches tall .my rpms are at 2600 when the speedo reads the o in hour.so yes i believe 2500 would be about 69mph.any mechanical speedo will have error eirther at low speeds or high speeds .due to tire growth most speedos are calibrated at 55 or 60 from the factory higher speed usually mean greater % of error

 

[sermis]

This will give you some tire sizes.

Army Military Tires

 

[gringeltaube]

.... some more info here: http://www.steelsoldiers.com/deuce/28757-tire-size-difference.html

G.

 

[OPCOM]

handy chart.

I believe the axle spacing on the deuce is 48". Therefore 48" tires will rub. I believe the biggest you can put on is the 395R20 type.

Not being a tire expert, the biggest thing I took away from those threads is not to use "MPT" tires on rims designed for regular 9.00x20 and 11.00x20 tires. It seems confusing, but there are these three similar-sounding tires sizes:
365/80R20 XZL MPT
365/85R20 XZL
395/85R20 XZL

So it is iportant to know whether the tire is an MPT type, as they don't fit right on non-MPT rms. I'm sure they can be put on, but their bead has a shape to it that does not fit. I'll reiterate those pictures to be clear.