Engine/Transmission (1994 - 1998) Speed Correction, how many tooth gear?

[gwizpro]

Hey guys,



Just put on some new rubber and now I need to correct my speedo.



Old tires were LT305 70 R16 and speedo was tight on.



New tires are LT315 75 R16 and speedo is off by 5MPH.



I E. Speedo says 70 -actual speed is 75.



Need to knwo how many tooth gear I need to get. Stealer was absolutely no help...



Thanks all

 

[Got Smoke?]

Originally posted by The RackMan

Stealer was absolutely no help...

Are you even the least little bit surprised?:-laf



I believe Rob Thomas posted something about this just recently? Have you done a search to see what you can come up with from previous post's?

 

[Dennis Garrett]

I went through the same thing with my 97. I changed the gear to get the odometer correct, but the speedo was off by 7 mph at that point. I took the bezel off the dash, took the lense out, and ran down the intestate with cruise set at 70. Then with my stopwatch, I figured actual speed for about 10 miles until I was sure I knew exactly how fast I was running by the watch. Then with out changing anything, I picked the needle off the speedo, and stuck it back on it's shaft at thecorrect speed. Worked fine, just took a firm pull to get it off. It's correct at all speeds now. Just be carefull, it's hard to drive & do this at the same time.

 

[gwizpro]

Hi Dennis,



That is a great way to correct the speed, But would the odometer & trip meter read correctly?



Is the odometer dependant on needle position or some other means. . ?? I. E. revlutions of a cable or a speed sensor. ?



Thanks for the input. If the odometer will read the correct miles driven, then this is what I'll do...

 

[illflem]

With the moving the needle method the odo will be off and the speedo will only be right at the mph you set it at. Could make your warranty last longer though.



http://www.thisoffice.com/calc_tire3.html#tire_code



Use this table to find the percent difference between your new tires and the ones where the speedo is correct. Multiply your present gear teeth by the percent difference for the number of teeth you need.

 

[gwizpro]

Thanks illflem,



I came up with a 5% difference. Will get to the Stealer in am.



One question tho, the tables calculated engine RPM's, but they do not match what I get for rpm at say a listed speed. I used a GPS for determining speed. Is it possibly do to the constant used for final drive ratios. Whereas the OD will be less than 1:1



Great help,

Thanks.

 

[Dennis Garrett]

I can adjust the speedometer by moving the needle on it's shaft, but I can't do anything about the odometer. That's why I re-geared it. I've tried three different gears, and never had the odo & speedo combination perfect. So I chose the gear that gives me about 10. 1 odo reading for 10 miles traveled, and picked the speedo needle off & re-set to the speed I was traveling. Odometer is real close, & and the speedometer is almost perfect. I've checked the speedometer at 50, 60, & 80, and it's within 1 mph at all speeds.

 

[gwizpro]

Thanks Dennis,



This is what I will be doing also.



Have a Great Day!

 

[MDKram]

When I put BFG 235/85 on, the speedo read 2. 5 mph (4 km/h)slow at 62 mph (100 km/h). I change the speedo drive gear to 1 tooth less and both the speedo and odometer are bang on. I verified this using both GPS and DOT distance markers along the highway. I imagine 2 teeth less would get you very close to the correct reading.

Marc