Gear swap
Posting Guru
Joined: Jul 2004
Posts: 1,627
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From: West Valley Utah
Mr. Bob, you need to remember when you put larger tires on your speedometer shows slower than you are going, unless you have the speedometer recalibrated.
With your 315's, I'd bet the speedometer is slow by about 15 mph or so.
With your 315's, I'd bet the speedometer is slow by about 15 mph or so.
Thread Starter
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Junior User
Joined: Nov 2006
Posts: 61
Likes: 0
So guys, with our C-6, and 31" tires, what should I be looking at (rpms & mph) with 4.10 or 4.11 gears? How about with 3.55's? Next time I'm in Alaska, I'll investigate the axle codes to be sure what we've got. Fuel savings is not the priority, as the truck is only used seasonally, but it would sure be nice to get 65 mph out of it without going deaf! I've thought about an overdrive unit, but the cost seems to be higher than switching gears? We're about due for new tires (next summer) so the first thing I'll do is go with the tallest tire that will fit (we have a non-stock bumber that's nowhere near the tires, so what's the tallest tire (narrow is okay) that will fit? It's a stock '88 F250 4x4 7.3 to refresh the memory! Also, both fuel gauges are not accurate, I assume the tanks have to be pulled to fix that? Thanks!
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 15
From: Nutter Fort, WV
A C6 will not mechanically lock, so the readings can vary from the ones I gave for a four speed.
The final drive ratio on a C6 is 1 to 1 like it is on a four speed or a five speed in fourth.
The numbers I gave above are mathmatical calculations, tire dimansions do vary with air pressure and different tire manufacturers.
The amount of sidewall flex does change the MPH at a given RPM, but it should not be off that far.
Tire diameter is more accurately the distance from the center of the axle to the ground times 2, with the truck setting on a flat level surface and normal loading.
When you are going for more MPG, the taller narrow tires are the best ones to run.
Wide tires have more wind and rolling resistance, so even though they are taller, they could actually cost you fuel mileage.
The final drive ratio on a C6 is 1 to 1 like it is on a four speed or a five speed in fourth.
The numbers I gave above are mathmatical calculations, tire dimansions do vary with air pressure and different tire manufacturers.
The amount of sidewall flex does change the MPH at a given RPM, but it should not be off that far.
Tire diameter is more accurately the distance from the center of the axle to the ground times 2, with the truck setting on a flat level surface and normal loading.
When you are going for more MPG, the taller narrow tires are the best ones to run.
Wide tires have more wind and rolling resistance, so even though they are taller, they could actually cost you fuel mileage.
