I have a '98 2x4 F150 XLT Supercab with 40,000 miles. It is time for new tires. My truck came with 235x70x16 but I'm thinking about going with a 255x70x16. In fact, I may be able to get 2001 F150 'take offs' cheap from a truck conversion company. I think they are General tires in the 255 size.

My question is will the larger tire screw up my speedometer calibration and/or will it affect the transmission i.e. shift points? If the answer is yes to either of the above how can I adjust/correct so I can use the 255 tires?

Thanks

Eddie

 

Going to a taller tire will change all that listed. BUT......... Since you are only looking at a 255/70/16's, it's not really that big of a change. Probably won't even notice it. I don't think you would be able to get the correct speedo gear to to compensate anyway.

Mine had a 20 tooth gear in it originally. The door jamb sticker said the tires for this rig is 255/70/16's, but what was actually on the rig was 235/70/16's. I originally got some 255/70/16's and didn't notice any difference in speedo or shifting. I later decided to go taller and went for the 265/75's. That's when I had to change the speedo gear to a 19 tooth. Now it's just right. If you changed to the 19 tooth gear, it would run "fast" on your speedo.

If you were going to a 265/75/16, that would change things alot.

 

Mitch,

Thanks for the feedback. Oh yea, very nice truck!!

Eddie

 

Check out this link I found in another wheel/tire post. It will tell you exactly what your speedometer and odometer error will be for a given tire size change.


http://www.dakota-truck.net/TIRECALC/tirecalc.html

 

You must have had the General 550 AS 235x70R16's. I have about 40K on my 97 F-150 and one of the Generals blew out on the highway a month back (I had a hole the size of you fist with shards of metal belt sticking out and a split up the inside sidewall).

Anyway, a friend has the same engine, transmission, and rear-end in his 97 F-150, but he had the Goodyear RT/S 255x70R16's. Since he had significantly better gas mileage than me, I decided to get the larger Goodyear tires.

I'm by no means a lead-foot at the ripe old age of 41, but my mileage got worse and the truck doesn't seem to have as much acceleration from a dead stop or while passing. Any suggestions?

Mark

 

Okay, I tried the calculator which told me that my speedometer and odometer would read low by 3.67% and that the effective gear ratio of my rearend would be reduced from 3.55 to 3.42.

Thinking things through, this makes sense and I should have realized this would happen. My mileage calculations are low (by 0.5 to 0.75 MPG for 14-18 MPG) because my speedometer is reading low. Performance seems sluggish because I'm running in a slightly higher gear. Let's put these observations in the "well duh, I should have known that" category.

I'm not really dissatisfied with the change. The tires do look marvelous, the effect loss of acceration doesn't bother me that much, I'm a little dissappointed that the economy hasn't improved as I was hoping. However, what still concerns me is that the shifting of my automatic transmission doesn't seem as smooth as it used to be.

I'm wondering since all I really did was change from the standard size tires available when I purchased my truck to the optional size tires, if the adjustments to compensate for the difference can be done with a software update at the dealer rather than a change in parts and hardware. With what little I know about the dynamics of this issue, it would seem that Ford would save money if they didn't have to install different parts on an assembly line simply because of a tire option. Perhaps the software change will help me realize the better economy I was thinking I'd get.

BTW. I was hoping that I might get 1 MPG out of it in the city and up to 2 on the highway. My last mixed city (2/3)/highway (1/3) average was 14.52 MPG corrected for the odometer error.

Mark

 

That's all you're gonna get really. 14.5 is not bad for your city/hwy average. Mine is the opposite of yours however. 2/3 hwy, 1/3 city. I am getting 15.0 mpg. On strictly hwy miles, I go up to about 16.5 mpg. I don't have a bed cover or cap. Run with the tailgate up too.

My tire combo did the same thing for the shifting and stuff, but still acceptable to me. The looks and performance of the bigger tires are worth it to me.