So I have a 95 7.3L 5spd manual on 35"tires I can't get the truck over 75mph and when going up small grades I lose 10mph I'm thinking it the injectors, the turbo, or speed limiter and if it's the limiter how do I adjust it and where would it be

 

Is your speedometer calibrated for those tires?

 

^^^ this^^^ if the speedometer is not calibrated to the 35 inch tires, it will be reading 10 mph lower than you are actually going.

 

Yeah I use a gps for that reason and also when I get a chance I use the speed signs that show you your speed and the gps is right on

 

The speedometer on the truck needs to be calibrated to your tire size. Needs to talk to transmission.

 

What is the gear ratio in the differential?

If it is relatively tall, and you have tall tires, and we know it is a tall truck, and it is pushing a lot of air, all of this can add up. Moreso if this is true and you are leaving it in overdrive.

What RPM are you at when actually going 75MPH on the GPS?

The way your question is worded, I can not divine if it used to be able to do these things you are looking for . . . or if it never did them. Could you elaborate?

How long have you owned this truck?

 

I've owned this truck for 3yrs, I bought it this was. When I'm doing 75mph on the gps I'm around 2000rpms, and the tack says im doing 60mpg. the od reads that it has 164000miles on it and for the gearing I have no clue

 

Any chance you have boost leaks?

 

I've been wondering the same thing

 

Okay so guessing it is set up for the original 235/85R16 tires so 31.728". New tires are 35". That makes the current tires 10.311% taller than stock. Doing the math, it should be reading 67.989MPH. Different than what we see in this thread.

Maybe it has 215/85R16 tires. If memory serves this is on the dually, and I don't know as of this thread whether this is an F250 or F350SRW or F350DRW. So 30.389". New tires are 35". That makes the current tires 15.170% taller than stock. Doing the math, it should be reading 65.121MPH. Still different than what we see in this thread.

So . . . I know 75MPH from the GPS, 2000RPM from your tach, probably 0.77:1 overdrive ratio if it doesn't have an oddball transmission, and 35" tires from your description. Doing the math this gets us a 3.606:1 rear end. 3.606:1 isn't a common ratio, but 35" is too round of a number.

If the 35" tire is really 34.456" tall (0.544" difference), you have a 3.55:1 rear end.

If the 35" tire is really 36.2031" tall (1.2031" difference), you have a 3.73:1 rear end.

I'll guess your tires are really 34.456" tall. Maybe the tread is worn down or something, easily accounting for a half inch.

Based on this, I believe you have a 3.55:1 rear end, which would have put this truck at about 69MPH at 2,000RPM with 235/85R16 tires on it. Cross checking math, this is very close to dead on considering the 10.311% taller tires you have.

It is possible in my mind that this thing is geared tall and has tall tires. It is also big and pushes a lot of air, and maybe a lot more air if these tires are extra wide and/or it is a dually with extra tall tires and extra fenders. I haven't asked if it has a tall topper or camper on it, that would drag more air. Maybe the engine isn't quite in perfect tune. Some or all of these things together would create an upper limit.

Maybe someone with more seat-of-pants experience on one of these with bigger tires etc. will chime in.

 

Not a dually and no topper. But wow Im impressed I don't know how to figure that stuff out that's cool. So from what everyone is saying I need to get the trany and the speedometer to talk get them synced up and that might help

 

What mods do you have on the truck? Downpipe intake chip?

with a manual it shouldn’t matter because you control the tranny not the computer

 

Good to know.

Aww shucks! Hopefully my math is right.

I can't imagine that correcting the speedometer would help with the fight between engine and everything the engine is pushing. Doing this would give you an accurate reading of the same actual performance. If I'm nearly correct that is.

 

I agree. And as CathedralCub mentioned before these trucks are big and push a LOT of air. Those 35s aren’t helping (even tho big tires are awesome don’t get me wrong ) if you’re using your gps for speed all you really need to worry about is engine mods. If your truck is completely stock I can understand it having that trouble. Throw a 3 inch downpipe in it a 6637 intake and a hydra and you’ll be good to go