Hey guys, I've got a question. I've got 3.55 gear ratios in my truck with the stock 18's. I'm planning on switching to the factory 20's and I was considering getting 4.10's for performance reasons.

I'd like to know how much this is going to hurt my fuel economy by switching to 4.10's because I drive around 35,000 miles a year. I'd say 70% highway and 30% city. If the truck won't get that much worse on the highway then I'll do it but with current fuel prices I'd figured I'd make a post to see responses. I've searched threads but not found too many that have switched that aren't running huge rims/tires.

What will my rpm's be at 80 with the 20's and 4.10's?


Any help appreciated.

Thanks.

 

I dont think switching form 18s to 20s is going to change your mpg much unless you do alot of towing. As long as you keep the same size tires.

 

We need to know tire sizes before we can give you RPM

 

I'm going to be switching to the OEM 20's with 275/55/20's Pirellis. Currently I have the OEM 18's with 265/60/18's.


Will switching to the 4.10's kill my fuel economy?

 

Negligible difference. No need to switch gears.

Use this site. Good tire size calculator.
http://www.miata.net/garage/tirecalcold.html

 

With the 18's and the 3.55 you should be turning about 2117 RPM at 80MPH.

With the 20's and the 4.10 you should be turning about 2419 RPM at 80MPH.

So you're engine will be turning ~14% faster which should result in better city mileage but worse highway mileage. Since you do more highway driving ... it will probably cost you more fuel overall with the new gears.


edit: I forgot to adjust overdrive ratio for the f150

 

265/65/18 with 3.55 @ 80 = 2,125RPM
265/65/18 with 4.10 @80 = 2,425RPM

275/55/20 with 3.55 @80 =2,100RPM
275/55/20 with 4.10 @80 =2,425RPM
275/55/20 with 4.56 @80 =2,700RPM

Your fuel economy already sucks. But no, 4.10s will not kill your fuel economy.

 

I have a Microsoft Excel spreadsheet that you just plug in the tire sizes and gear ratios and it computes the engine RPM and speed at all speeds.

It's made for the 4r75 trans.

 

Cat's out of the bag now.

 

I had to put that in there for crediblilty. LOL Anyone that wants the spread sheet, just email me and I'll send it to ya.

 

Thanks for the replies. I'm going to get the 4.10's and eventually get the OEM 20's. Can't wait to see the difference!