Larger Tire Shifting Points

[pedal2themedal]

I am wanting to install 31" tires, but if the truck is not going to shift right it is out of the question. So, will 31" tires affect the shifting at all? This is on a 2000 merc mountaineer. Thanks, Robert

 

[brickwall]

I just put a set of 265/75R16(31.7) on my 2000 sport with no issues, speedo's only off 5km/h(3mph) at 100km/h(60mph)

 

[Fredness]

Shift points are a critical thing called NV in the tune.
N/V is the tire Revs Per Mile x 60, x the rear gear ratio /60 /60
or ((((RPM*60)*Axle)/60)/60)

This comes out to a calculated NV between 40 and 50, factory being 50.
If this number is off, your shifts suffer.

Any good tuner can correct for gear and tire changes and get you added shift pressure to boot.

 

[pedal2themedal]

Shift points are a critical thing called NV in the tune.
N/V is the tire Revs Per Mile x 60, x the rear gear ratio /60 /60
or ((((RPM*60)*Axle)/60)/60)

This comes out to a calculated NV between 40 and 50, factory being 50.
If this number is off, your shifts suffer.

Any good tuner can correct for gear and tire changes and get you added shift pressure to boot.

Any good tuners around Emporia KS that anybody knows about?

 

[Divemaster191307]

Shift points are a critical thing called NV in the tune.
N/V is the tire Revs Per Mile x 60, x the rear gear ratio /60 /60
or ((((RPM*60)*Axle)/60)/60)

This comes out to a calculated NV between 40 and 50, factory being 50.
If this number is off, your shifts suffer.

Any good tuner can correct for gear and tire changes and get you added shift pressure to boot.

How far "off" should one start looking at it as a potential problem?

Using your formula, my OEM setup (2007 Ex) comes out to about 41.77 (706RPM * 60 * 3.55 / 60 / 60). I'm getting ready to swap in some 265/70R17, which would give a N/V of about 38.99 (659RPM, according to Firestone's web site).

 

[hitman46mod]

My 2000 Mounty I don't have the tires yet.
Dunlop Radial Mud Rover 31 X 10.5 R15
30.8" over all diameter
3.73 gears
692 Revs per mile 692x60x3.73/60/60= 43.01

So what is this in reference to, you want a ratio of 50. So does a lower number mean higher or lower shift points? I could understand if your shift points changed radically how it could hurt the trans but if were only talking a 2 to 4 hundred RPM then it should not matter.

 

[tommygun]

Any good tuners around Emporia KS that anybody knows about?

I am with MC Racing in Overland Park, KS. We can tune for gear ratio as well as other mods. PM me if I can help you out.

 

[KevinZwick]

did any of you experience a dramatic loss in mpg with larger tires. well i didnt make a big difference. 235/70/16 to a 255/65/16. after this i noticed about a 2 mpg drop. went from 380 to almost empty to about 320. Around the same time i had a thermostat stuck open that i replaced. Can anyone help me get 17mpg back? or atleast let me know how i lost it. I can settle for a little bit skinnier stock tire again lol

 

[explorerguy89]

any tuners for not to much i got 31" tires now and dun have hundreds to blow on a tuner

 

[Divemaster191307]

did any of you experience a dramatic loss in mpg with larger tires.

I went to 265/70R17 and I lost roughly 1.5 mpg... I have noticed that even a minor uphill will make it drop down in gear, while it used to just chug along with the OEM setup.

 

[hitman46mod]

An electronic control system for scheduling gearshifts in a four speed automatic transmission includes storing in electronic memory accessible to a microprocessor, six functions relating vehicle speed to throttle position, one function being related to each upshift and downshift. Gearshifts are made by probing computer memory with current values for throttle position and vehicle speed, and determining on the basis of the stored shift schedule whether an upshift of downshift is required. Upshift points are determined also on the basis of the maximum engine speed when a wide open throttle condition is detected. Electronic memory continually accessible to the microprocessor is updated with new calculated values of the ratio NOV divided by base NOV. Vehicle speed from a stored schedule of gearshift values is corrected with the calculated ratio to account for changes in the driveline components from their condition when originally installed and differences from a calculated base NOV value.

I'm not sure but if this is correct than the comp will relearn the NV values.

As for the people who lost MPGs. I would be willing to bet that you all have 3.55 gears. A truck with 3.73 or 4.10 won't suffer as much from the larger tires.

 

[XLT]

Yeah i threw 31" tires on my sport which had 255/65-r16 on stock and i didnt see any drop in mpg or shift points. But i also have 4.10 gears. Even my spedometer is right on. I have tested a couple of times and was very suprised and delighted. I assume you have the v8 so you should be good to go.

 

[explorerguy89]

i got a 4.0 and the speedo is off by 5 mph at 60 mph which isnt much but still