Hmmm, I've noticed with the Cooper ATR 245/70's, that it seems to take an extra 100 rpm to keep the same mph on the speedo. Coopers are supposedly 3/10ths inch larger diameter than the Goodyears, that might account for the discrepancy.

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Mitchell Oates
'87 MB 300D Diamond Blue Metallic
'87 MB 300D - R.I.P. 12/08
'05 Sport CRD Stone White
Provent CCV Filter/AT2525 Muffler
Stanadyne 30 u/Cat 2 u Fuel Filters
Fumoto Drain/Fleetguard LF3487 Oil filter
V6 Airbox/Amsoil EAA Air Filter
Suncoast TC/Shift Kit/Aux Cooler
Kennedy Lift Pump/Return Fuel Cooler

I wonder how that's possible? The speed distance sensor just reads axle speed, which should match RPM consistently. The drivetrain doesn't know what the outer diameter of your tires is.

- Chris

Shouldn't larger diameter tires result in lower RPM's when traveling at the same speed (assuming the speedo reading is accurate, of course.)

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2006 Liberty CRD Limited
Mopar engine, transmission, transfer case skids
245/70/16 Michelin Latitude X-Ice (winter)
235/75/16 Firestone Destination ATs (summer)
Thule roof rack, cargo box
V6 airbox mod
Flowmaster 50 2.5 inch muffler
Edge EZ module (set for fuel economy)
SEGR
TDIWagonGuy CCV filter
B99 (summer), B20 (winter)

By external measures (timing mile markers, GPS) but not by the vehicle's speedometer.

- Chris

Update: Up to Sevierville yesterday 85mph all the way 24mpg hand calculated. On the way back 56mph (oh so painful) accelerate to 61 then drop down to 56 and it will stay in 5th gear= 36mpg hand calculated. Fuel is going to have to get very hard to come by in order for for me to drive 56 for more than 5 minutes though. I now have a new highest MPG!

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Manure green 2005 CRD sport4x4, GDE Hot tune, Cat Gut, OE skids, Draw tight hitch, Duramax lift pump, 160K on multiple varieties of fuel, XM radio, Escort live with Redline, fog light mod, GPS, Icom IC7000 all band radio call sign KC9QPF, Grabber AT2s on Soft 8s, FIA grill blanket.

Oh, and we don't get winter diesel in Tennessee, it just doesn't get cold enough to require it.

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Manure green 2005 CRD sport4x4, GDE Hot tune, Cat Gut, OE skids, Draw tight hitch, Duramax lift pump, 160K on multiple varieties of fuel, XM radio, Escort live with Redline, fog light mod, GPS, Icom IC7000 all band radio call sign KC9QPF, Grabber AT2s on Soft 8s, FIA grill blanket.

This size difference will account for around 25rpm @ 70mph