I just got home after the initial 20 mile drive from the shop with new 4.88s. They replaced 4.10s and I have 35.4 " tires on my 2005 F250. I generally run a 5Star 87 perf/tow tune. I talked to Mike and he told me to return to the stock tune before the gear swap. I forgot to do that. The battery was clearly disconnected while the work took place because the stereo reverted to default settings. Now, a couple things have me confused.
1) It was my intent to not exceed 60 MPH on the way home but I looked down at the speedo and it read 70...no way was I going 70. I passed two of those road side "slow down!" signs with the digital read-out of your speed and both confirmed that the truck was reading about 10 MPH high. Could the truck have reset to stock settings somehow? If I hook up the X-3 will it tell me which tune is currently installed? Should I return it to stock setting if it isn't?
2) Assuming the speedo is off, at 60 MPH (actual speed) the tach was just a hair over 2000. Does that seem right? I expected it to be higher.
Thanks for you input.
Jeff

 

Some stand alone GPS devices show speed .

 

You have to reflash the truck to change the tune. Disconnecting the battery will not change the tune in the vehicle. If Mike told you to go back to stock go ahead and do that.

Your speedo is going to be off because of the large tires not the gears. If you have a GPS use that to tell your actual speed. Your tachometer will still show the correct rpm.

 

The tune that was (is?) on the truck accounted for the tires. On Wednesday, I drove the truck to the shop the speedo was close to dead on. All they did was install new front and rear gears. Everything else was untouched. I drove the truck home today and the speedo was off by 10 MPH. I guess I will take it back to stock tomorrow and see what happens.

 

On my EX with 4.88s and 35.28" tires the RPMs run at 2055 at a GPS verified 60 MPH. So at least that matches up.
Why was Mike having you revert back to a stock tune? Just for the speedo correction or to take it that much easier on the new gears until they are seasoned a little?
I don't think my SCT X3 will let me adjust the tire size to correct the speedo, but mine is a 2 valve V-10. I just know my speed is always 10% higher than my speedo shows, and I don't don't drive too fast, in the EX anyway.

 

Good to know about the RPMs. Thanks for that info. I loaded a speedometer app on my phone this morning. Reviews said it was very accurate. I will take it out on the freeway today to reconfirm yesterday's findings. I'm not sure why Mike suggested taking the tune back to stock. I have exchanged a few emails with him over the years and he is a man of few words. Always prompt and helpful but right to the point.

 

4.30 gears: I'm seeing 1800 rpm @ 60 mph with 35.2" rubber

sounds like your RPM's are right about where'd I'd expect with 4.88's. My speed is 5-7 mph "off" at highway speeds as well. annoying, but price to pay for a cool truck. Speedo is 2-3 mph off at speeds <40 mph

 

Reflashed the computer to stock settings. Speedometer now reads 6 MPH less than actual at 65. That is what it should read with 35" tires. I now remember that it was exactly the same before I got the custom tunes. I still have no clue why it was reading 10 MPH over yesterday with the 87 perf/tow tune installed. Anyways, I will contact Mike and see if we can get things straightened out because I miss my tune already. Shifts are simply not as crisp and throttle response, although better than with the 4.10s, is still not quite up to par. I feel good about the gears so far but I really want to mash the go pedal to see what she can do. I am committed to babying the gears for the prescribed break-in period so I will behave.

 

Mike sent me a revised 87 perf/tow tune this morning. Went straight to the parking lot and loaded it up. I couldn't wait so I baled on work for a little test drive. Speedo appears to be correct, go pedal is a hair trigger and running through the gears as I entered the freeway was strong and seamless. Tach sits right around 2200 at 65. Can't wait to tow!

 

I calculated my mileage with the new gears for the first time. I traveled 308 miles since my last fill up and I put in 30.5 gallons today...right at 10 MPG! In the two years that I have owned the truck, I've never even attained 9 MPG. I certainly didn't swap the gears to gain economy but this has me very pleased.

 

In theory, at least, your engine is most efficient at it's torque peak, and in theory, that is the rpm you would want to cruise at (or tow at). You may still be somewhat low-geared for the V10, but sounds like a step in the right direction.

 

The 4.88 ratio was chosen because of my 35" tires. I had read time and again in this forum that the absolute best ratio for these trucks is 4.30. My effective ratio with the oversize tires calculates out to about 4.36. I have not done any towing yet, but I can tell you that from a "seat of your pants" standpoint, I feel like I have a new truck while cruising around town.

 

I can't wait to get a 5 star tune. Been waiting for years though, just so many other things to spend money on!

I hear a lot how the gear swap or tune doesn't hurt mileage like you'd think, and many times actually better, but in all those times the person reporting the gain is getting super crappy mileage already. Even towing, I've never had one tank below 10MPG. Heck I could probably count on my fingers how many tanks below 12 I've ever got. I usually average 13.5-14.5 at least in the summer. That's all stock w/ 4.10s. I'd love to go 35's I just can't believe I'd be giving up so much MPG going from 32" to 35" lol.

 

I think the biggest hogs are the old 2-valve pre-'05 V10s that made less power, thus your foot is to the floor more often. I just bought a 2010 F250 SuperCab longbed FX4 Lariat 2 weeks ago, that rolled off the line with 18" wheels, and some moron changed out to 20"s......so it has 34" tires. No towing yet, but 2 tanks of mostly highway driving and I'm getting 10.3 mpg. Not complaining, because it runs great unloaded, and I'm hopping off the deisel bandwagon. But I'm going to put 17"s on it before I tow anything heavier than a 20' boat, because it is geared with 4.10s, and the tall tires keep the engine rpms too low. The V10 wants to rev, and it's torque peak is at 3250, which is where the engine is running most efficiently. This ideally would be the rpms at highway cruise for best mileage.

 

It generates maximum torque at that rpm but that is not where its most efficient for mileage (where you want the minimum power generated and least fuel that overcomes drivetrain losses and wind resistance, etc, maybe 50hp on the highway?). I've found as low as I can go gives me best mpg, keeping it no higher than 1800 rpm even while accelerating gave best mpg. If you think your going to get better mileage over 3000 rpms on the highway just downshift and drive like that if you can, will be closer to your rpms but I can guarantee your not going to get good mpg.